Fact-checked by Grok 2 weeks ago

Protozoa

Protozoa are microscopic, unicellular eukaryotic organisms that exhibit animal-like characteristics, including motility via structures such as pseudopodia, cilia, or flagella, and heterotrophic nutrition requiring preformed organic compounds.^[1] Ranging in size from approximately 1 to 150 micrometers, they inhabit a wide array of environments, including freshwater, marine, and terrestrial habitats, where many function as free-living predators or decomposers, while others live as symbionts or parasites within multicellular hosts.^[1] The term "protozoa" is an informal, polyphyletic designation traditionally applied to these diverse protists, reflecting their historical classification rather than a single evolutionary lineage.^[2] In classical taxonomy, protozoa are divided into seven major phyla based on locomotor and morphological features: Sarcomastigophora (flagellates and amoeboids), Labyrinthomorpha, Apicomplexa (sporozoans like Plasmodium), Ciliophora (ciliates like Paramecium), Microspora, Ascetospora, and Myxozoa.^[1] However, advances in molecular biology have dispersed these organisms across several eukaryotic supergroups, such as Amoebozoa, Excavata, and Alveolata, highlighting their evolutionary diversity and challenging the monophyletic view of the group.^[3] Reproduction in protozoa occurs mainly through asexual binary fission, though sexual cycles are common in parasitic forms, enabling alternation between hosts and environmental stages.^[1] Ecologically, protozoa play crucial roles in nutrient cycling and food webs, acting as bacterivores that regulate microbial populations in aquatic and soil ecosystems.^[4] Parasitic protozoa, however, pose significant threats to human and animal health, with species like Plasmodium falciparum causing malaria and Entamoeba histolytica leading to amoebic dysentery; these infections often spread via contaminated water, food, or arthropod vectors and can multiply rapidly within hosts.^[5] Many protozoan species engage in symbiotic relationships, underscoring their broad impact on biodiversity and disease dynamics.^[2]

Classification and History

Historical Development

The discovery of protozoa began with the pioneering microscopic observations of Antonie van Leeuwenhoek in 1674, who first described these single-celled organisms as "animalcules" in samples from rainwater, well water, and dental plaque, using handmade single-lens microscopes that magnified up to 270 times.^[1] These early sightings marked the initial recognition of microscopic life forms, though Leeuwenhoek viewed them primarily as diminutive animals without formal classification.^[6] In the early 19th century, Georg August Goldfuss introduced the term "Protozoa" in 1818, establishing it as a class within the animal kingdom to encompass these "first animals," including sponges and other low forms initially. Christian Gottfried Ehrenberg advanced this in 1838 with his seminal work Die Infusionsthierchen als vollkommene Organismen, classifying protozoa—termed "Infusoria" or animalcules—as complete, organized animals with distinct organs, based on detailed microscopic studies of over 300 species from infusions and sediments.^[6] Carl Theodor von Siebold refined the concept in 1845, redefining Protozoa strictly as unicellular animals in his Lehrbuch der vergleichenden Anatomie der wirbellosen Thiere, excluding multicellular forms and aligning them with emerging cell theory by emphasizing their single-celled nature.^[7] Ernst Haeckel further elevated their status in 1866 by proposing the kingdom Protista in Generelle Morphologie der Organismen, grouping protozoa with unicellular algae and fungi as primitive, cell-based organisms intermediate between plants and animals, influenced by cytology and evolutionary theory.^[8] By the early 20th century, classifications shifted toward cytology and parasitology, with Louis Léger's 1902 contributions emphasizing locomotor structures and life cycles, dividing protozoa into groups like flagellates, ciliates, and sarcodines based on morphological and developmental traits observed in parasitic forms.^[9] This culminated in Richard R. Kudo's influential 1954 system in Protozoology, which standardized four major classes—Sarcodina (pseudopodial), Mastigophora (flagellate), Sporozoa (spore-forming parasites), and Ciliata (ciliated)—drawing on advances in microscopy and staining techniques to highlight cellular organelles and reproductive modes, though later recognized as paraphyletic.^[10] These frameworks reflected growing integration of protozoa into broader biological sciences, transitioning from simplistic animal-like groupings to more nuanced understandings of their diversity.^[6]

Modern Phylogenetic Framework

Protozoa are traditionally defined as heterotrophic, unicellular eukaryotic organisms lacking cell walls and exhibiting diverse modes of motility, but molecular phylogenetic analyses since the 1980s have established them as a paraphyletic assemblage rather than a monophyletic clade.^[11] This recognition of their artificial nature stems from revisions in protist taxonomy, particularly those by Thomas Cavalier-Smith, who initially classified protozoa as a kingdom but later highlighted their polyphyletic origins through ultrastructural and genetic evidence.^[12] The integration of protozoa into broader classificatory systems began with Robert Whittaker's five-kingdom proposal in 1969, which placed them within the kingdom Protista as a diverse group of unicellular eukaryotes alongside algae and fungi-like organisms. This framework was superseded by Carl Woese's three-domain system in 1990, which reorganized cellular life into Bacteria, Archaea, and Eukarya based on 16S/18S rRNA sequences, firmly situating all protozoa within the domain Eukarya as basal or derived unicellular forms. Contemporary understanding disperses protozoa across major eukaryotic supergroups, as delineated in phylogenetic revisions such as Adl et al. (2019), reflecting their evolutionary divergence.^[13] The supergroup Excavata includes heterotrophic flagellates like diplomonads (e.g., Giardia) and euglenozoans (e.g., Euglena and Trypanosoma). The SAR clade encompasses alveolates (e.g., apicomplexans such as Plasmodium) and rhizarians (e.g., foraminifera). Amoebozoa comprises lobose and filose amoebae, while Opisthokonta features choanoflagellates as closest protist relatives to animals and fungi. Key evidence for this polyphyly derives from 18S rRNA gene sequencing initiated in the 1980s by Mitchell Sogin and colleagues, which revealed deep divergences among protozoan lineages; for instance, analyses of Naegleria demonstrated the polyphyletic origins of amoeboid forms.^[14] Trypanosoma, a kinetoplastid parasite, exemplifies this by nesting within Euglenozoa of Excavata, distant from other flagellate protozoa. Approximately 92,000 protozoan species have been described,^[2] with taxonomy managed under codes like the International Code of Zoological Nomenclature (ICZN) for animal-like groups, underscoring their non-monophyletic status in modern systematics.^[15]

Morphological and Physiological Characteristics

Cellular Structure and Size

Protozoa exhibit a unicellular eukaryotic organization, characterized by the presence of a membrane-bound nucleus containing chromatin, mitochondria for energy production (or reduced mitosomes in anaerobic species such as Giardia lamblia), a Golgi apparatus involved in secretory processes, and an endoplasmic reticulum for protein and lipid synthesis.^[1] The cytoplasm is typically divided into an outer ectoplasm, which is often clear and gel-like, and an inner endoplasm rich in granules and organelles, enabling complex metabolic activities within a single cell.^[1] Lysosomes and vacuoles are also common, aiding in digestion and osmoregulation, while ribosomes are distributed throughout the cytoplasm for protein synthesis.^[16] Unlike plant cells, most protozoa lack a rigid cell wall, relying instead on a flexible plasma membrane for protection and shape maintenance; however, many possess specialized external coverings such as a pellicle, a proteinaceous layer reinforced by microtubules that provides structural support and flexibility, as seen in ciliates like Paramecium and flagellates like Euglena.^[2] Some groups feature protective tests or shells, including siliceous skeletons in radiolarians (Radiolaria), which form intricate lattice-like structures for buoyancy and defense, or calcareous tests in certain foraminiferans.^[17] Loricae, rigid external cases often composed of organic material or sediment particles, are prominent in tintinnid ciliates and some dinoflagellates, serving as protective enclosures that enhance survival in planktonic environments.^[18] Internal skeletal elements further contribute to cellular integrity in select protozoa; for instance, heliozoans like Actinosphaerium utilize axopodia—long, slender pseudopodia supported by axial bundles of microtubules—for structural reinforcement and prey capture support.^[2] Microtubular arrays also form supportive frameworks in various groups, such as the axostyle in symbiotic flagellates like Trichonympha.^[2] Protozoan cell sizes vary dramatically, reflecting adaptations to diverse ecological niches, with most free-living species measuring 10–100 μm in diameter, though parasitic forms are often smaller at 1–50 μm.^[1] The smallest known protozoa include kinetoplastids, with species such as Leishmania spp. measuring 2–6 μm and Bodo saltans ranging from 5–12 μm, while the largest, such as xenophyophores (Xenophyophorea), form multinucleate aggregations up to 20 cm in length on deep-sea floors.^[19]^[20]^[21] Radiolarians typically span 0.1–2 mm, their siliceous tests influencing overall dimensions.^[17] Size is influenced by environmental factors like nutrient availability, which can limit growth in nutrient-poor habitats or promote expansion in resource-rich ones.^[22]

Motility and Locomotion

Protozoa exhibit diverse motility mechanisms adapted for navigation in aquatic or host environments, primarily through flagella, cilia, pseudopodia, or gliding, each powered by ATP hydrolysis via molecular motors like dynein or myosin. These structures enable active locomotion, often triggered by environmental cues such as chemotactic gradients toward nutrients or away from toxins.^[23]^[24] Flagellar motility involves 1 to 8 whip-like flagella, typically arranged with a 9+2 microtubule axoneme where nine outer doublet microtubules surround two central singles, anchored by a basal body of nine triplets. Dynein arms on the doublets generate sliding forces that produce undulating or planar waves, propelling the cell forward at speeds up to several body lengths per second, as seen in the kinetoplastid Trypanosoma brucei, where a single posterior flagellum enables serpentine swimming. This ATP-dependent mechanism relies on axonemal dynein ATPase for bending, with intraflagellar transport maintaining the structure.^[23]^[1]^[25] Ciliary motility features thousands of short, hair-like cilia covering the cell surface, coordinated in metachronal waves that sweep in unison to generate fluid currents or propel the organism. In Paramecium caudatum, for instance, up to 2,500 cilia arranged in rows beat with a power stroke followed by recovery, achieving speeds of 1-2 mm/s through planar or helical waves, regulated by the rotating central microtubule pair in the 9+2 axoneme. This dynein-powered system, also ATP-driven, allows precise control and reversibility of direction.^[23]^[1] Amoeboid movement relies on dynamic pseudopodia formed by cytoplasmic streaming and actin-myosin contractions, where polymerizing actin filaments push the plasma membrane outward while myosin II pulls the cell body forward. In Entamoeba histolytica, this enables crawling at rates of 5-10 μm/min across substrates, with bleb-like pseudopods nucleating via localized actin assembly for directional migration. Subpellicular microtubules in some forms provide rigidity during extension.^[26]^[27]^[1] Other motility types include gliding, observed in gregarines like Gregarina spp., where actin-myosin interactions translocate surface adhesins rearward along the cell body without external appendages, achieving slow, substrate-dependent movement influenced by cortical folds and environmental conditions. All these mechanisms are fueled by ATP generated through glycolysis or mitochondrial respiration, with chemotaxis modulating direction via receptor-mediated signaling in species like Giardia lamblia.^[28]^[29]^[24]

Nutrition and Feeding Mechanisms

Protozoa are predominantly heterotrophic organisms, relying on the consumption of organic matter for energy and growth, though some exhibit mixotrophic capabilities combining heterotrophy with autotrophy.^[30] Their feeding mechanisms are adapted to diverse environments, enabling efficient nutrient acquisition through various strategies such as phagocytosis and absorption.^[31] Phagotrophy represents a primary heterotrophic mode in many protozoa, involving the engulfment of solid food particles like bacteria, algae, or other microorganisms. In amoeboid protozoa such as naked amoebae, pseudopodia—extensions formed by actin polymerization—surround and internalize prey, forming a food vacuole within the cytoplasm.^[31] Ciliates and flagellates employ similar processes via cytostomes or oral grooves, where prey is drawn in by ciliary or flagellar action.^[32] Osmotrophy, another key heterotrophic strategy, entails the direct absorption of dissolved organic nutrients across the plasma membrane, particularly prevalent in parasitic protozoa lacking phagocytic apparatus. For instance, parabasalids like Trichomonas vaginalis acquire amino acids, sugars, and other solutes from host tissues through membrane transporters, bypassing the need for particle ingestion.^[33] This mode supports rapid nutrient uptake in nutrient-rich, anaerobic environments.^[33] Some protozoa display mixotrophy, integrating phagotrophy or osmotrophy with photosynthesis via retained chloroplasts. Euglena gracilis, a euglenid, exemplifies this by performing both autotrophy under light conditions and heterotrophy through ingestion of organic particles or absorption of dissolved organics, allowing metabolic flexibility in varying light and nutrient availability.^[30]^[34] The digestive process in phagotrophic protozoa begins with endocytosis, where the plasma membrane invaginates to enclose prey in a phagosome. This vesicle fuses with lysosomes, acidifying the compartment to a pH of approximately 4-5 via proton pumps like V-type H⁺-ATPases, which activates hydrolytic enzymes such as proteases and lipases for breakdown of proteins, lipids, and other macromolecules.^[35]^[36] Nutrient monomers are then released into the cytoplasm for assimilation, while undigested residues are expelled through exocytosis.^[32] Specialized adaptations enhance feeding efficiency in certain protozoa. Raptorial ciliates, such as those in the Haptoria group, deploy toxicysts—extrusome organelles that discharge toxins upon prey contact—to immobilize targets before phagocytosis, supported by gene duplications for membrane transporters and hydrolytic enzymes.^[37] In parasitic forms, expanded plasma membrane surface area facilitates osmotrophic absorption, optimizing nutrient extraction from hosts.^[38]

Reproduction Strategies

Protozoa exhibit diverse reproduction strategies, primarily asexual and sexual, which enable rapid population growth and genetic adaptation. Asexual reproduction predominates under favorable conditions, allowing for quick proliferation without the need for mates, while sexual reproduction often arises in response to environmental stressors, promoting genetic diversity through recombination. These mechanisms vary across protozoan groups, reflecting their evolutionary adaptations to different ecological niches.^[1]^[39] Asexual reproduction in protozoa occurs mainly through binary fission, multiple fission, and budding. Binary fission, the most common method, involves the duplication of organelles followed by cytokinesis, resulting in two genetically identical daughter cells; in flagellates, division is typically longitudinal, whereas in ciliates, it is transverse. Multiple fission, or schizogony, is characteristic of apicomplexans, where the nucleus undergoes repeated divisions to produce numerous merozoites from a single schizont, as seen in Plasmodium species during the erythrocytic stage, potentially infecting up to 10% of red blood cells and yielding around 400 million parasites per milliliter of blood. Budding occurs in certain peritrich ciliates like Vorticella, where a smaller daughter cell (swarmer) develops externally on the parent and detaches after maturation. Under optimal conditions, such as adequate nutrients and temperature around 20-22°C, Paramecium species can undergo binary fission every 8-12 hours, dividing 2-3 times per day.^[1]^[1]^[40]^[41] Sexual reproduction in protozoa facilitates genetic exchange and is triggered by environmental stresses like nutrient scarcity or temperature fluctuations, which favor modes enhancing variability for survival. Syngamy, the fusion of gametes to form a zygote, occurs in apicomplexans such as Plasmodium, where microgametes fertilize macrogametes during the mosquito stage to initiate sporogony. Conjugation, prevalent in ciliates like Paramecium, involves temporary pairing of two individuals, followed by the exchange of haploid micronuclei through a cytoplasmic bridge, restoring diploid micronuclei and promoting recombination without gamete production. Some protozoa, including opalinids, employ parthenogenesis—development from unfertilized eggs—or hermaphroditism, where individuals produce both male and female gametes, further diversifying reproductive options in stable or isolated environments.^[39]^[1]^[42]^[43]

Life Cycles and Aging

Protozoan life cycles vary significantly, ranging from simple direct cycles completed within a single host or environment to complex indirect cycles requiring multiple hosts or stages for completion. In direct life cycles, typical of many free-living or monoxenous parasitic protozoa, development occurs without host alternation; for example, Amoeba proteus reproduces asexually through binary fission in a single aquatic environment, transitioning between active trophozoite and dormant cyst stages as needed for survival.^[44] In contrast, indirect life cycles involve obligatory passage through intermediate and definitive hosts, as seen in the apicomplexan Plasmodium species responsible for malaria; here, the cycle includes sporozoite injection by a mosquito vector into a human host, followed by liver-stage merozoites that invade erythrocytes to produce more merozoites and sexual gametocytes, which are then taken up by the mosquito to form new sporozoites.^[45] These cycles ensure transmission and adaptation to diverse ecological niches.^[46] A key adaptation in many protozoan life cycles is encystment, the formation of a resistant cyst stage that enables dormancy and survival under adverse conditions such as desiccation or nutrient scarcity. In Giardia lamblia, a flagellate parasite, trophozoites in the host's intestine differentiate into cysts upon exposure to bile and pH changes in the lower gut, forming a protective wall that allows cysts to persist in water or feces for weeks to months.^[47] Excystment occurs when cysts are ingested and encounter stomach acid and intestinal bile salts, triggering the release of viable trophozoites to resume the active phase. This process is crucial for environmental transmission in both free-living and parasitic species.^[48] Protozoan life cycles often exhibit haplontic or diplontic patterns, reflecting variations in ploidy dominance. Most protozoa follow a haplontic pattern, where the haploid phase is dominant and meiosis occurs zygotically, as in many amoebae and flagellates that maintain haploid vegetative cells throughout most of their cycle.^[1] Some alveolates, such as certain dinoflagellates like Noctiluca, display diplontic patterns with a predominant diploid phase, where mitosis occurs in diploid cells and haploid stages are brief.^[49] These patterns support genetic diversity through sexual reproduction in select species. Aging in protozoa manifests as replicative senescence or clonal deterioration, particularly in ciliates, where repeated asexual divisions lead to declining fitness. In Tetrahymena thermophila, telomere shortening during macronuclear divisions contributes to senescence, as progressive erosion of telomeric repeats impairs chromosome stability after numerous replications.^[50] Clonal aging arises from accumulated mutations in the somatic macronucleus, reducing growth rates and viability over generations in the absence of sexual reorganization.^[51] However, some ciliates achieve apparent immortality through macronuclear reorganization during conjugation, where the old macronucleus is resorbed and a new one is generated from the unaltered micronucleus, purging deleterious mutations and resetting cellular age.^[52] Life cycle durations differ markedly between free-living and parasitic protozoa, influencing population dynamics and transmission. Free-living species like Tetrahymena often complete generations in hours, with division times of 2-3 hours under optimal conditions, allowing rapid proliferation in stable environments.^[53] Parasitic cycles, such as that of Plasmodium, can span years due to dormant stages like hypnozoites in the liver, which may reactivate months to decades after initial infection, sustaining chronic transmission.^[45]

Habitats and Distribution

Free-Living Niches

Free-living protozoa thrive in diverse natural environments, predominantly as independent organisms in aquatic, marine, and terrestrial settings. These unicellular eukaryotes are found in nearly every conceivable habitat, from freshwater bodies to ocean depths and soil moisture films, where they contribute to nutrient cycling and microbial food webs.^[1]^[54] In freshwater habitats such as ponds, ditches, and shallow puddles, protozoa inhabit nutrient-rich, sunlit waters that support ample sunlight and organic matter. These environments provide ideal conditions for heterotrophic protozoa, with species adapting to hypotonic conditions through osmoregulatory mechanisms. Contractile vacuoles in freshwater protozoa, such as those in amoebae and ciliates, actively expel excess water and ions to maintain cellular homeostasis, preventing osmotic lysis in dilute surroundings.^[55]^[56]^[2] Marine habitats host planktonic free-living protozoa, including radiolarians and phaeodarians, which are holoplanktonic organisms with intricate silica skeletons that sink upon death, contributing significantly to the ocean's silica flux and carbon export. These protists dominate in open ocean waters, where their biomineralization plays a pivotal role in geochemical cycles, with global carbon demand from flux-feeding phaeodarians estimated at 0.46 Pg C per year.^[57]^[58] These protozoa exhibit cosmopolitan distributions but show variations in abundance tied to productivity gradients.^[59] Terrestrial free-living protozoa inhabit moisture films in soils and leaf litter, where naked amoebae such as gymnamoebae predominate as bacterivores in the rhizosphere and detrital layers. These environments, often oligotrophic and variable in moisture, support high diversity, with over 200 species reported in some temperate forest soils.^[60] Adaptations to terrestrial life include cysts for desiccation resistance and rapid encystment in drying conditions.^[61]^[62] Certain free-living protozoa demonstrate thermal tolerance, with some thermophilic ciliates enduring temperatures up to 100°C in hydrothermal vent sediments, facilitated by heat-stable enzymes and membrane adjustments. In extreme environments like Antarctic lakes, protozoa exhibit biogeographic patterns, including endemics such as the ciliate Euplotes focardii in oligotrophic coastal sediments, contrasting with cosmopolitan species in more temperate zones. Climate change is altering these distributions, with warming temperatures potentially shifting ranges and affecting community structures in aquatic and soil habitats.^[63]^[64]^[65]^[66] Abundances of free-living protozoa in productive waters, such as eutrophic lakes or wastewater systems, typically range from 10⁴ to 10⁶ individuals per liter, reflecting their role as key predators in microfood webs through bacterivory and algivory. This density underscores their ecological importance in energy transfer across trophic levels in these dynamic habitats.^[67]^[68]

Parasitic and Symbiotic Environments

Protozoa exhibit a wide array of host-associated lifestyles, ranging from parasitism to mutualism and commensalism, where they depend on interactions within the gastrointestinal tracts, tissues, or body fluids of their hosts for survival and reproduction. In parasitic relationships, protozoa often exploit host resources at the expense of the host's health, utilizing specialized adaptations to colonize diverse niches. For instance, intracellular parasitism is exemplified by Toxoplasma gondii, an obligate intracellular protozoan that invades nucleated cells of warm-blooded vertebrates, including mammals and birds, by forming a parasitophorous vacuole to evade lysosomal degradation.^[69] In contrast, extracellular parasitism occurs in species like Leishmania, which reside in the midgut and other tissues of sandfly vectors (Phlebotomus and Lutzomyia spp.), multiplying as promastigotes before transmission to vertebrate hosts.^[70] Parasitic protozoa employ sophisticated adaptations to persist in hostile host environments, such as antigenic variation, which allows them to alter surface glycoproteins and evade immune recognition. This mechanism is prominently featured in trypanosomes like Trypanosoma brucei, where variant surface glycoprotein (VSG) genes are sequentially expressed from telomeric expression sites, enabling the parasite to switch coats and avoid antibody-mediated clearance in vertebrate bloodstreams.^[71] Such strategies highlight the evolutionary pressures driving protozoan diversification in parasitic niches. Commensal protozoa, which neither significantly benefit nor harm their hosts, often inhabit the gastrointestinal tract and subsist on undigested host materials or microbial byproducts. A representative example is Balantidium coli, a ciliated protozoan that resides as a commensal in the large intestine of pigs, feeding on undigested carbohydrates and bacteria without causing pathology in healthy individuals.^[72] In mutualistic associations, protozoa provide essential services to their hosts, particularly in nutrient processing. Flagellate protozoa such as Trichonympha sphaerica in the hindgut of lower termites (e.g., Zootermopsis) form obligate symbioses, where the protozoa ingest wood particles and host endosymbiotic bacteria to ferment cellulose into acetate, a key energy source for the termite host.^[73] This multilayered symbiosis underscores the protozoa's role in enabling termites to exploit lignocellulosic diets. The host ranges of protozoa span multiple taxa, including infections of other protists (e.g., gregarines parasitizing invertebrate protist cells), invertebrates like insects and mollusks, and vertebrates such as mammals and fish. Transmission between hosts frequently involves biological vectors; for example, Trypanosoma species are cyclically transmitted from vertebrates to tsetse flies (Glossina spp.), where they undergo developmental stages before being inoculated into new mammalian hosts during blood meals.^[74] Environmental factors within host habitats, such as gastrointestinal pH gradients, influence protozoan distribution and survival, with many species tolerating a broad range from acidic (pH 4) to near-neutral (pH 8) conditions along the intestinal tract. Immune evasion strategies beyond antigenic variation include molecular mimicry and modulation of host cytokine responses, allowing protozoa like Leishmania to suppress macrophage activation and persist extracellularly in vector and host tissues.^[75]

Ecological and Evolutionary Roles

Ecosystem Contributions

Protozoa act as key predators in microbial ecosystems, primarily targeting bacteria and archaea to regulate their populations and prevent uncontrolled blooms. In aquatic environments, ciliates and flagellates consume bacteria at rates ranging from 1 to several dozen per cell per minute, depending on species and conditions, thereby maintaining balance in microbial communities and promoting diversity. This predation exerts significant top-down control, with protozoan grazing responsible for up to 50-100% of bacterial mortality in some systems, influencing the structure of the entire microbial food web.^[76]^[77]^[78] Through their metabolic activities, protozoa drive nutrient regeneration by remineralizing essential elements such as nitrogen and phosphorus via excretion, converting organic matter into inorganic forms readily available to primary producers. In oceanic systems, protozoan excretion of ammonium, for instance, supports a substantial portion of phytoplankton growth, contributing to 20-50% of regenerated nitrogen that fuels primary production in nutrient-limited waters. This process enhances nutrient cycling efficiency, closing the loop between microbial decomposition and autotrophic uptake.^[79]^[80] In pelagic food webs, protozoa facilitate carbon flux by grazing on picoplankton and transferring organic carbon to higher trophic levels, including metazoan zooplankton, thus bridging the microbial loop to classical grazing chains. This intermediary role ensures efficient energy propagation, with protozoan-mediated carbon flows accounting for a major portion of biomass transfer in oligotrophic oceans. On land, soil-dwelling protozoa, such as amoebae, bolster ecosystem contributions by accelerating organic matter decomposition and aiding soil aeration through their pseudopodial movement and burrowing behaviors, which improve soil porosity and nutrient availability for plants.^[81]^[82] Additionally, protozoan communities function as sensitive bioindicators of environmental health, particularly in aquatic systems where high ciliate diversity correlates with unpolluted conditions, such as clean streams with diverse microbial habitats. Variations in protozoan assemblage composition and abundance reflect changes in water quality, offering a rapid and reliable metric for monitoring ecosystem integrity.^[83]^[84]

Pathogenic Impacts

Protozoa encompass a diverse group of single-celled eukaryotes, several of which are significant pathogens causing diseases in humans and animals worldwide. These pathogenic protozoa often exploit specific transmission routes and host interactions to establish infection, leading to substantial morbidity and mortality, particularly in tropical and subtropical regions. Key examples include species from genera such as Plasmodium, Entamoeba, Giardia, Cryptosporidium, and Trichomonas, which collectively impose a heavy global health burden through mechanisms like tissue invasion and immune evasion.^[85] Among the most devastating protozoan diseases is malaria, caused by Plasmodium species, primarily P. falciparum, with an estimated 263 million cases and 597,000 deaths in 2023, predominantly among children under five in the WHO African Region.^[86] Transmission occurs via the bite of infected female Anopheles mosquitoes, which inject sporozoites into the human bloodstream during blood meals, initiating the parasite's liver and erythrocyte stages. Amoebiasis, induced by Entamoeba histolytica, affects nearly 50 million people annually with symptomatic infections, causing intestinal ulceration and liver abscesses through fecal-oral transmission in areas with poor sanitation.^[85] Giardiasis, resulting from Giardia lamblia infection, is a common waterborne illness spread through contaminated drinking or recreational water, leading to prolonged diarrhea and malabsorption, especially in travelers and young children.^[87] Similarly, cryptosporidiosis, caused by Cryptosporidium species, follows a fecal-oral route via oocysts in water or food, posing risks to immunocompromised individuals.^[88] Pathogenesis varies by species but often involves direct host cell damage; for instance, in cerebral malaria, P. falciparum-infected erythrocytes sequester in brain microvasculature through cytoadherence, causing vascular obstruction, inflammation, and neurological impairment without direct neuronal invasion.^[89] In contrast, E. histolytica actively invades intestinal mucosa via amoebic motility and proteolytic enzymes, while Trichomonas vaginalis contributes to vaginitis through contact-dependent cytotoxicity and secretion of proteins like TVSAPLIP12, which exhibit pore-forming and hemolytic activities akin to toxin effects.^[90] Protozoan diseases account for over 600,000 deaths annually, with the majority concentrated in tropical regions due to endemic transmission and limited healthcare access; emerging drug resistance, such as partial artemisinin resistance in P. falciparum characterized by delayed parasite clearance, further exacerbates control efforts in Southeast Asia and Africa.^[86] Control strategies rely on antiprotozoal drugs like chloroquine, which accumulates in the parasite's food vacuole to inhibit heme detoxification in sensitive Plasmodium strains, though resistance has limited its use in many areas.^[91] Preventive measures include vector control via insecticide-treated nets and improved sanitation to interrupt transmission cycles, while vaccines remain in development; for example, the RTS,S/AS01 malaria vaccine targets sporozoite invasion and has shown modest efficacy in reducing severe cases among children. Additionally, the R21/Matrix-M vaccine, recommended by WHO in 2023, is being introduced in several African countries as of 2025, demonstrating higher efficacy in clinical trials and supporting expanded immunization programs.^[92]^[93] Ongoing research emphasizes integrated approaches to mitigate resistance and enhance vaccine immunogenicity against these resilient pathogens.^[94]

Mutualistic and Commensal Interactions

Protozoa engage in mutualistic interactions with various hosts, where both partners derive benefits from the association. In the rumen of herbivorous mammals, ciliates such as Entodinium caudatum form symbiotic relationships by fermenting plant material, particularly cellulose and hemicellulose, into volatile fatty acids (VFAs) like acetate, propionate, and butyrate, which serve as a primary energy source for the host.^[95] These protozoa collaborate with rumen bacteria to enhance nutrient breakdown, contributing significantly to the host's caloric intake without causing harm.^[96] Another prominent example occurs in coral reefs, where dinoflagellate protozoa of the genus Symbiodinium (commonly known as zooxanthellae) live intracellularly within cnidarian hosts such as reef-building corals. These symbionts perform photosynthesis to produce carbon-rich photosynthates, which can supply up to 90% of the host's daily energy requirements, enabling calcification and growth while receiving protection and nutrients like carbon dioxide and nitrogen from the coral.^[97] Commensal interactions involve protozoa that benefit from the host without providing advantages or causing detriment. Planktonic and sessile ciliates like Trichophyra species inhabit the gills of freshwater fish, attaching to the gill surfaces and feeding on mucus, debris, and bacteria without impairing respiration or overall health under normal conditions. Similarly, in the hindguts of invertebrates such as cockroaches, ciliates of the genus Nyctotherus act as commensals by scavenging undigested particles and bacteria, utilizing the anaerobic environment for survival while exerting no measurable impact on the host's digestion or physiology.^[98] These associations highlight protozoa's opportunistic exploitation of host microhabitats for nutrient acquisition. The evolutionary roots of protozoan symbioses trace back to ancient endosymbiotic events, such as the incorporation of bacterial ancestors into eukaryotic cells, which gave rise to organelles like mitochondria and chloroplasts; however, extant interactions among protozoa often mirror these dynamics through stable, non-organelle-forming partnerships.^[99] For instance, modern ciliates frequently harbor bacterial endosymbionts that aid in metabolic processes, reflecting the persistent selective pressures favoring cooperative microbial associations in diverse environments. Such interactions underscore the protozoa's role in bridging free-living and symbiotic lifestyles across evolutionary timescales. These symbiotic relationships can be disrupted by environmental stressors, leading to breakdowns in mutualistic bonds. In coral systems, thermal stress induces the expulsion of Symbiodinium from host tissues through mechanisms like exocytosis and host digestion, resulting in coral bleaching where the loss of photosynthates causes energy starvation and increased mortality.^[100] This process, exacerbated by rising ocean temperatures, compromises reef ecosystems by halting the energy transfer that sustains 90% of coral productivity.^[97]

References

[1]
Protozoa: Structure, Classification, Growth, and Development - NCBI
Protozoa are microscopic unicellular eukaryotes that have a relatively complex internal structure and carry out complex metabolic activities.
[2]
Protozoa - Lander University
“ Protozoa” is an informal collective term used in reference to a polyphyletic assemblage of animal-like protists. Their more plant-like counterparts are ...
[3]
Veterinary parasitologists: the time has come to talk about the use of ...
Apr 14, 2025 · Modern approaches now classify eukaryotes into several supergroups, with "protozoa" now dispersed among different groups.
[4]
Protozoa - Soil Ecology Wiki
May 6, 2022 · Protozoans are motile, heterotrophic organisms that exhibit animal like behaviors, such as predation. Body sizes range from 5 to 500 μm.
[5]
About Parasites - CDC
Protozoa are microscopic, one-celled organisms that can be free-living or parasitic in nature. They are able to multiply in humans, which contributes to their ...
[6]
[PDF] Protozoology from the Perspective of Science Theory: History and ...
Jun 29, 2001 · 1): Leeuwenhoek for the past history, Ehrenberg for the early history and initial phase, Bütschli for the constitution phase and Hartmann for ...
[7]
A brief history of the origin of Kingdoms Protozoa, Protista and ...
Aug 5, 2025 · The unicellular nature of protists was first determined by von Siebold in 1845 (Scamardella, 1999) . Ehrenberg was also unconvinced by ...
[8]
Ernst Haeckel and Protista - Oxford Academic
Aug 31, 2023 · Haeckel's kingdom Protista was influential and controversial in equal measure. Haeckel encountered the animal-plant boundary during his studies on radiolaria.Generelle Morphologie (1866) · New classes of Protista · Four kingdoms of life
[9]
[PDF] HAECKEL'S Kingdom Protista and Current Concepts in Systematic ...
In 1866, he inclu- ded such major groups as the Bacteria (his. Moneres), naked and some testaceous rhizo- pod amoebae, slime molds, the radiolarians,.
[10]
Details - Protozoology - Biodiversity Heritage Library
Jul 3, 2008 · Title. Protozoology. By. Kudo, Richard R. (Richard Roksabro), 1886-1967. Type. Book. Material. Published material. Publication info.
[11]
Protist phylogeny and the high-level classification of Protozoa
Protist large-scale phylogeny is briefly reviewed and a revised higher classification of the kingdom Protozoa into 11 phyla presented.
[12]
The phagotrophic origin of eukaryotes and phylogenetic ... - PubMed
I discuss the relationship between the 13 protozoan phyla recognized here and revise higher protozoan classification by updating as subkingdoms Lankester's 1878 ...
[13]
(PDF) Small-subunit riboso-mal RNA sequence from Naegleria ...
Aug 7, 2025 · We have sequenced the small-subunit ribosomal RNA gene of the amoebo-flagellate protozoan Naegleria gruberi. Comparison of this sequence with ...
[14]
A newly revised classification of the protozoa - PubMed
Seven phyla of PROTOZOA are accepted in this classification--SARCOMASTIGOPHORA, LABYRINTHOMORPHA, APICOMPLEXA, MICROSPORA, ASCETOSPORA, MYXOSPORA, and ...
[15]
Cell fractionation of parasitic protozoa: a review - SciELO
The nucleus of the protozoan is centrally located. The cytoplasm contains randomly distributed ribosomes and profiles of endoplasmic reticulum. The Golgi ...
[16]
Elemental composition and ultrafine structure of the skeleton in shell ...
Radiolarians (Polycystinea) are microplanktonic Protozoa characterized by a delicate skeleton of opaline silica. They are holoplanktonic and reckoned to be ...
[17]
Tintinnid | - The Evergreen State College
Nov 19, 2015 · Tintinnids are heterotrophic plankton with trumpet-shaped shells (loricae), ranging from 20-200 micrometers in size. They are part of the ...
[18]
Bodo Morphology
The World of Protozoa, Rotifera, Nematoda and Oligochaeta. Isolation ... Previously reported size ranges: 6-12 um (Larsen & Patterson 1990), 5-12 um ...
[19]
[PDF] class xenophyophorea - International Society of Protistologists
They range from a few millimeters to 25 em in size, making them among the largest known protists. Initially described more than a century ago as either sponges ...
[20]
[PDF] Can Protozoa Prove the Beginning of the World? - FireScholars
Apr 16, 2020 · Irritability is demonstrated in protozoa by their use of pseudopodia, flagella, or cilia for motility; it has been shown that such locomotors ...
[21]
The evolution of eukaryotic cilia and flagella as motile and sensory ...
Eukaryotic cilia and flagella are motile organelles built on a scaffold of doublet microtubules and powered by dynein ATPase motors.
[22]
Chemotactic behaviour of Giardia lamblia and Trichomonas ...
Jun 25, 2025 · Chemotaxis is the phenomenon of sensing external concentration gradients by cells and the cellular movement towards or away from the cells.
[23]
Biology and Mechanism of Trypanosome Cell Motility - ASM Journals
Microbial pathogens employ a variety of mechanisms for cell locomotion, including passive movement within their host's circulation, cooptation of host ...
[24]
Amoeboid movement in protozoan pathogens - ScienceDirect.com
Entamoeba histolytica, the causative agent of amoebiasis, is a protozoan parasite characterised by its amoeboid motility, which is essential to its survival ...
[25]
From Molecules to Amoeboid Movement: A New Way for ... - NIH
Dec 11, 2024 · Along with actin, myosin plays a huge role in cell motility, especially in amoeboid movement, providing the contractility of the filaments ...
[26]
The enigma of eugregarine epicytic folds: where gliding motility ...
Sep 22, 2013 · Our data suggest that gregarines utilize several mechanisms of cell motility and that this is influenced by environmental conditions.
[27]
Origin and arrangement of actin filaments for gliding motility ... - Nature
Aug 9, 2023 · Gliding is powered by actomyosin motors that translocate host-attached surface adhesins along the parasite cell body.
[28]
Glycosylated proteins in the protozoan alga Euglena gracilis
Introduction. Euglena are a class of mixotrophic protozoa that live in predominantly freshwater aquatic environments (Buetow 1968). Most possess a green ...
[29]
Naked amoebae - Soil Ecology Wiki
May 1, 2025 · Phagocytosis involves the process of the amoeba extending its pseudopods and then creating a vacuole of the food within its cytoplasm, which it ...
[30]
Transport into the Cell from the Plasma Membrane: Endocytosis
In protozoa, phagocytosis is a form of feeding: large particles taken up into phagosomes end up in lysosomes, and the products of the subsequent digestive ...
[31]
Food selectivity of anaerobic protists and direct evidence for ... - NIH
Apr 27, 2020 · Nutrition and growth characteristics of Trichomitopsis termopsidis, a cellulolytic protozoan from termites. Appl Environ Microbiol. 1985;49 ...
[32]
A Uniquely Complex Mitochondrial Proteome from Euglena gracilis
Euglenids are versatile organisms that possess a variety of nutritional strategies, including eukaryotrophy, bacteriotrophy, and osmotrophy (Leander et al.
[33]
Protozoal food vacuoles enhance transformation in Vibrio cholerae ...
May 16, 2022 · Protozoa package bacteria into food vacuoles (phagosomes) that become acidified and filled with toxic components that aid digestion, including ...
[34]
Intracellular Calcium Channels in Protozoa - PMC - PubMed Central
Acidic compartments such as the acidocalcisome and the PLV contain enzymes involved in their acidification e.g. the H+- ATPase (i) and the vacuolar-H+ ...
[35]
Genomic insights into the cellular specialization of predation in ...
Among protozoan predators, raptorial Haptorian ciliates are particularly fascinating as they possess offensive extrusomes known as toxicysts, which are rapidly ...
[36]
Are Colpodella Species Pathogenic? Nutrient Uptake and ... - NIH
Apicomplexans utilize apical phagotrophy, phagocytosis, osmotrophy, pinocytosis, and endocytosis for nutrient uptake, with the cytostome and micropore ...
[37]
Diversity Generator Mechanisms Are Essential Components of ...
Feb 13, 2018 · Like HGT in prokaryotes, sexuality occurs principally in reaction to stressful environmental conditions (Bernstein and Bernstein, 2010). In ...Missing: mode | Show results with:mode
[38]
Vorticella: A Protozoan for Bio-Inspired Engineering - PMC
When Vorticella reproduces through budding, a daughter Vorticella leaves its mother zooid and becomes a telotroch. In other cases, a sessile stalked Vorticella ...
[39]
Reproduction in Paramecium - Microbe Notes
Aug 3, 2023 · By binary fission, Paramecium caudatum divides 2-3 times in a day. The process of binary fission requires 2 hours to complete. About 600 ...Missing: optimal | Show results with:optimal
[40]
Possible Third Step Preventing Conjugation between Different ... - NIH
Jan 12, 2023 · Conjugation is a mode of sexual reproduction in ciliates that usually occurs within the same species; interspecific conjugation is ...
[41]
Protozoan Sexuality - ResearchGate
The dominance of one phase and suppression of another has been suggested to be due to the common occurrence in algae of apogamy, apomeiosis and parthenogenesis.
[42]
Free-Living Protozoa Causing Human Disease - Tulane University
May 22, 2018 · The life cycle consists of trophozoite and cyst stages and the trophozoite stage can be either ameboid or flagellated (Figure). The ameboid ...
[43]
DPDx - Malaria - CDC
. Some parasites differentiate into sexual erythrocytic stages (gametocytes) ... Inoculation of the sporozoites into a new human host perpetuates the malaria life ...
[44]
Life Cycles – Concepts in Animal Parasitology
Additionally, life cycles can be categorized as simple or direct where a parasite only infects a single host in its life cycle, or as complex or indirect life ...
[45]
Encystation of Giardia lamblia: A model for other parasites - PMC
Cysts are dormant, yet “spring-loaded for action” to excyst upon ingestion. Giardial encystation has been studied from morphological, cell-biological, ...
[46]
Intestinal Protozoa - Tulane University
Jun 24, 2021 · The cyst wall functions to protect the organism from desiccation in the external environment as the parasite undergoes a relatively dormant ...
[47]
Plasmodium—a brief introduction to the parasites causing human ...
Jan 7, 2021 · The Plasmodium life cycle begins when parasites known as sporozoites produced in the insect vector enter the blood of the vertebrate host ...
[48]
Tetrahymena mutants with short telomeres - PMC - NIH
In Tetrahymena thermophila, telomeres become long at 30 degrees, and growth rate slows. A slow-growing culture with long telomeres is often overgrown by a ...
[49]
Mutation accumulation in Tetrahymena - PMC - PubMed Central - NIH
Nov 15, 2010 · In this study we carry out a mutation accumulation experiment with the ciliate protozoan Tetrahymena thermophila with the aim to determine its ...
[50]
Aging and longevity in the simplest animals and the quest for ...
Aging in ciliate protozoa – clonal immortality, regeneration, and dedifferentiation/differentiation contribute to longevity. The protozoa are one-celled ...
[51]
Tetrahymena thermophila - microbewiki - Kenyon College
May 6, 2015 · In lab T. thermophila's generation time is between 2-3 hours. It requires 11 essential amino acids, six B-complex vitamins, Fe3+ ...Description and Significance · Genome Structure · Cell Structure · Metabolism
[52]
Protozoan - an overview | ScienceDirect Topics
Free-living protozoa are ubiquitous and found in planktonic, benthic, and soil biomes and a wide variety of protozoa are symbionts with several being important ...
[53]
Volvox - an overview | ScienceDirect Topics
Volvox is defined as a genus of green algae commonly found in ponds, ditches, and shallow puddles, particularly in deep ponds and lagoons that receive ample ...
[54]
Contractile Vacuole - an overview | ScienceDirect Topics
Protozoa living in freshwater exist in a hypotonic environment. Water flows across their plasma membrane since their cytosol is hypertonic to the environment.
[55]
Diversity and ecology of Radiolaria in modern oceans - PMC
All remaining radiolarians consistently biomineralize opaline silica ... radiolarians in marine food web models, precluding our understanding of their role there.Missing: protozoa | Show results with:protozoa
[56]
Global census of the significance of giant mesopelagic protists to the ...
Apr 29, 2024 · Globally, the carbon demand of mesopelagic, flux-feeding Phaeodaria reaches 0.46 Pg C y−1, representing 3.8 to 9.2% of gravitational carbon ...
[57]
[PDF] Protistan Skeletons: A Geologic History of Evolution and Constraint
Like animals, however, radiolarians really emerged as important components of the marine silica ... Over time, then, silica flux may have increased even as.
[58]
Soil Protozoa - an overview | ScienceDirect Topics
Several groups of soil protozoa are recognised. Amoebae are amongst the most abundant of soil protozoa (Foissner, 1999), with about 60 species known from soils ...
[59]
Ecology of free-living amoebae - PubMed
Small free-living amoebae (FLA) are the main predators controlling bacterial populations in soils. They are distributed in the rhizospheric zone and the ...
[60]
Free-Living Amoebas in Extreme Environments: The True Survival in ...
Oct 18, 2022 · The gymnamoebas, naked amoebas and a significant number of testate amoebas are grouped here. The Rhizaria supergroup amoebae have very fine ...
[61]
[PDF] Diversity and Distributional Patterns of Ciliates in Guaymas Basin ...
The objectives of this study were to investigate and compare the diversity and distributional patterns of metabolically active ciliates in mat environments and ...
[62]
Microbial Consortium Associated with the Antarctic Marine Ciliate ...
This ciliate is a free-swimming protozoan endemic of the oligothrophic coastal sediments of Terra Nova Bay, in Antarctica.
[63]
The biodiversity and ecology of Antarctic lakes: models for evolution
Typically, continental Antarctic lakes and cryoconites are dominated by plankton composed of photosynthetic and heterotrophic protozoa, a few algae, fungi ...
[64]
Short-term harmful effects of ammonia nitrogen on activated sludge ...
In microfauna communities, ciliated protozoa are usually one of the major groups, with densities of around 104–106 individuals/ml. Several reports have ...
[65]
Bacterivore - an overview | ScienceDirect Topics
Accordingly, low concentrations of bacteria (104–107 per milliliter, usually 105–106) can persist even in stationary-phase cultures of bacterivorous protozoa.
[66]
Invasion and Intracellular Survival by Toxoplasma - NCBI - NIH
gondii remains an obligate intracellular parasite. Go to: Actin-Based Motility and Cell Invasion. Apicomplexan parasites are equipped with a unique form of ...Summary · Host Cell Recognition and Entry · Vacuole Modification and...
[67]
Sand flies: Basic information on the vectors of leishmaniasis ... - Nature
Apr 4, 2022 · We discuss the biology, distribution, and life cycle, the blood-feeding process, and the Leishmania-sand fly interactions that govern parasite ...
[68]
Common strategies for antigenic variation by bacterial, fungal and ...
We review how bacterial, protozoan and fungal pathogens from distant evolutionary lineages have evolved surprisingly similar mechanisms of antigenic variation.
[69]
Prevalence of Balantidium coli (Malmsten, 1857) infection in swine ...
B. coli (Malmsten, 1857), a ciliated protozoan, belonging to the family Balantidiidae [7], is considered a commensal of the intestine of several mammalian ...
[70]
Cellulose Metabolism by the Flagellate Trichonympha ... - Science
Continuous axenic cultures were established of Trichonympha sphaerica, a cellulose-digesting symbiotic protozoon in the gut of a termite.
[71]
DPDx - Trypanosomiasis, African - CDC
The only known vector for each is the tsetse fly (Glossina spp.). Geographic Distribution. T. b. gambiense is endemic in West and Central Africa. T. b.
[72]
Modulation of Host-Pathogen Communication by Extracellular ...
Apr 10, 2019 · Leishmania genus protozoan parasites have developed various strategies to overcome host cell protective mechanisms favouring their survival ...
[73]
Bacterivory in ciliates isolated from constructed wetlands (reed beds ...
The highest mean grazing rates were recorded for Paramecium spp (1.85 FLB/cell/min), which was the largest ciliate used in the study, followed by oxytrichids ( ...
[74]
Associational Resistance to Predation by Protists in a Mixed Species ...
Jan 19, 2023 · Grazing by protozoa is one of the main mortality factors for bacteria in natural environments (1), and thus predation plays an important role in ...
[75]
Microbial food webs in hypertrophic fishponds: Omnivorous ciliate ...
Jul 29, 2019 · Cell-specific grazing rates (bacteria ciliate−1 h−1) ... Cyclops predation on ciliates: Species-specific differences and functional responses.Microbial Food Webs In... · Materials And Methods · Results
[76]
influence of marine protozoa on nutrient regeneration1 - ASLO
By constituting a nutrient source for protozoa, bacteria are indirectly involved in this increased regeneration. component of the marine fauna.Missing: primary | Show results with:primary
[77]
N remineralization in planktonic protozoa - ASLO
The association of bacteria and protozoa play a significant role in nutrient recycling in ma- rine environments (Johannes 1965; Fenchel and Harrison 1976).
[78]
Protozoa-driven micro-food webs shaping carbon and nitrogen ...
Aug 22, 2025 · By mediating trophic cascades that regulate bacterial and algal populations, protozoa influence nutrient remineralization and energy .Missing: phosphorus | Show results with:phosphorus
[79]
Flows of biogenic carbon within marine pelagic food webs
Feb 17, 2018 · They identified 5 main fluxes, i.e. (1) photosynthetic fixation of carbon in organic matter, (2) respiration, (3) transfer to large organisms ...
[80]
Protozoan communities serve as a strong indicator of water quality ...
Jul 16, 2024 · This study demonstrates that protozoans can be a potential bioindicator of water quality status in this subtropical freshwater river system.
[81]
[PDF] Protozoa as bioindicators in running waters. - wilhelm foissner
protozoa as bioindicators in streams and rivers. Pros and cons are discussed, showing that micro-organisms have several unique bio- logical and ...
[82]
Entamoeba histolytica Infection - StatPearls - NCBI Bookshelf - NIH
Although 90 percent of E. histolytica infections are asymptomatic, nearly 50 million people become symptomatic, with about 100,000 deaths yearly.
[83]
More malaria cases and deaths in 2020 linked to COVID-19 ...
Dec 6, 2021 · According to WHO's latest World malaria report, there were an estimated 241 million malaria cases and 627 000 malaria deaths worldwide in 2020. ...
[84]
[PDF] GIARDIA: DRINKING WATER FACT SHEET
Giardia must be removed or killed. How important is waterborne transmission of giardiasis? A risk assessment has estimated that in the United States as many ...
[85]
DPDx - Cryptosporidiosis - CDC
Oocysts are infectious upon excretion, thus enabling direct and immediate fecal-oral transmission. Extracellular stages have been reported, but their relevance ...
[86]
cerebral malaria due to Plasmodium falciparum in an adult traveler ...
Dec 20, 2023 · Contrary to its name, cerebral malaria does not involve P. falciparum invading the brain tissue. Other commonly observed systemic ...
[87]
Production and Functional Characterization of a Recombinant ...
Sep 29, 2020 · ... toxic proteins expressed by T. vaginalis (Mercer and Johnson, 2018) ... Trichomonas vaginalis: pathogenesis, symbiont interactions, and host cell ...
[88]
One Million Deaths by Parasites - Speaking of Medicine and Health
Jan 16, 2015 · Parasitic Disease, Global Deaths in 2013 ; Malaria, 854,600 ; Leishmaniasis (Kala-azar), 62,500 ; Cryptosporidiosis, 41,900 ; Amoebiasis, 11,300.<|separator|>
[89]
Chloroquine against malaria, cancers and viral diseases - PMC
Chloroquine (CQ), a protonated, weakly basic drug, exerts its antimalarial effect mainly by increasing pH and accumulating in the food vacuole of the parasites.Missing: antiprotozoal | Show results with:antiprotozoal
[90]
Malaria vaccines: the 60-year journey of hope and final success ...
May 17, 2023 · Malaria vaccines have been in development for almost 60 years. The RTS,S/AS01 vaccine has now been approved, but cannot be a stand-alone solution.
[91]
Recent perspectives in clinical development of malaria vaccines
Apr 15, 2025 · For example, chloroquine is no longer effective against the most lethal malaria parasite strain Plasmodium (P.)
[92]
The transcriptome of the rumen ciliate Entodinium caudatum reveals ...
Dec 21, 2019 · As a fermentative ciliate, E. caudatum ferments sugars to volatile fatty acids (VFA) and to produce ATP. As indicated by the transcripts ...Missing: herbivore | Show results with:herbivore
[93]
Metabolic influence of core ciliates within the rumen microbiome
May 11, 2023 · Ruminants operate in symbiosis with their intrinsic rumen microbiome, which is responsible for the degradation of forage into nutrients, in the ...
[94]
Transcriptomic Analysis of Thermally Stressed Symbiodinium ...
Feb 28, 2017 · As photosynthate transfer from Symbiodinium to coral hosts provides up to 90% of a coral's daily energy requirements, the implications of ...
[95]
Some Aspects of the Physiology of the Nyctotherus velox, a ... - MDPI
Apr 29, 2023 · Nyctotherus species are considered commensal protozoa. In addition, little is known about the ability of Nyctotherus species and bacteria to ...
[96]
Endosymbiotic associations within protists | Philosophical ... - Journals
Mar 12, 2010 · This review focuses on endosymbionts in protists (unicellular eukaryotes). Selected examples illustrate the incorporation of various new biochemical functions.
[97]
Moderate Thermal Stress Causes Active and Immediate Expulsion ...
Dec 10, 2014 · This result indicates that corals more actively digest and expel damaged Symbiodinium under thermal stress conditions, likely as a mechanism for coping with ...