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Microalgae

Microalgae are a diverse group of predominantly unicellular, photosynthetic microorganisms that include both prokaryotic forms, such as cyanobacteria, and eukaryotic algae, inhabiting a wide array of environments from marine and freshwater systems to hypersaline and soil habitats.^[1] They contain chlorophyll a and perform oxygenic photosynthesis, enabling rapid growth rates and efficient conversion of sunlight, carbon dioxide, and inorganic nutrients into biomass, often doubling in population within hours under optimal conditions.^[2] Unlike macroalgae, microalgae lack differentiated tissues and vascular structures, existing primarily as single cells, simple colonies, or short filaments, which allows them to thrive in diverse ecological niches.^[3] In aquatic ecosystems, microalgae function as foundational primary producers, forming the base of food webs and supporting higher trophic levels including zooplankton, fish, and ultimately humans.^[4] They contribute substantially to global biogeochemical cycles, generating approximately 50% of Earth's atmospheric oxygen through photosynthesis and playing a pivotal role in carbon sequestration by fixing approximately 50 billion tons of carbon annually into organic compounds.^[5]^[6] Microalgal blooms, while essential for ecosystem productivity, can sometimes lead to imbalances such as oxygen depletion or toxin production by certain species, impacting water quality and biodiversity.^[4] The biotechnological potential of microalgae has garnered significant attention due to their versatility in producing high-value bioproducts, including biofuels like biodiesel and bioethanol derived from their lipid and carbohydrate content, as well as nutraceuticals such as polyunsaturated fatty acids (e.g., omega-3s) and proteins.^[7] Commercially important genera like Chlorella and Spirulina are cultivated on large scales for human and animal nutrition, offering antioxidant, anti-inflammatory, and antimicrobial compounds that support applications in food supplements and pharmaceuticals.^[8] Additionally, microalgae facilitate sustainable processes such as wastewater remediation, heavy metal biosorption, and CO₂ mitigation from industrial emissions, positioning them as key players in circular bioeconomies and environmental engineering.^[9]

Characteristics

Morphology and Structure

Microalgae are primarily unicellular eukaryotic or prokaryotic organisms, with cell diameters typically ranging from 1 to 100 micrometers, though some species form loose colonies or chains that can reach up to 0.2 mm in aggregate size.^[1]^[10] This unicellular structure allows for rapid reproduction and adaptability in diverse aquatic environments, while colonial forms, such as those in certain green algae like Volvox, provide protective clustering without true multicellularity.^[1] The cell wall of microalgae varies significantly across groups, influencing rigidity, flexibility, and ecological roles. In diatoms, the wall, known as a frustule, is composed primarily of amorphous silica (SiO₂), forming intricate, nanopatterned structures that provide mechanical support and protection.^[11] Green algae often feature walls made of polysaccharides such as cellulose, hemicellulose, and pectins, resulting in flexible yet sturdy envelopes, as seen in Chlorella vulgaris.^[12] In contrast, prokaryotic cyanobacteria possess a Gram-negative-type wall with a thick peptidoglycan layer between inner and outer membranes, conferring rigidity similar to other bacteria.^[13] These compositions lead to rigid walls in silica-bearing diatoms for structural integrity, while flexible or absent walls, like the mucilaginous coat in Dunaliella salina, facilitate osmoregulation in fluctuating salinities.^[14] Internally, eukaryotic microalgae contain chloroplasts housing photosynthetic pigments, including chlorophyll a universally, alongside chlorophyll b in green algae or c in groups like diatoms and dinoflagellates.^[15] Pyrenoids, dense proteinaceous structures within chloroplasts, facilitate carbon fixation by concentrating ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), enhancing CO₂ capture efficiency.^[16] Eyespots, carotenoid-rich organelles in the chloroplast, enable phototaxis in motile species by directing light perception.^[17] Vacuoles, often contractile, maintain cellular turgor and perform osmoregulation by expelling excess water via ion-driven mechanisms.^[18] Motility in microalgae is achieved through diverse mechanisms, with many species non-motile but others exhibiting flagella or gliding. Dinoflagellates, for instance, possess two dissimilar flagella—a transverse one encircling the cell for rotation and a trailing longitudinal one for propulsion—enabling complex swimming patterns at speeds up to 1 body length per second.^[19] Some diatoms and cyanobacteria glide over surfaces using mucilage secretion or twitching motility, while coccolithophores like Emiliania huxleyi are typically non-motile, relying on calcium carbonate scales (coccoliths) for buoyancy and protection rather than active movement.^[20] Key structural adaptations include lipid bodies, neutral lipid-rich organelles that serve as energy reserves, accumulating triacylglycerols under nutrient stress to support survival and reproduction.^[21] In buoyant cyanobacteria such as Anabaena, gas vacuoles—hollow, protein-sheathed cylinders—provide adjustable flotation by collapsing under pressure, optimizing light exposure for photosynthesis.^[22] These features underscore the morphological diversity enabling microalgae to thrive in varied conditions.

Physiology and Metabolism

Microalgae primarily acquire energy through photosynthesis, a process divided into light-dependent reactions and the light-independent Calvin cycle. In the light-dependent reactions, photosystems I and II capture photons to split water molecules, releasing oxygen and generating ATP via photophosphorylation and NADPH through electron transport. These reducing equivalents power the Calvin cycle in the chloroplast stroma, where ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) fixes CO₂ into organic compounds, ultimately yielding glucose as represented by the equation:

$6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2

^[23] Nutrient uptake in microalgae supports metabolic processes, with nitrogen assimilation occurring primarily via the nitrate reduction pathway in most species, where nitrate (NO₃⁻) is sequentially reduced to nitrite (NO₂⁻) by nitrate reductase and then to ammonium (NH₄⁺) by nitrite reductase for incorporation into amino acids. Phosphorus is assimilated as orthophosphate or polyphosphate, stored in vacuoles or polyphosphate bodies to meet demands for nucleic acids, phospholipids, and ATP synthesis. Trace metals such as iron, magnesium, and zinc are essential cofactors for enzymes like RuBisCO and photosystems, absorbed via specific transporters to prevent deficiencies that impair growth. In cyanobacteria, a subset of prokaryotic microalgae, nitrogen fixation occurs through the nitrogenase enzyme complex, converting atmospheric N₂ to ammonia under anaerobic conditions within specialized heterocysts to avoid oxygen inhibition.^[24]^[25] Microalgal growth progresses through distinct phases in batch cultures: the lag phase, where cells adapt to environmental conditions with minimal division; the exponential phase, characterized by rapid cell division driven by optimal nutrient and light availability; the stationary phase, where growth plateaus due to nutrient limitation or waste accumulation; and the death phase, marked by cell lysis and population decline. Division rates vary by species and conditions, with some achieving doubling times as short as a few hours under optimal light, temperature, and nutrient regimes.^[26]^[27]^[28] Secondary metabolism in microalgae produces diverse compounds beyond primary biosynthetic needs, including pigments like carotenoids (e.g., β-carotene and astaxanthin) that serve in photoprotection by dissipating excess light energy and scavenging reactive oxygen species. Certain species, particularly dinoflagellates such as Alexandrium, synthesize toxins like saxitoxins, potent neurotoxins that may deter grazers or provide ecological advantages, though their exact physiological roles remain under study.^[29]^[30] In addition to photosynthesis, microalgae perform aerobic respiration at night or under low light, oxidizing organic compounds via the tricarboxylic acid cycle and electron transport chain in mitochondria to generate ATP. Many species exhibit mixotrophic metabolism, simultaneously utilizing light for photosynthesis and organic carbon sources like glucose for respiration, enhancing growth rates. Under complete darkness or high organic substrate availability, heterotrophic modes predominate, relying solely on organic carbon assimilation for energy and biomass production.^[31]

Classification

Prokaryotic Groups

Prokaryotic microalgae encompass photosynthetic microorganisms that lack a membrane-bound nucleus and organelles, distinguishing them as true prokaryotes, and they conduct oxygenic photosynthesis using thylakoid membranes derived from the cytoplasmic membrane. These organisms are primarily represented by the phylum Cyanobacteria, formerly known as blue-green algae, which are Gram-negative bacteria capable of fixing carbon dioxide and nitrogen under diverse environmental conditions. Their cellular structure includes a thick peptidoglycan layer and, in some cases, a mucilaginous sheath, enabling them to form colonies or filaments while maintaining prokaryotic simplicity.^[32] The taxonomic classification of cyanobacteria relies on a polyphasic approach integrating morphological, molecular, and ecological data, resulting in several major orders that highlight their morphological diversity. Chroococcales includes unicellular or colonial coccoid forms, such as Synechococcus, which are often planktonic and exhibit simple division without filament formation. In contrast, Oscillatoriales comprises non-heterocystous filamentous types, exemplified by Arthrospira (commonly known as Spirulina), which form coiled trichomes adapted to alkaline environments. Other notable orders include Nostocales, featuring heterocystous filaments for specialized nitrogen fixation, and Synechococcales, encompassing marine picocyanobacteria like Prochlorococcus. This classification underscores the evolutionary adaptations within cyanobacteria, with over 8,000 species estimated across approximately 100 genera, though around 5,700 have been formally described as of 2023.^[33]^[32]^[34] Key physiological traits of cyanobacteria include oxygenic photosynthesis, which utilizes chlorophyll a and phycobilins to produce oxygen, a process pivotal to their ecological impact. Specialized structures such as heterocysts—thick-walled cells in filamentous species like Anabaena—enable nitrogen fixation by providing an anaerobic microenvironment, while akinetes serve as dormant spores for survival under stress conditions like nutrient limitation or desiccation. These features have contributed to their ancient lineage, with fossil evidence in stromatolites dating back over 3.5 billion years, and their activity drove the Great Oxidation Event around 2.4 billion years ago by elevating atmospheric oxygen levels through sustained photosynthetic output.^[32]^[33]^[35] Representative examples illustrate cyanobacterial diversity and significance: Arthrospira platensis is cultivated commercially for its nutritional value due to high protein content, while Microcystis aeruginosa is notorious for forming harmful algal blooms in eutrophic waters, producing hepatotoxic microcystins that impact aquatic ecosystems. These species exemplify the range from beneficial to problematic roles, rooted in their prokaryotic adaptations for photosynthesis and survival.^[32]

Eukaryotic Groups

Eukaryotic microalgae are single-celled or colonial organisms classified as eukaryotes, possessing membrane-bound organelles such as a nucleus enclosing genetic material and mitochondria for energy production through oxidative phosphorylation. This structural complexity contrasts with prokaryotic microalgae like cyanobacteria, which lack these organelles and rely on simpler cellular organization. The chloroplasts in eukaryotic microalgae have diverse evolutionary origins. Primary endosymbiosis, where an ancestral eukaryotic cell engulfed a photosynthetic cyanobacterium, occurred once and gave rise to the Archaeplastida supergroup (including Chlorophyta and Rhodophyta), integrating the endosymbiont as a chloroplast and enabling oxygenic photosynthesis in these lineages. Other eukaryotic microalgae acquired plastids through secondary endosymbioses, engulfing eukaryotic algae (typically red or green algae) that already possessed primary plastids; for example, stramenopiles like diatoms engulfed red algae, while euglenoids engulfed green algae, and dinoflagellates often involve tertiary endosymbioses.^[36]^[37] Eukaryotic microalgae exhibit remarkable diversity, with approximately 44,000 described species contributing to the broader algal tally, though estimates suggest a total of 72,500 algal species exist, predominantly eukaryotic. For microalgae specifically, described species number around 40,000, part of an estimated 200,000 to 800,000 total microalgal species, highlighting vast undescribed biodiversity in marine and freshwater environments.^[38]^[39] Key phyla include Chlorophyta (green algae), which store photosynthetic products as starch in pyrenoid structures, as exemplified by Chlorella vulgaris with its cup-shaped chloroplast containing starch grains.^[40] Bacillariophyta (diatoms) feature elaborate cell walls called frustules composed of opaline silica, providing structural rigidity and contributing to their role as major phytoplankton components; their plastids derive from secondary endosymbiosis of a red alga.^[41] Other prominent divisions encompass Dinophyta (dinoflagellates), characterized by cellulose-based thecal plates forming a rigid armor and bioluminescent properties in over 18 genera via luciferin-luciferase reactions in scintillons; their plastids often result from tertiary endosymbioses.^[42]^[43] Haptophyta includes coccolithophores, which produce intricate calcified scales (coccoliths) from calcium carbonate, often external to organic base plates, enabling biomineralization unique among microalgae; like diatoms, they have secondary red algal-derived plastids.^[44] Rhodophyta (red algae) features unicellular microalgae like Porphyridium species, which are marine and possess phycoerythrin pigments for light harvesting in deeper waters; as part of Archaeplastida, they have primary plastids.^[45] Certain eukaryotic microalgae display specialized nutritional strategies, such as mixotrophy in Euglenophyta (euglenoids), where species like Euglena combine autotrophy via chloroplasts (from secondary green algal endosymbiosis) with heterotrophy through phagocytosis or osmotrophy, adapting to variable light and nutrient conditions.^[46] These features underscore the evolutionary divergence and adaptive versatility of eukaryotic microalgae beyond prokaryotic counterparts.

Ecology

Habitats and Distribution

Microalgae inhabit a wide array of aquatic environments, with the vast majority of their global biomass occurring in marine systems, where they form the foundation of open ocean phytoplankton communities.^[47] In these oligotrophic waters, species such as Prochlorococcus dominate, contributing significantly to primary production across vast expanses of the world's oceans. Freshwater habitats, including lakes and rivers, support diverse microalgal assemblages adapted to lower salinity conditions, while brackish systems—transitional zones between freshwater and marine environments—host euryhaline species capable of tolerating variable salinities. Beyond aquatic realms, microalgae colonize terrestrial and extreme environments, demonstrating remarkable adaptability. In soil crusts, they contribute to biocrust formation in arid regions, stabilizing surfaces and facilitating nutrient cycling. Thermophilic cyanobacteria thrive in hot springs, where temperatures often exceed 45°C, forming colorful microbial mats in geothermal pools. Polar ice environments harbor psychrophilic species, such as Plocamiomonas psychrophila in Arctic sea ice, which endure subzero conditions and low light. Hypersaline ponds, with salt concentrations surpassing 100 g/L, support halophilic microalgae like Dunaliella salina, which accumulate compatible solutes to maintain cellular function.^[48]^[49]^[50]^[51] Distribution patterns of microalgae reflect their cosmopolitan nature, with certain species achieving widespread prevalence. Prochlorococcus, the most abundant photosynthetic organism on Earth, numbers approximately 3 × 10^{27} cells globally and is distributed across over 75% of the ocean's surface, particularly in tropical and subtropical waters.^[47]^[52] Vertical stratification in water columns confines most microalgal activity to the euphotic zone, where light penetration supports photosynthesis, typically extending 20–200 meters depending on water clarity. Diatoms, a key eukaryotic group, exemplify broad distribution in both marine and freshwater settings.^[53]^[54] Abundance estimates underscore the ecological prominence of microalgae, particularly in marine settings, where they generate roughly 50% of Earth's atmospheric oxygen through photosynthesis. Seasonal blooms are common in temperate zones, driven by nutrient upwelling and warming waters, leading to rapid population surges that can color coastal waters and influence local biogeochemistry.^[55]^[56] Factors such as temperature, salinity, and pH profoundly influence microalgal distribution and growth. Optimal temperatures for many species range from 20–30°C, though extremophiles extend this spectrum; salinity tolerance spans 0–35 parts per thousand (ppt) for euryhaline forms; and pH levels between 6 and 9 support metabolic processes across diverse taxa. These parameters interact to shape niche occupancy, with deviations often limiting proliferation.^[57]^[58]

Environmental Roles

Microalgae, particularly phytoplankton, serve as primary producers at the base of aquatic food webs, converting solar energy into organic matter through photosynthesis and supporting higher trophic levels from zooplankton to fisheries. They contribute approximately 45-50% of global net primary productivity, equivalent to about 50 gigatons of carbon per year, predominantly in marine environments where they outpace terrestrial vegetation in efficiency due to rapid growth rates and nutrient uptake.^[59] This productivity underpins the transfer of energy and biomass throughout ocean and freshwater ecosystems, influencing biodiversity and carbon flow. In addition to fueling food webs, microalgae play a pivotal role in oxygen production, generating an estimated 50% of Earth's atmospheric oxygen through photosynthetic activity, with marine phytoplankton responsible for the majority. This process, occurring primarily in sunlit surface waters, releases O₂ as a byproduct while fixing CO₂, maintaining the planet's oxygen balance essential for aerobic life. Sources vary slightly, with some estimates reaching up to 70-80% when accounting for historical contributions and oceanic dominance.^[55]^[60] However, ongoing ocean warming poses risks to key species like Prochlorococcus, potentially altering global productivity patterns as of 2025.^[61] Microalgae are integral to nutrient cycling, facilitating the recycling of essential elements like nitrogen and phosphorus through uptake, assimilation, and remineralization processes that sustain ecosystem productivity. Diatoms, a key microalgal group, deposit biogenic silica in their frustules, contributing to the global silica cycle and influencing water chemistry in silica-limited regions. They also drive carbon sequestration via the biological pump, where sinking organic matter exports carbon to deep oceans as "blue carbon," locking away atmospheric CO₂ for centuries.^[62]^[63] Symbiotically, certain microalgae, such as dinoflagellates known as zooxanthellae, form mutualistic relationships with corals, providing photosynthetic products that supply up to 90% of the host's energy needs in exchange for nutrients and protection. However, environmental stressors like warming waters can lead to the expulsion of these symbionts, causing coral bleaching and widespread reef degradation. Conversely, microalgae can exert harmful effects through blooms; for instance, Karenia brevis produces neurotoxins during red tides, leading to mass fish kills, shellfish contamination, and hypoxic "dead zones" from oxygen depletion as biomass decomposes.^[64]^[65]^[66]^[67] Through these dynamics, microalgae regulate climate by drawing down CO₂; the ocean absorbs approximately 25-30% of annual anthropogenic CO₂ emissions, with microalgae contributing to this via photosynthesis and the biological carbon pump exporting organic carbon to the deep sea for long-term storage, mitigating acidification and warming while highlighting their potential in global carbon budgets.^[68]

Applications

Nutritional and Pharmaceutical Uses

Microalgae serve as a rich source of nutrients, offering high-quality proteins that typically constitute 40-70% of their dry weight, with species like Spirulina platensis providing all essential amino acids in balanced proportions suitable for human consumption.^[69] Their lipid content, ranging from 1-40% dry weight, includes valuable omega-3 polyunsaturated fatty acids such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), particularly abundant in heterotrophic species like Schizochytrium sp., which can yield up to 35-50% DHA-rich oils.^[70] Additionally, certain microalgae, such as Chlorella vulgaris, are notable for their vitamin B12 content.^[71] Beyond basic nutrition, microalgae produce bioactive compounds classified as nutraceuticals, leveraging their unique metabolic pathways for health benefits. Astaxanthin, extracted from Haematococcus pluvialis, acts as a potent antioxidant, exhibiting activity up to 500 times stronger than vitamin E in quenching singlet oxygen and protecting against oxidative stress in cellular models.^[72] Similarly, phycocyanin from Spirulina platensis demonstrates anti-inflammatory effects by inhibiting pro-inflammatory cytokines and reducing oxidative damage in vitro and in animal studies.^[73] In pharmaceutical applications, microalgae-derived compounds show promise for targeted therapies. Porous silica structures in diatom frustules enable efficient drug delivery systems, offering biocompatibility and controlled release for encapsulating anticancer agents due to their nanoscale pores and high surface area.^[74] Cyanobacteria, such as those producing dolastatin analogs, yield anticancer peptides that disrupt microtubule dynamics in tumor cells, inspiring synthetic derivatives like monomethyl auristatin E used in antibody-drug conjugates for clinical oncology.^[75] Several microalgae products have received regulatory approval, enhancing their commercial viability. The U.S. Food and Drug Administration (FDA) grants Generally Recognized as Safe (GRAS) status to Spirulina and Chlorella species for use in food and supplements, supporting their integration into dietary products.^[76] The global market for microalgae-based supplements reached approximately $0.85 billion in 2025, driven by demand for natural health products.^[77] Safety considerations are paramount, as wild-harvested or bloom-associated microalgae may contain contaminants like microcystins, hepatotoxic peptides produced by certain cyanobacteria that pose risks of liver damage at elevated exposures.^[78] Purification processes, including filtration and toxin-binding technologies, are essential to ensure supplement safety and compliance with regulatory limits.^[79]

Bioenergy and Industrial Uses

Microalgae serve as a promising feedstock for biofuel production due to their high lipid accumulation and rapid growth rates. Biodiesel is primarily derived from triacylglycerols extracted from microalgal biomass through transesterification, a process where triglycerides react with methanol to produce fatty acid methyl esters and glycerol.^[80] Certain species, such as Botryococcus braunii, can accumulate triacylglycerols up to 60% of their dry weight under stress conditions, making them particularly suitable for lipid-based fuels.^[81] Additionally, bioethanol is produced via fermentation of starch-rich biomass from species like Chlorella vulgaris, while biogas, including methane, is generated through anaerobic digestion of the residual biomass post-lipid extraction.^[82]^[83] The lipid content in microalgae typically ranges from 20% to 50% of dry biomass, enabling higher energy yields compared to traditional crops; theoretical productivity can reach 10 times that of terrestrial oleaginous plants per unit land area due to efficient photosynthesis and non-arable growth requirements.^[84]^[85] The transesterification reaction for biodiesel production is represented as:

\text{Triglyceride} + 3\text{[Methanol](/page/Methanol)} \rightarrow 3\text{[Fatty Acid](/page/Fatty_acid) Methyl Esters} + \text{[Glycerol](/page/Glycerol)}

This equation highlights the stoichiometric conversion, with yields optimized by catalysts like sodium hydroxide.^[86] In practical applications, strains such as Nannochloropsis gaditana have been engineered for enhanced oil production, supporting trials for renewable jet fuel; for instance, the ExxonMobil-Synthetic Genomics collaboration in the 2010s demonstrated doubled oil content in modified algae, targeting scalable biofuel outputs, though the program was discontinued in 2023 without reaching commercial scale.^[87]^[88]^[89] Beyond energy, microalgae biomass contributes to industrial applications, including bioplastics production via polyhydroxyalkanoates (PHAs). Microalgal residues serve as low-cost feedstocks for bacterial PHA synthesis, as seen in co-cultures with Cupriavidus necator, yielding biodegradable polymers that rival petroleum-based plastics in tensile strength.^[90] Dead microalgal biomass also acts as an effective adsorbent for wastewater remediation, removing dyes like methylene blue through biosorption mechanisms involving cell wall functional groups.^[91] Regarding sustainability, integrating flue gas CO₂ into microalgal cultivation can achieve negative carbon emissions, as the biomass sequesters industrial CO₂ while producing biofuels with a net footprint as low as -4 g CO₂e per MJ when using biogenic sources.^[92] This closed-loop approach enhances the overall environmental viability of microalgal bioenergy systems.^[93]

Cultivation

Methods and Systems

Microalgae cultivation employs various modes depending on the carbon source and light availability, broadly categorized as autotrophic, heterotrophic, or mixotrophic. In photoautotrophic mode, microalgae fix inorganic carbon dioxide using sunlight as the energy source through photosynthesis, which is the most common approach for large-scale production due to its reliance on abundant natural resources. Heterotrophic cultivation, in contrast, occurs in darkness with organic carbon substrates like glucose serving as both carbon and energy sources, enabling higher biomass densities but requiring sterile conditions to prevent bacterial contamination. Mixotrophic growth combines elements of both, allowing simultaneous utilization of light and organic carbon, which can enhance productivity under controlled conditions. Open systems represent the simplest and most cost-effective method for microalgae cultivation, primarily consisting of raceway ponds and circular ponds. Raceway ponds are shallow, elongated channels (typically 0.2–0.4 m deep) agitated by paddlewheels to circulate the culture and prevent sedimentation, achieving biomass densities of 0.1–0.3 g/L while exposing the culture to atmospheric conditions; however, they are susceptible to evaporation, weather variability, and contamination by airborne microbes or predators. Circular ponds, agitated by rotating arms, offer similar low-cost operation but are less common due to uneven mixing and lower scalability for high-volume production. Closed systems, such as photobioreactors (PBRs), provide controlled environments that mitigate contamination risks and optimize resource use, making them suitable for high-value products. Tubular PBRs feature long, transparent tubes arranged horizontally or vertically to maximize light exposure, while flat-plate designs use vertical panels for efficient illumination and reduced fouling; these systems attain biomass densities of 1–5 g/L and up to 80% light utilization efficiency through better photon distribution. Hybrid airlift PBRs integrate pneumatic mixing via air bubbles for gentle circulation, combining the benefits of closed containment with energy-efficient operation without mechanical parts. Optimal growth conditions are species-specific but generally include light intensities of $100-200\ \mu\mathrm{mol}\ \mathrm{photons}\ \mathrm{m}^{-2}\ \mathrm{s}^{-1} to support photosynthesis without photoinhibition, CO_2 enrichment at 1–5% in the aeration gas to boost carbon fixation rates, temperatures between 15–35°C to maintain metabolic activity, and pH levels of 7–9 to facilitate nutrient uptake and prevent precipitation. These parameters are adjusted based on strain physiology, with deviations potentially reducing yields by 20–50%. Large-scale implementations demonstrate the feasibility of these methods, such as the SCALE biorefinery in Baillargues, France, which opened in July 2025 and utilizes photobioreactor technology for producing over 100 tons/year of microalgae-based active ingredients.^[94]

Harvesting and Processing

Harvesting microalgae biomass from culture media is a critical step that accounts for a significant portion of production costs, often 20-30% due to the dilute nature of suspensions (typically 0.5-5 g/L) and the small cell sizes (2-30 μm). Common techniques include centrifugation, which achieves high recovery rates of 90-100% and purity but is energy-intensive, consuming up to 25 kWh per kg of dry biomass, making it suitable for high-value products rather than large-scale biofuel production.^[95]^[3]^[96] Flocculation offers a more economical alternative, promoting cell aggregation for sedimentation, with chemical flocculants like chitosan or aluminum sulfate achieving 80-95% recovery at lower energy costs (around 0.5-2 kWh/kg), though residual chemicals may require downstream removal to avoid contamination in food or pharmaceutical applications.^[97]^[98] Bioflocculation using bacteria or fungi provides a sustainable option with minimal additives, yielding 70-90% efficiency, while filtration methods such as microfiltration or ultrafiltration enable continuous processing with 85-95% recovery but can suffer from membrane fouling, necessitating periodic cleaning.^[97]^[99] Post-harvesting, drying reduces moisture content to below 10% for stability and extraction efficiency, with spray drying being a rapid convective process that preserves bioactive compounds like pigments and proteins through short exposure times (seconds) at inlet temperatures of 150-200°C, though it demands 1-3 MJ/kg energy.^[100] Solar drying, conversely, leverages natural sunlight for low-cost operation (near-zero energy input) in open systems, achieving comparable quality for bulk biomass but extending drying times to days and risking contamination from environmental factors.^[101]^[102] Extraction of intracellular components requires cell disruption to access lipids, proteins, and carbohydrates, often via mechanical methods like ultrasonication, which uses high-frequency waves to achieve 70-90% disruption efficiency, or bead milling, offering scalable 80-95% release at 0.5-1.5 MJ/kg energy use.^[99] Solvent extraction with non-polar solvents such as hexane targets lipids for biodiesel, yielding 15-40% of dry weight, while supercritical CO₂ extraction, operating at 30-50 MPa and 40-60°C, enables selective recovery of high-value compounds like carotenoids without solvent residues, with efficiencies up to 90% for non-polar fractions.^[103]^[104] Integrated biorefinery approaches enhance overall efficiency by sequential extraction, first isolating lipids via solvent or supercritical methods, followed by protein and carbohydrate recovery from the residue using aqueous or enzymatic processes, achieving near-zero waste and improving economic viability with total yields of 80-95% of biomass value.^[105]^[106] This cascade minimizes energy imbalances, where harvesting and processing typically require 1-5 MJ/kg biomass, offset by product revenues in high-impact applications.^[99]

Sustainability and Challenges

Environmental Impacts

Microalgae play a significant role in mitigating atmospheric CO2 through cultivation, sequestering approximately 1.8 kg of CO2 per kg of biomass produced via photosynthesis.^[107] In natural aquatic systems, microalgae serve as primary producers, supporting biodiversity by forming the base of food webs and facilitating nutrient cycling that sustains diverse microbial and faunal communities.^[108] These positive contributions highlight their potential in carbon capture and ecological stability when managed appropriately. However, certain microalgal species can form harmful algal blooms (HABs) that lead to widespread fish kills and bioaccumulation of toxins in marine food chains, producing various toxins with dozens of distinct types identified across species.^[109] Cultivation activities may exacerbate nutrient pollution if effluents rich in nitrogen and phosphorus are discharged without treatment, contributing to localized eutrophication in receiving waters.^[110] Open pond systems, commonly used for large-scale production, demand substantial water resources, typically 10-20 m³ per kg of biomass, though closed photobioreactors enable up to 90% water recycling, reducing overall consumption.^[111] Life cycle assessments of microalgal biofuel production reveal variable net energy ratios (NER) ranging from 0.5 to 10, depending on cultivation method, harvesting efficiency, and downstream processing, often indicating energy-intensive operations that may not always yield net positive returns.^[112] Nutrient runoff from cultivation sites poses an additional eutrophication risk, potentially elevating phosphorus and nitrogen levels in adjacent ecosystems and promoting unintended algal overgrowth.^[113] In response to intensified HAB events in the Gulf of Mexico during the 2020s, including persistent red tides, U.S. regulatory frameworks have been strengthened by 2025, with increased funding for monitoring and response under NOAA programs to curb ecological damage; as of 2025, international efforts including EU monitoring enhancements under the Water Framework Directive also address HAB risks globally.^[66]

Technological Advancements

Recent advancements in genetic engineering have significantly enhanced microalgae productivity, particularly through CRISPR-Cas9 editing to boost lipid yields for biofuel applications. In Nannochloropsis species, environmental stresses such as nitrate limitation have achieved lipid contents up to 68% of dry cell weight under optimized conditions. CRISPR-mediated modifications, including targeted gene knockouts, have demonstrated relative increases in lipid production by up to 68-110% compared to wild-type strains. Synthetic biology approaches further optimize metabolic pathways, such as introducing heterologous genes for enhanced fatty acid desaturation, as reviewed in comprehensive toolkits for microalgae genome engineering.^[114]^[115]^[116]^[117] Innovations in cultivation systems integrate artificial intelligence (AI) with photobioreactors (PBRs) to optimize growth parameters, including LED lighting for spectral tuning that boosts biomass yields by up to 20% through real-time adjustments to light intensity and wavelength. AI-driven image processing enables automated monitoring of cell density and health, facilitating predictive control in closed-loop systems and reducing operational costs. Vertical farming integrations incorporate these AI-optimized PBRs into multi-layer setups, enhancing space efficiency for urban or indoor production while maintaining high photosynthetic efficiency.^[118]^[119] Progress in microalgae biorefineries emphasizes cascading processes that maximize resource recovery, with integrated systems achieving over 90% organic carbon removal alongside nutrient extraction from biomass. These multi-step approaches sequentially harvest lipids, proteins, and carbohydrates, minimizing waste and improving economic viability. In 2025, EU-funded pilots like the SusAlgaeFuel project advance algal-derived sustainable aviation fuels through cascading biorefinery designs, targeting scalable production from waste streams with multifunctional catalysts.^[120]^[121] Omics technologies, including genomics and metabolomics, support strain selection by revealing genetic bases for desirable traits like high lipid accumulation. The 10KP project, with ongoing analyses as of 2025, sequences over 1,000 green algal genomes to catalog diversity and identify engineering targets, complemented by databases like pico-PLAZA for comparative analyses. These resources enable precise metabolic modeling and pathway engineering for optimized strains.^[122]^[123] Market trends indicate robust growth, with the global microalgae industry projected to reach $10 billion by 2030, driven by diversified applications in biofuels and nutraceuticals. Breakthroughs in 2025 include heterotrophic fermentation techniques that enable year-round production independent of sunlight, yielding up to 25% higher protein content in strains like Chlorella through nutrient manipulation in controlled bioreactors. These innovations address scalability gaps, fostering integration with existing bioenergy infrastructures.^[124]^[125]

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Diatoms—From Cell Wall Biogenesis to Nanotechnology
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A eukaryotic microalgae cell contains a nucleus, chloroplast, endoplasmic reticulum, mitochondrion, vacuole, Golgi body, cytoplasm and cell wall, with each ...
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Microalgae: A Promising Source of Valuable Bioproducts - MDPI
Primary carotenoids (chlorophylls, β-carotenoids, violaxanthin and voheriaxanthin) are synthesized under normal favorable microalgae growth conditions, ...Microalgae: A Promising... · 5. Microalgae Lipids · 7. Microalgae Pigments And...<|separator|>
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The pyrenoid: the eukaryotic CO2-concentrating organelle - PMC
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Chlamydomonas reinhardtii cellular compartments and their ...
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Contractile Vacuoles: Expanding Understanding
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How Dinoflagellates Swim - ScienceDirect.com
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Biofuel Applications. LDs derived from plants and microalgae store neutral lipids as a source of energy and carbon and serve as potential substitutes to fossil ...
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Gas vacuoles are found in all of the waterbloom-forming cyanobacteria and are responsible for buoying them to the water surface (Reynolds and Walsby, 1975).
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Potential and Challenges of Improving Photosynthesis in Algae - PMC
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Chlorella vulgaris: a perspective on its potential for combining high ...
C. vulgaris has a single, cup-shaped chloroplast with or without the presence of pyrenoids (storing starch grains). As a non-motile microalga, C ...
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What are Diatoms?
Diatoms are algae that live in houses made of glass. They are the only organism on the planet with cell walls composed of transparent, opaline silica.
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Cellulose from dinoflagellates as a versatile and environmentally ...
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Exploring the Diversity of Red Microalgae for Exopolysaccharide ...
The red marine microalgae Porphyridium or Rhodella produce sulphated EPS, exhibiting some biological activities with potential interest in the pharmaceutical ...
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Editorial: Mixotrophic, Secondary Heterotrophic, and Parasitic Algae
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Integrated overview of stramenopile ecology, taxonomy, and ...
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Microalgae‐based bioactive compounds, including protein, peptides, pigments, lipids, and polyphenols, have a positive impact on the inflammatory process and ...
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Bioactive substances of cyanobacteria and microalgae
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Unveiling the rise of microalgae-based foods in the global market
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Lipid Composition of a Green Alga, Botryococcus braunii
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Unveiling the dual potential of microalgae and seaweed biomass for ...
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Bioengineering strategies of microalgae biomass for biofuel ...
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Microalgae for the production of lipid and carotenoids: a review with ...
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[PDF] Microalgae lipid and biomass for biofuel production
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Microalgal Biomass as Feedstock for Bacterial Production of PHA
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Direct flue gas capture for algae cultivation and subsequent ...
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Minimizing carbon footprint via microalgae as a biological capture
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Harvesting economics and strategies using centrifugation for cost ...
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Harvesting of microalgae by centrifugation for biodiesel production
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A comprehensive review on harvesting of microalgae using ...
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Exploration of Microalgae Biorefinery by Optimizing Sequential ...
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Integrated microalgae-based biorefinery for wastewater treatment ...
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[PDF] Algae-based Beneficial Re-use of Carbon Emissions Using a Novel ...
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Geographical assessment of open pond algal productivity and ...
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The net energy ratio of microalgae biofuels production based on ...
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Lipid Enhancement in Oleaginous Nannochloropsis sp. under ...
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Nannochloropsis as an Emerging Algal Chassis for Light-Driven ...
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Comprehensive Genome Engineering Toolbox for Microalgae ...
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Application of Artificial Intelligence and Image Processing for the ...
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Tubular photobioreactors illuminated with LEDs to boost microalgal ...
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Microalgae-biorefinery with cascading resource recovery design ...
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SusAlgaeFuel - Innovative approaches to enable microalgae ...
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Microalgae biofuels: illuminating the path to a sustainable future ...
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Unveiling Algae Products Market Industry Trends
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Turning Microalgae into a Sustainable High Protein Food Solution
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