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Halotolerance

Halotolerance refers to the ability of certain organisms, particularly microorganisms, to tolerate and grow in environments with high salt concentrations that exceed those necessary for their optimal growth, distinguishing it from halophily where salt is required.^[1] These organisms, known as halotolerant, can adapt to saline conditions ranging from moderate (up to 5-10% NaCl) to extreme levels without salt-dependent metabolism, enabling survival in diverse habitats such as hypersaline lakes, coastal soils, and salted food products. Halotolerance is observed not only in microorganisms but also in plants (halophytes) and animals adapted to saline environments.^[2] Halotolerant microorganisms encompass a wide range of bacteria, archaea, and fungi, classified based on their salt tolerance thresholds rather than strict obligate requirements.^[1] For instance, non-halophilic bacteria like Staphylococcus saprophyticus (up to 20% NaCl) and Bacillus species (up to 7-10% or higher) exhibit halotolerance, while some archaea can endure higher salinities without dependency.^[1] ^[3] Key examples include Aerobacter (now Enterobacter) bacteria from salted environments and certain actinomycetes like Streptomyces species, which thrive in fluctuating saline soils.^[2] This adaptability contrasts with obligate halophiles, as halotolerant species maintain functionality across low- to high-salt gradients through versatile physiological responses.^[4] The primary mechanisms of halotolerance involve osmotic balance, ion homeostasis, and cellular protection to counteract salt-induced dehydration and toxicity.^[1] Organisms accumulate compatible solutes such as ectoine, glycine betaine, or trehalose to maintain turgor pressure without disrupting proteins, while specialized transporters (e.g., BCCT and ABC systems) regulate sodium efflux and potassium influx.^[1] Additional strategies include modifications to cell membranes for reduced permeability, biosynthesis of protective polysaccharides, and activation of DNA repair pathways like the SOS response to mitigate salt-induced damage.^[1] These adaptations often rely on alternative metabolic pathways, such as the Entner-Doudoroff route in archaea, which function efficiently under ionic stress.^[2] Beyond ecological significance, halotolerant microorganisms hold substantial biotechnological potential due to their robust enzymes and metabolites stable in saline conditions.^[4] Applications include the production of salt-tolerant hydrolases and isomerases for industrial processes, compatible solutes as protein stabilizers in pharmaceuticals, and biosurfactants for enhanced oil recovery in saline reservoirs.^[4] In food biotechnology, they facilitate fermentation of salted products like soy sauce and cheese, while in environmental remediation, they aid in degrading pollutants in hypersaline waste sites.^[4] Ongoing research into their genomics further uncovers novel genes for engineering salt tolerance in crops and microbes.^[1]

Fundamentals

Definition and Scope

Halotolerance refers to the capacity of living organisms to endure and proliferate in environments characterized by elevated salt concentrations, generally exceeding 0.5 M NaCl, while being able to grow optimally in the absence of such conditions.^[5] This adaptation enables survival in saline habitats like coastal marshes or evaporative ponds without the obligatory salt dependency seen in halophiles. In contrast, halophily denotes a requirement for high salinity for growth, whereas osmoadaptation encompasses the general physiological responses to osmotic imbalances beyond just salt.^[6] The study of halotolerance originated in the 19th century with observations of salt-tolerant algae, such as Dunaliella salina, which were noted for causing red pigmentation in hypersaline lakes as early as the 1830s.^[7] A pivotal advancement came in the mid-20th century through the work of Benjamin Elazari-Volcani, who isolated halotolerant bacteria from Dead Sea sediments in the 1940s, demonstrating microbial life in one of the world's most saline bodies of water.^[8] Assessment of halotolerance typically involves quantitative metrics like the EC₅₀ value—the NaCl concentration that inhibits growth by 50%—and monitoring growth rates along salinity gradients, such as from 0 to 5 M NaCl, to quantify tolerance thresholds.^[9] These measures highlight halotolerance's distinction from other environmental stresses, such as xerotolerance (adaptation to desiccation) or thermotolerance (resistance to elevated temperatures), as it primarily counters salinity-induced ionic disequilibrium.^[10]

Types of Halotolerant Organisms

Halotolerant organisms are classified based on their degree of salt tolerance, typically measured in terms of sodium chloride (NaCl) concentration. Slight halotolerants thrive in low to moderate salinity, such as 0.5–1.5 M NaCl, and include many common soil bacteria like species of Bacillus that inhabit saline agricultural soils.^[10] Moderate halotolerants endure higher levels, around 1.5–4 M NaCl, exemplified by bacteria such as Staphylococcus species.^[10] Extreme halotolerants withstand concentrations exceeding 4 M NaCl, such as the green alga Dunaliella salina, which accumulates glycerol to maintain cellular balance in hypersaline environments.^[10] Across taxonomic groups, halotolerant prokaryotes are prominent, encompassing bacteria like Bacillus species isolated from coastal saline soils.^[11] Among eukaryotes, fungi including Wallemia sebi demonstrate tolerance in salt marshes, while plants like Salicornia europaea (glasswort) grow in coastal salt flats, and animals such as the brine shrimp Artemia salina inhabit hypersaline lagoons.^[12]^[13] This diversity spans domains of life, reflecting adaptations to varied saline niches without strict salt dependency.^[14] Halotolerants are distinguished as facultative, the majority, which grow optimally without salt but tolerate elevated levels up to several molar concentrations, such as many Pseudomonas species in estuarine sediments.^[10] Halotolerant microbes are widespread in saline-influenced environments like soils and waters, contributing to ecosystem resilience.^[5] In contrast, halophytic plants, which are vascular species tolerant of high salinity, represent less than 1% of all vascular plants worldwide.^[15]

Physiological Mechanisms

Osmotic Adjustment

Osmotic adjustment is a fundamental physiological process in halotolerant organisms, enabling them to maintain cellular turgor pressure and water balance under high external salt concentrations. This involves the accumulation of osmolytes within cells to lower the internal water potential, thereby counteracting the external osmotic stress and preventing water loss. The total water potential (ψ) of a cell is given by the equation ψ = ψ_s + ψ_p, where ψ_s represents the solute potential (negative due to solutes) and ψ_p is the pressure potential (turgor pressure). By adjusting ψ_s to match or exceed the external osmotic potential, organisms sustain positive turgor, essential for cell expansion and metabolic function.^[15] Halotolerant species employ two main strategies for osmotic adjustment: the "salt-in" approach using inorganic ions or accumulation of organic compatible solutes. In microorganisms, the salt-in strategy often involves uptake of potassium ions (K⁺) coupled with organic anions like glutamate to balance external salinity, maintaining cytoplasmic ionic strength without Na⁺ accumulation. This is energetically efficient and common in bacteria such as Halomonas elongata. In contrast, some eukaryotes, including fungi and plants, may use Na⁺ and Cl⁻ for osmotic balance, compartmentalizing them away from sensitive cytoplasmic regions. The solute potential (ψ_s) is quantitatively described by the van't Hoff equation: ψ_s = -RT Σ c_i, where R is the gas constant, T is the absolute temperature, and c_i are the molar concentrations of individual solutes. Achieving osmotic adjustment requires energy, primarily for ion or solute transport via ATP-dependent pumps, highlighting the trade-off between homeostasis and growth under salt stress.^[16]^[17] By facilitating osmotic adjustment, halotolerant organisms prevent plasmolysis, the shrinkage of the protoplast away from the cell wall due to excessive water efflux. This mechanism is particularly critical during environmental shifts, such as exposure to sudden salinity increases, where rapid osmolyte accumulation enables survival and growth.^[18]

Ion Management and Compatible Solutes

Halotolerant organisms employ ion exclusion and homeostasis strategies to mitigate the toxicity of sodium ions (Na⁺) under high salinity conditions. In microorganisms, Na⁺ is often extruded from the cytoplasm via Na⁺/H⁺ antiporters like NhaA in bacteria, powered by proton gradients from H⁺-ATPases. Complementary K⁺ uptake occurs through systems such as Kdp, helping maintain low cytosolic Na⁺ levels (typically below 10-50 mM) and high K⁺/Na⁺ ratios to protect enzymatic activities. These processes are essential for preventing ionic disruption in diverse halotolerant bacteria and archaea.^[19]^[1] To achieve osmotic adjustment without heavy reliance on inorganic ions, halotolerant organisms synthesize or uptake compatible solutes—non-ionic, organic osmolytes that do not interfere with cellular metabolism. Common examples include proline, glycine betaine, trehalose, and ectoine, which accumulate in the cytoplasm to balance external osmotic pressure while preserving protein function. Glycine betaine biosynthesis typically proceeds via the oxidation of choline: first, choline monooxygenase (CMO) converts choline to betaine aldehyde, followed by oxidation to glycine betaine catalyzed by betaine aldehyde dehydrogenase (BADH); this pathway is upregulated in response to salinity in bacteria and plants. Ectoine, prevalent in halotolerant bacteria like Halomonas species, is synthesized from aspartate semialdehyde through a three-step pathway involving EctA (DABA acetyltransferase), EctB (DABA aminotransferase), and EctC (ectoine synthase), providing robust protection at high salinities. Uptake of external compatible solutes is facilitated by specialized transporters, including BCCT (betaine-carnitine-choline transporter) family members for secondary active transport and ABC (ATP-binding cassette) systems for primary active uptake, ensuring rapid osmoprotection.^[20]^[21]^[22]^[23] Compatible solutes exert protective effects beyond osmosis by acting as chemical chaperones that stabilize protein structures and prevent denaturation under saline stress. For instance, ectoine and glycine betaine exclude water from protein surfaces, maintaining native conformations and enhancing enzyme stability in high-Na⁺ environments; concentrations of 0.5-1 M can support functionality in media up to 2-3 M NaCl. Similarly, trehalose and proline shield macromolecules from ionic perturbations, reducing aggregation and preserving membrane integrity. These actions contribute to overall osmotic and ionic stress tolerance.^[20]^[24] The genetic underpinnings of these mechanisms involve key regulators and genes responsive to salinity. The badh and ectABC gene clusters encode enzymes critical for glycine betaine and ectoine production, with overexpression enhancing halotolerance in various organisms. Ion management is coordinated by stress-responsive pathways, integrating osmotic and ionic responses across taxa.^[25]^[22]

Halotolerance in Microorganisms

Bacterial Halotolerance

Bacterial halotolerance refers to the ability of prokaryotic cells to maintain cellular integrity and function in environments with elevated salt concentrations, typically up to 10-15% NaCl for moderately halotolerant species. Key groups include the Gammaproteobacteria such as Vibrio species, which thrive in marine and estuarine settings by accumulating compatible solutes like ectoine and betaine to counter osmotic stress.^[20] Similarly, Halomonas species, isolated from saline soils and hypersaline waters, exemplify moderate halotolerance through ectoine biosynthesis, a cyclic amino acid derivative that stabilizes proteins and membranes without disrupting cellular processes.^[26] Ectoine synthesis proceeds via a five-enzyme pathway starting from L-aspartate: L-aspartate kinase (Ask), L-aspartate-β-semialdehyde dehydrogenase (Asd), L-2,4-diaminobutyrate aminotransferase (EctB), L-2,4-diaminobutyrate acetyltransferase (EctA), and ectoine synthase (EctC), enabling rapid osmoprotection upon salt exposure.^[27] To manage ion imbalances, halotolerant bacteria employ specialized transport systems. Potassium-sodium antiporters, such as those encoded by the kdp operon, facilitate high-affinity K⁺ uptake to restore turgor during hyperosmotic shock, as observed in Escherichia coli and extended to halotolerant strains like Halomonas elongata.^[28] Na⁺ extrusion occurs via Na⁺/H⁺ antiporters, often functioning with K⁺ selectivity to maintain cytoplasmic ion homeostasis.^[29] Aquaporins modulate transmembrane water flux during osmotic shifts.^[30] In response to sudden hyperosmotic stress, genes such as the proU locus are rapidly upregulated, encoding an ABC transporter for proline and betaine uptake, which bolsters intracellular osmolyte pools within minutes.^[31] In true bacteria, high salinity triggers biofilm formation, as seen in Halomonas and rhizobacterial isolates, where extracellular polymeric substances create a protective matrix that retains water and shields cells from ionic stress.^[32] This communal strategy enhances survival in fluctuating saline habitats. Genomic analyses reveal that horizontal gene transfer (HGT) disseminates halotolerance genes, such as those for ectoine biosynthesis and ion transporters, across bacterial lineages in saline niches, as evidenced in Martelella strains from coastal environments.^[33] Studies from the 2020s highlight HGT's role in expanding adaptive repertoires, with metagenomic surveys of hypersaline microbiomes showing frequent acquisition of osmolyte and stress-response operons.^[34] Additionally, CRISPR-Cas systems in halotolerant bacteria contribute to adaptation by integrating spacers from saline-specific phages, conferring immunity that supports persistence in competitive, high-salt microbial communities.^[35]

Archaeal Halotolerance

Archaea represent a significant group of halotolerant microorganisms, particularly in extreme hypersaline environments. Unlike many bacteria that rely on compatible solutes, halotolerant archaea such as Haloferax species often employ a "salt-in" strategy, accumulating high intracellular K⁺ concentrations to balance external NaCl, while adapting proteins to function in high ionic strength.^[1] Extreme examples include Halobacterium salinarum, which tolerates up to saturation (∼5.2 M NaCl) through acidified proteins with increased aspartate and glutamate residues for stability, and specialized retinal-based phototrophy for energy in low-oxygen, high-salt conditions. These adaptations enable archaea to dominate hypersaline microbial communities, such as those in the Dead Sea or solar salterns, and distinguish them from bacterial mechanisms by their unique membrane lipids (e.g., bacterioruberins for protection). Genomic studies reveal extensive gene clusters for ion pumps like Mrp antiporters and compatible solute synthesis as secondary strategies in moderately halotolerant archaea.^[14]

Fungal Halotolerance

Fungal halotolerance encompasses a diverse array of species adapted to high-salinity environments, including species such as Aspergillus spp. and the obligate halophile Wallemia ichthyophaga, which thrives in salt concentrations up to 5.2 M NaCl by maintaining cellular integrity under extreme osmotic pressure.^[36]^[37] These fungi primarily accumulate glycerol as an osmolyte to counter water loss, a process regulated by the high osmolarity glycerol (HOG1) MAPK signaling cascade, which activates glycerol-3-phosphate dehydrogenase upon salt exposure to restore turgor.^[38]^[39] This pathway, conserved across fungi, enables rapid adaptation without disrupting cytoplasmic functions, distinguishing eukaryotic responses from prokaryotic mechanisms.^[40] Cell wall modifications play a crucial role in fungal resilience to ionic stress, with halotolerant species exhibiting increased chitin deposition and melanization to enhance structural rigidity and impermeability.^[41]^[42] Chitin, a key β-1,4-linked polysaccharide, forms a reinforced scaffold that limits sodium ion influx, while melanin pigments bind to chitin fibrils, providing a barrier against oxidative damage from salt-induced reactive oxygen species (ROS).^[43] Additionally, spore dormancy serves as a survival strategy in hypersaline conditions, allowing fungi like Wallemia to remain viable for extended periods by suspending metabolism until salinity decreases, thereby preserving propagules in fluctuating environments.^[44] Fungi also manage ions through vacuolar sequestration, compartmentalizing toxic Na⁺ and Cl⁻ to maintain cytosolic homeostasis.^[45] Under salt stress, fungi undergo metabolic reprogramming, upregulating trehalose and polyols such as mannitol to stabilize proteins and membranes against dehydration.^[46] Trehalose accumulation, in particular, mitigates osmotic shock by acting as a compatible solute that preserves enzymatic activity.^[47] Concurrently, salt exposure elevates ROS levels, prompting the induction of antioxidant enzymes like catalases to decompose hydrogen peroxide and prevent cellular damage.^[48] These shifts ensure metabolic continuity in saline niches. Ecologically, halotolerant fungi colonize saline soils, contributing to nutrient cycling and microbial community stability in arid, salt-affected ecosystems.^[49] Recent studies highlight their symbiotic potential, with 2023 research demonstrating that endophytic fungi isolated from salt-adapted plants enhance host salt tolerance by modulating osmolyte production and reducing Na⁺ uptake in crops like wheat grown in saline conditions.^[50]^[51] Such interactions underscore fungi's role in mitigating soil salinization impacts on agriculture.^[52]

Halotolerance in Higher Organisms

Plant Halotolerance

Halophytes, salt-tolerant plants adapted to high-salinity environments, are classified into several categories based on their salt management strategies. Recretohalophytes actively excrete excess salts through specialized structures like salt glands or bladders, as exemplified by Tamarix species, which possess multicellular salt glands on leaves that secrete sodium chloride to maintain internal ion balance. In contrast, euhalophytes accumulate ions in their tissues while diluting salt concentrations through succulent growth, such as in Suaeda and Salicornia species, enabling them to thrive in highly saline soils without active excretion. Additionally, halophytes can be distinguished as succulent types, which store water in swollen leaves or stems to dilute salts (e.g., Salicornia), versus non-succulent types like certain grasses (Spartina) that rely on other mechanisms such as ion exclusion at the root level.^[53]^[54]^[55] At the cellular level, halophytes exhibit photosynthetic adjustments to cope with salinity-induced water stress, including the adoption of crassulacean acid metabolism (CAM) in some species to minimize transpiration; for instance, facultative CAM in succulents like Mesembryanthemum crystallinum allows nocturnal CO₂ fixation, reducing daytime stomatal opening and water loss by up to 90% under saline conditions. Root-shoot signaling plays a crucial role in coordinating these responses, with abscisic acid (ABA) synthesized in roots under salt stress translocating to shoots to induce stomatal closure and gene expression for tolerance, while cytokinins modulate shoot growth and delay senescence to maintain photosynthetic efficiency. Plants also employ osmotic adjustment to sustain leaf turgor, accumulating organic solutes in response to salinity.^[56]^[57]^[58] Ion homeostasis in halophytes involves compartmentalizing toxic ions like Na⁺ into vacuoles via tonoplast transporters such as NHX proteins, preventing cytoplasmic damage while using Na⁺ as an osmoticum in older tissues. Proline accumulation serves dual roles in osmoprotection, stabilizing proteins and membranes, and as a reactive oxygen species (ROS) scavenger to mitigate oxidative stress from salinity; levels can increase 10- to 100-fold under high salt, correlating with enhanced survival. Salinity impacts yield variably, with some halophytes showing up to 50% growth reduction at 200 mM NaCl due to impaired nutrient uptake and photosynthesis, though many maintain productivity at levels lethal to glycophytes.^[59]^[60]^[61] Breeding efforts leverage genetic tools like quantitative trait locus (QTL) mapping to identify salt tolerance genes, such as those controlling ion exclusion and osmotic regulation in crops like rice and tomato, facilitating marker-assisted selection for saline-adapted varieties.^[62]

Animal Halotolerance

Halotolerance in animals primarily manifests in invertebrates, particularly extremophiles inhabiting hypersaline environments such as salt lakes and coastal saline puddles, where osmoregulation is crucial for maintaining cellular integrity against osmotic stress.^[63] These organisms employ specialized physiological and behavioral strategies to counteract the dehydrating effects of high salt concentrations, often exceeding those tolerable by most metazoans. Invertebrates like crustaceans, nematodes, and tardigrades exemplify this adaptation, relying on ion transport mechanisms and osmolyte accumulation rather than the sessile ion exclusion seen in plants.^[64] Brine shrimp (Artemia spp.), iconic halotolerant crustaceans, thrive in salinities up to 300 g/L through ionoregulatory gills that actively excrete excess sodium via Na⁺,K⁺-ATPase pumps, enabling survival in environments where sodium concentrations surpass 4 M.^[63] Similarly, nematodes such as Caenorhabditis elegans demonstrate tolerance to saline conditions in ephemeral puddles, withstanding salinities that induce osmotic stress through behavioral avoidance and limited ion regulation during short exposures.^[65] Extremophile tardigrades, known for their resilience, tolerate NaCl concentrations up to approximately 600 mOsm kg⁻¹ (about 1.75% NaCl) in some species, such as Ramazzottius oberhaeuseri for direct transfers, entering a protective tun state to mitigate osmotic damage during dehydration-like conditions in saline habitats.^[66] Osmoregulatory organs play a pivotal role in animal halotolerance; for instance, Malpighian tubules in insects facilitate salt excretion by secreting ions into the hindgut, maintaining hemolymph balance under saline stress as seen in Drosophila responses to dietary salt.^[67] In crustaceans and other arthropods, hemolymph adjustments involve elevating free amino acids—such as glycine and taurine—as compatible osmolytes to preserve cell volume without disrupting protein function, paralleling but distinct from proline accumulation in plants.^[68] These organic osmolytes counteract ionic imbalances, allowing intracellular osmotic equilibrium in hypersaline media.^[69] Behavioral adaptations further enhance survival, including burrowing to evade surface salt accumulation in intertidal or evaporative zones, as observed in some marine invertebrates that reduce exposure to fluctuating salinities.^[70] In Artemia, eggs enter diapause as cysts, enduring high salinity and desiccation for extended periods until conditions ameliorate, a strategy that ensures population persistence in variable saline environments.^[71] Physiological limits are exemplified by Artemia's upper threshold of 300 g/L NaCl, beyond which survival declines due to impaired osmoregulation.^[72] Molecular facilitators like aquaglyceroporins enable glycerol flux across membranes, supporting osmolyte dynamics in tolerant species such as tardigrades during osmotic challenges.^[73]

Ecological and Applied Aspects

Natural Habitats and Ecology

Halotolerant organisms inhabit a range of extreme saline environments, including salt marshes, hypersaline lakes, coastal sabkhas, and evaporite deposits. Salt marshes, often found in intertidal zones, feature fluctuating salinities influenced by tidal inundation, supporting communities adapted to periodic hypersalinity. Hypersaline lakes, such as the Great Salt Lake in Utah, exhibit salinities reaching up to 27%, far exceeding the 3.5% of seawater, creating athalassohaline or thalassohaline conditions that select for specialized microbial and plant life. Coastal sabkhas, arid saline flats in regions like the Arabian Peninsula, form through evaporation and host evaporitic minerals alongside halotolerant microbes, while evaporite deposits represent both modern and ancient accumulations of salts like halite, preserving microbial signatures in buried layers.^[74]^[75]^[76]^[77] In these habitats, halotolerant organisms fulfill essential ecological roles that sustain ecosystem function. As primary producers, halotolerant algae such as Dunaliella species form dense microbial mats in hypersaline lakes and sabkhas, driving carbon fixation and serving as the foundation for trophic webs in otherwise nutrient-limited settings. These mats oxygenate sediments and support grazers like brine shrimp. Halotolerant microbes also function as decomposers in saline soils, breaking down organic matter to recycle nutrients like nitrogen and phosphorus, thereby maintaining soil fertility despite high osmotic stress. Salt marshes act as biodiversity hotspots, with some supporting over 100 halophyte species, fostering complex plant-microbe interactions that enhance overall ecosystem resilience.^[78]^[79]^[80] Ecological interactions among halotolerants are vital for community stability. Symbiotic partnerships, such as those between salt-tolerant rhizobia bacteria and halophyte legumes, facilitate nitrogen fixation in saline conditions, converting atmospheric nitrogen into bioavailable forms and bolstering plant growth in nutrient-poor soils. According to a 2024 FAO assessment, nearly 1.4 billion hectares of land globally are affected by salinity, comprising about 10.7% of the total land area, with projections indicating further expansion due to climate change.^[81] Climate change exacerbates habitat dynamics, with rising sea levels projected to increase saline intrusion, for example by 10-27% in regions like the Mekong Delta by 2050, potentially expanding hypersaline zones and altering species distributions. Evolutionarily, niche partitioning along salinity gradients enables coexistence, as different halotolerant taxa occupy specific osmotic niches, from moderate to extreme levels. Fossil records from Permian salt deposits, dating back 250 million years, reveal ancient halotolerant communities, including archaea and bacteria preserved in halite crystals, underscoring the long-term persistence of these adaptations.^[82]^[83]^[84]^[85]

Applications in Biotechnology and Agriculture

Halotolerant plants, such as quinoa (Chenopodium quinoa), serve as promising crops for cultivation on saline lands, where they can tolerate sodium chloride concentrations up to 200 mM without significant yield loss.^[86] This adaptability makes quinoa suitable for reclaiming salt-affected soils, which comprise approximately 20% of global arable land and pose a major threat to conventional agriculture.^[87] In addition, halotolerant microorganisms applied as biofertilizers enhance crop yields in these environments by promoting nutrient uptake and stress alleviation, with reported improvements in plant growth and productivity under saline conditions.^[88] In biotechnology, enzymes derived from halotolerant bacteria, including halostable proteases, are utilized in detergent formulations due to their stability and activity in high-salt and alkaline environments.^[89] These proteases effectively break down protein-based stains even in saline wash conditions, offering eco-friendly alternatives to traditional enzymes.^[90] Furthermore, compatible solutes like ectoine, produced by halophilic bacteria, are incorporated into cosmetics for their protective effects on skin cells, shielding against dehydration, UV damage, and surfactant-induced irritation.^[91] Industrial applications of halotolerance include bioleaching processes in saline mining operations, where halotolerant acidophilic microbes facilitate metal extraction from ores in water-scarce, high-salinity regions.^[92] These microorganisms maintain oxidative activity under chloride stress, enabling efficient recovery of base metals like copper without freshwater dependency.^[93] In wastewater treatment, halotolerant algae such as Dunaliella species remove nutrients like nitrogen and phosphorus from saline effluents, achieving high efficiencies such as over 80% for total nitrogen removal while producing valuable biomass.^[94] Emerging research in synthetic biology focuses on engineering salt tolerance in staple crops, such as rice, through the insertion of glycine betaine biosynthetic genes, which enhance osmotic adjustment and yield under salinity stress.^[95] Recent trials demonstrate improved survival and productivity in saline fields.^[96] Economically, halophyte-based farming is projected to drive the saline agriculture market to USD 814.59 million by 2030, supporting sustainable production on marginal lands amid rising salinization.^[97]

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The primary response to osmotic stress mentioned above is a temporary response, as potassium glutamate increases intracellular osmolarity, inducing a negative ...
[33]
Characterization of a novel two-component Na+(Li+, K+)/H+ ... - Nature
Jun 26, 2017 · In this study, genomic DNA was screened for novel Na+/H+ antiporter genes from Halomonas zhaodongensis by selection in Escherichia coli ...
[34]
Aquaporin AqpZ Is Involved in Cell Volume Regulation and ...
Water flux occurs mainly through water-permeable channels, including aquaporins ... This decrease in water loss coincided with an increase in tolerance to ...
[35]
Isolation and sequencing of Escherichia coli gene proP ... - PubMed
Jan 5, 1993 · Transporters encoded in genetic loci putP, proP and proU mediate proline and/or betaine accumulation by Escherichia coli K-12.
[36]
Biosynthesis of the Stress-Protectant and Chemical Chaperon Ectoine
Two of the ectoine/5-hydroxyectoine biosynthetic enzymes, namely, the ectoine synthase EctC and the ectoine hydroxylase EctD, have already been studied in ...
[37]
Halotolerant biofilm-producing rhizobacteria mitigate seawater ...
Apr 4, 2022 · Halotolerant biofilm-producing rhizobacteria reduced sea water-induced salinity stress parameters and increased antioxidant defense systems in ...
[38]
Genomic Analysis of Halotolerant Bacterial Strains Martelella ... - NIH
Previous studies have suggested that horizontal gene transfer may occur within bacteria and methanogenic archaea by comparing the phylogenetic relationships ...
[39]
Genomic insights into novel extremotolerant bacteria isolated from ...
May 12, 2025 · Isolation and characterization of bacteria capable of tolerating the extreme conditions of clean room environments. Appl Environ Microbiol ...
[40]
In-silico study of molecular adaptations in halophilic Cas9
Jan 20, 2025 · Haloferax volcanii's CRISPR–Cas system has been adapted for activity in saline environments and has substantial implications for genetic ...
[41]
Population Genomics of an Obligately Halophilic Basidiomycete ...
One of only a handful of true fungal halophiles is Wallemia ichthyophaga, one of the most extremotolerant fungal species with regard to low water activity ...Missing: xerohalophytes | Show results with:xerohalophytes
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Halotolerant and halophilic fungi | Request PDF - ResearchGate
Aug 9, 2025 · Only a few fungal species display true halophilic behaviour (Zalar et al. 2005), such as Debaryomyces hansenii, Hortaea werneckii, and Wallemia ...Missing: xerohalophytes | Show results with:xerohalophytes
[43]
The HOG pathway and the regulation of osmoadaptive responses in ...
In yeast, the high-osmolarity glycerol (HOG) pathway is responsible for the response to high osmolarity. Activation of the Hog1 stress-activated protein kinase ...
[44]
Adaptation to high salt concentrations in halotolerant/halophilic fungi
The novel and intricate molecular mechanisms used by these fungi to combat high salt concentrations, which differ in many aspects between the extremely ...Missing: xerohalophytes | Show results with:xerohalophytes
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Reconstruction of the High-Osmolarity Glycerol (HOG) Signaling ...
In yeast and fungi, increases in environmental osmolarity are sensed by the evolutionarily conserved high-osmolarity glycerol (HOG) signaling pathway (Krantz et ...
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Structural adaptation of fungal cell wall in hypersaline environment
Nov 4, 2023 · Chitosan and β-glucans are tightly associated with chitin and each other, but a high concentration of salt weakens these packing interactions ...
[47]
Functions of fungal melanin beyond virulence - PMC - NIH
Melanin deposition in the fungal cell wall involves intimate molecular interactions with chitin structures (Baker et al., 2007; Banks et al., 2005; Bull ...
[48]
Structural Polymorphism of Chitin and Chitosan in Fungal Cell Walls ...
Aug 24, 2021 · This study provides high-resolution information on the structure of chitin and chitosan in their cellular contexts.
[49]
Salt needed: Tolerance lessons from a Dead Sea fungus
May 9, 2014 · Some organisms thrive in salty environments by lying dormant when salt concentrations are very high. Other organisms need salt to grow.
[50]
Vacuolar control of subcellular cation distribution is a key parameter ...
Vacuoles play a role in autophagy, osmoregulation and storage of amino acids and ions, and it was previously reported that hypoosmotic conditions promote ...
[51]
Response to Salt Stress of the Halotolerant Filamentous Fungus ...
Glycometabolism products, including trehalose, show marked changes under NaCl stress and their genes are expressed in response to NaCl stimulation and play a ...
[52]
Intracellular Protective Functions and Therapeutical Potential of ...
May 1, 2024 · It has been shown that trehalose can alleviate oxidative stress by reducing H2O2-induced ROS accumulation and preserving the activity of the ...
[53]
Growth Phase-Dependent Changes in the Carbohydrate ... - MDPI
New information could prove beneficial in forecasting the behaviour of fungal pathogens such as plant pathogens in agriculture and human pathogens in medicine.
[54]
Metabolic Potential of Halophilic Filamentous Fungi—Current ... - NIH
Apr 10, 2022 · The best-known fungi considered to be model halotolerant and halophilic organisms include Hortaea werneckii, Wallemia ichthyophaga, Debaryomyces ...Missing: xerohalophytes | Show results with:xerohalophytes
[55]
Salt tolerant fungal endophytes alleviate the growth and yield of ...
In this study, we assessed the endophytic fungal associates of salt-tolerant KRL-19, KRL-213 and KRL-213 wheat genotypes and their tolerance to salinity stress.
[56]
Fungal endophytes boost salt tolerance and seed quality in quinoa ...
This study investigates the contributions of quinoa-associated endophytes to salinity tolerance and seed quality in different genotypes.
[57]
Halotolerant Endophytic Fungi: Diversity, Host Plants, and ... - MDPI
Among these, 55 studies specifically focused on endophytic fungi's contributions to improving plants' salt–alkali tolerance capabilities. Based on the ...
[58]
Adaptive Mechanisms of Halophytes and Their Potential in ...
Oct 3, 2021 · Recretohalophytes have unique salt-secreting structures, such as salt glands and salt bladders, whereas euhalophytes have the ability to dilute ...
[59]
Progress in Studying Salt Secretion from the Salt Glands in ...
Jun 30, 2016 · Amongst these halophytes there is a small number that are able to secrete salt from their leaves, the so-called recretohalophytes (Flowers et al ...
[60]
Using euhalophytes to understand salt tolerance and to develop ...
An example is the euhalophytic herb Suaeda salsa. Euhalophytes, which can dilute salt within their succulent leaves or stems and thus have high salt tolerance, ...
[61]
Ecophysiology of Crassulacean Acid Metabolism (CAM) - PMC
... halophytes are not CAM plants and CAM plants are not halophytes. There are some isolated reports that CAM plants may cope well with mild NaCl stress, e.g. ...
[62]
ABA control of plant macroelement membrane transport systems in ...
Nov 28, 2013 · During water deficit stress, ABA was suggested to act as a long-distance signal between roots and shoots (Sauter et al., 2001; Wilkinson & ...
[63]
Insights into plant salt stress signaling and tolerance - ScienceDirect
Actually, rapid accumulation of cytoplasmic osmolytes for osmotic adjustment is one efficient strategy for halophytes to adapt to saline soils (Slama et al., ...Review · Salt Stress Perception In... · Plant Salt Responses And...
[64]
Sodium chloride toxicity and the cellular basis of salt tolerance in ...
Dec 1, 2014 · Conclusions The data reviewed here suggest that halophytes tolerate cytoplasmic Na+ and Cl− concentrations of 100–200 mm, but whether these ions ...
[65]
Reproductive Physiology of Halophytes: Current Standing - Frontiers
In general, the larger seeds are the main type showing rapid germination in most halophytes with dimorphic seeds, such as Atriplex prostrata (Wertis and Ungar, ...<|control11|><|separator|>
[66]
Salt tolerance of selected halophytes at the two initial growth stages ...
May 13, 2021 · Halophytes are plants that have the ability to complete its life cycle in salt concentrations of not less than 200 mM NaCl. Halophytes have been ...
[67]
QTLs and Genes for Salt Stress Tolerance: A Journey from Seed to ...
Apr 14, 2024 · Genetics of Salt Tolerance and QTL Mapping. Many agronomically important traits in crop plants are polygenic. Each of the contributing genes ...
[68]
Technological advancements in the CRISPR toolbox for improving ...
Nov 19, 2024 · The utilization of CRISPR can streamline the incorporation of novel genes from salt-tolerant species into agricultural crops. This transfer of ...
[69]
A Na pump with reduced stoichiometry is up-regulated by brine ...
Brine shrimp (Artemia) are the only animals to thrive at sodium concentrations above 4 M. Salt excretion is powered by the Na+,K+-ATPase (NKA), a heterodimeric ...
[70]
Transcriptomic analysis of brine shrimp Artemia franciscana across ...
Halophilic and halotolerant organisms, well-adapted to high-salinity waters, maintain their internal osmotic equilibrium to thrive in harsh environments. The ...
[71]
Tolerance of the nematode Caenorhabditis elegans to pH, salinity ...
In this study, we established the tolerance range of the nematode Caenorhabditis elegans for pH, salinity and hardness using 24- (without food source) and 96-h ...
[72]
Osmotic stress tolerance in semi-terrestrial tardigrades
Nov 22, 2016 · Direct transfers into NaCl solutions showed an upper tolerance level of around 600 mOsm kg−1 in R. oberhaeuseri and 200 mOsm kg−1 in E. testudo.Missing: halotolerance NaCl
[73]
Salty dog, an SLC5 symporter, modulates Drosophila response to ...
Adult Drosophila were exposed to dietary salt stress, and their physiological, survival, and gene expression responses monitored.Missing: halotolerance | Show results with:halotolerance
[74]
Changes in free amino acids in the hemolymph of giant freshwater ...
Moreover, to cope with high salinities, some crustaceans increase their hemolymph free amino acids (via synthesis or the breakdown of hemolymph/muscle proteins) ...Missing: adjustments halotolerance
[75]
Osmoadaptative Strategy and Its Molecular Signature in Obligately ...
Organic osmolytes are strong “water structure-formers” that are excluded from protein hydration shells, and thus prevent cell desiccation while being compatible ...
[76]
Adapting to marine habitats - Science Learning Hub
Stenohaline animals rely on behavioural adaptations such as moving out of the area, burrowing in the sand and closing their shells or physiological adaptations ...Missing: diapause high
[77]
Combined effects of salinity and temperature on survival and ...
They noted that although salinity resistance is high, a tolerance range from 3 ppt to 300 ppt, as generalized for the genus Artemia, is not valid for all ...
[78]
The genome of the extremophile Artemia provides insight into ...
Aug 31, 2021 · Artemia is one of the few known animal halophiles and is even considered an extreme halophile, tolerating salinities over 300 g/L [12]. Being ...Missing: threshold | Show results with:threshold
[79]
Comparative functional analysis of aquaporins/glyceroporins in ...
Aquaporins (AQPs) are water-selective members of the MIP family, whereas aquaglyceroporins (GLPs) transport both water and organic compounds such as glycerol, ...
[80]
[PDF] Great Salt Lake, Utah - USGS Publications Warehouse
Salinity south of the causeway has ranged from 6 percent to 27 percent over a period of 22 years (2 to 7 times saltier than the ocean). The high salinity ...
[81]
Hypersaline environments as natural sources of microbes with ... - NIH
Apr 26, 2022 · "Hypersaline environments" are those with higher salt concentrations than seawater (around 3.5% w/v in seawater vs. up to 35% w/v in brines).
[82]
Halophile - an overview | ScienceDirect Topics
One of the ways halophilic and halotolerant bacteria, fungi, yeast, and algae employ is a compatible solute strategy to maintain an isoosmotic balance inside ...
[83]
Astrobiological significance of the sabkha life and environments of ...
This research deals with halite, gypsum, and other evaporite minerals that have accumulated in saline lakes and their associated microorganisms. The present ...
[84]
Characterization of eukaryotic microbial diversity in hypersaline ...
Dunaliella are considered potentially important in hypersaline systems as both primary producers ... Ecology of algal mats from hypersaline ponds in the British ...
[85]
Effect of salinization on soil properties and mechanisms beneficial to ...
Halotolerant bacteria can promote nutrient cycling in salinized soil and improve the bioavailability and supply capacity of soil nutrients. Soil column ...
[86]
The case of Sivash Bay (the Sea of Azov), the largest hypersaline ...
Currently, hypoxic and anoxic events often occur in Sivash Bay under floating algal mats, and this also may suppress taxon diversity in macrobenthos more than ...Missing: producers | Show results with:producers
[87]
Rhizobium-Legume Symbiosis and Nitrogen Fixation under Severe ...
Recent reports support the finding that some rhizobia have the potential to form a successful symbiosis with legumes under salt stress. Most of the work ...
[88]
The growing trend of saltwater intrusion and its impact on coastal ...
Feb 25, 2025 · In the Mekong Delta, salinity-affected areas are projected to increase by 10–27 % by 2050 due to anthropogenically induced factors such as ...
[89]
Comparative genomics reveals adaptations of a halotolerant ...
Aug 8, 2014 · We show that the local archaeal community of five different BSI habitats (with up to 18.2% salinity) is composed mostly of a single, highly abundant ...
[90]
Halophilic Archaea: Life with Desiccation, Radiation and Oligotrophy ...
The first cultivations of halophilic microorganisms from Permian salt sediments (about 250 million years old) were reported in the 1960s [2,3] and raised ...Missing: records halotolerant
[91]
[PDF] Chenopodium quinoa Willd. A new cash crop halophyte for saline ...
Nov 17, 2003 · The limit of salinity resistance was estimated at 200 mM NaCl (~20 dSm-1) based on seed yield production. Depending on the results obtained from ...
[92]
Soil salinity: A serious environmental issue and plant growth ...
It has been estimated that worldwide 20% of total cultivated and 33% of irrigated agricultural lands are afflicted by high salinity. ... affected by saline lands.
[93]
Halotolerant plant growth-promoting rhizobacteria improve soil ...
In general, high soil salinity inhibits seed germination (Wu et al., 2019). This could be attributed to suppression of water potential, protein content, and ...
[94]
Alkaline and Halophilic Protease Production by Bacillus luteus H11 ...
These properties make the protease suitable for applications in food, detergent and pharmaceutical industries, and also in environmental bioremediation. Key ...
[95]
Review Operative utility of salt-stable proteases of halophilic and ...
The commercially-compatible protease should show stability and altered specificity in organic solvents, and be economically as well as ecologically sustainable.
[96]
The multifunctional role of ectoine as a natural cell protectant
Our present data show that the compatible solute ectoine protects the cellular membrane from damage caused by surfactants.
[97]
Progress in bioleaching: fundamentals and mechanisms of microbial ...
The application of halotolerant microorganisms in bioleaching became more relevant in the last decade, e.g., for bioleaching in regions where freshwater is ...
[98]
Salt-tolerant microorganisms potentially useful for bioleaching ...
May 1, 2015 · However, the efficacy of bioleaching is compromised in the presence of saline and brackish waters as acidophilic microorganisms are generally ...
[99]
Bioremoval of Co(II) by a novel halotolerant microalgae Dunaliella ...
Apr 29, 2024 · In this study, we found that the halotolerant microalga named Dunaliella sp. FACHB-558 could grow under a salinity condition with 0.5–4.5 M NaCl.
[100]
Microalgae systems - environmental agents for wastewater ...
Jul 1, 2023 · Microalgal systems remove up to 99% of total nitrogen and phosphorus. Microalgae systems remove up to 93% of the most prevalent antibiotics.
[101]
(PDF) Metabolic engineering of rice leading to biosynthesis of ...
Aug 7, 2025 · Metabolic engineering of rice leading to biosynthesis of glycinebetaine and tolerance to salt and cold ... genes conferring salt stress tolerance ...
[102]
[EPUB] A comprehensive review on rice responses and tolerance to salt stress
Mar 31, 2025 · The overexpression of glycine betaine biosynthetic genes has resulted in rice plants exhibiting enhanced growth and productivity in saline soils ...
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Saline Agriculture Market By Size, Share & Forecast 2030F
Global Saline Agriculture Market was valued at USD 587.26 Million in 2024 and is expected to reach USD 814.59 Million by 2030 with a CAGR of 5.61% during ...Missing: projection | Show results with:projection