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Labyrinthulomycetes
Labyrinthulomycetes
Labyrinthulomycetes
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Labyrinthulomycetes
Cell with network of ectoplasmic filaments (Aplanochytrium sp.)
Scientific classification Edit this classification
Domain: Eukaryota
Clade: Sar
Clade: Stramenopiles
Phylum: Bigyra
Subphylum: Sagenista
Class: Labyrinthulomycetes
Arx, 1970, Dick, 2001
Orders[1]
Synonyms
  • Labyrinthulomycota Whittaker, 1969
  • Labyrinthomorpha Page in Levine et al., 1980
  • Labyrinthulea Olive, 1975
    ex Cavalier-Smith, 1989

Labyrinthulomycetes (ICNafp) or Labyrinthulea[2] (ICZN) is a class of protists that produce a network of filaments or tubes,[3] which serve as tracks for the cells to glide along and absorb nutrients for them. The two main groups are the labyrinthulids (or slime nets) and thraustochytrids. They are mostly marine, commonly found as parasites on algae and seagrasses or as decomposers on dead plant material. They also include some parasites of marine invertebrates and mixotrophic species that live in a symbiotic relationship with zoochlorella.[4][5][6]

Characteristics

[edit]

Although they are outside the cells, the filaments of Labyrinthulomycetes are surrounded by a membrane. They are formed and connected with the cytoplasm by a unique organelle called a sagenogen or bothrosome. The cells are uninucleated and typically ovoid, and move back and forth along the amorphous network at speeds varying from 5-150 μm per minute. Among the labyrinthulids, the cells are enclosed within the tubes, and among the thraustochytrids, they are attached to their sides.

Evolution

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Evolutionary origin

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Labyrinthulomycetes are not fungi, but a monophyletic group of eukaryotes within the Stramenopiles. They belong to the phylum Bigyra, which contains other heterotrophic microorganisms such as the bicosoecids. Considering that the plastids from Stramenopiles are possibly the result of an event of endosymbiosis in their last common ancestor, the bicosoecids and the labyrinthulomycetes could have originated from a mixotrophic algal common ancestor that secondarily lost their plastids.[3]

Some characteristics of the labyrinthulomycetes can be explained by their origin from ancestral plastids. They produce omega-3 poly-unsaturated fatty acids using a desaturase usually present in chloroplasts. The zoospores of labyrinthulids have an eyespot composed of membrane-bound granules that resembles eyespots of photosynthetic stramenopiles, which are either within a plastid or believed to be derived from a plastid.[3]

Within Bigyra, the labyrinthulomycetes are the sister group to Eogyrea, a class containing the species Pseudophyllomitus vesiculosus and the environmental clade called MAST-4. Together they compose the subphylum Sagenista.[7][8]

Stramenopiles

Classification

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Labyrinthulomycetes or Labyrinthulea used to compose the defunct fungal phylum Labyrinthulomycota.[9] They were originally considered unusual slime moulds, although they are not very similar to the other sorts. The structure of their zoospores and genetic studies show them to be a primitive group of heterokonts, but their classification and treatment remains somewhat unsettled.

This class usually contained two orders, Labyrinthulales and Thraustochytriales (ICBN), or Labyrinthulida and Thraustochytrida (ICZN), but a different classification has recently been proposed.[6][10][11][1][9]

Genetic code

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The labyrinthulomycete Thraustochytrium aureum is notable for the alternative genetic code of its mitochondria which use TTA as a stop codon instead of coding for Leucine.[13] This code is represented by NCBI translation table 23, Thraustochytrium mitochondrial code.[14]

Genetic code Translation
table		 DNA codon RNA codon Translation
with this code		 Standard code
(Translation table 1)
Thraustochytrium mitochondrial 23 TTA UUA STOP = Ter (*) Leu (L)
[edit]

References

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[edit]
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Labyrinthulomycetes

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from Grokipedia
Labyrinthulomycetes are a class of heterotrophic, belonging to the stramenopiles (Heterokonta), characterized by their fungus-like morphology and lifestyles, including the production of ectoplasmic nets or threads for and nutrient absorption via osmotrophy. These organisms typically exhibit a life cycle involving walled cells, amoeboid or spindle-shaped forms, and biflagellate zoospores, with cells often covered in scales of dictyosomal origin and featuring bothrosomes—specialized organelles that generate the ectoplasm. Ubiquitous in coastal, estuarine, and open environments, they thrive in organic-rich waters and can achieve levels exceeding those of bacterioplankton, underscoring their ecological significance as decomposers. Taxonomically, Labyrinthulomycetes form a monophyletic group within the stramenopiles, historically misclassified as fungi or slime molds due to superficial resemblances but now firmly placed in the lineage based on , such as 18S rRNA gene analyses. The class encompasses approximately 25 genera and over 50 described species, organized into five orders: Thraustochytrida (e.g., Thraustochytrium, Aurantiochytrium, Schizochytrium), Labyrinthulida (e.g., Labyrinthula, Aplanochytrium), Amphitremida, Amphifilida, and Oblongichytriales, with ongoing revisions revealing high undescribed diversity through environmental sequencing. Key distinguishing features include tubulocristate mitochondria and the absence of chloroplasts, reflecting evolutionary loss from photosynthetic ancestors, positioning them phylogenetically near groups like Bicosoecida. Ecologically, Labyrinthulomycetes primarily function as saprotrophs, breaking down from , plants, and animals to recycle nutrients in marine food webs, though some act as parasites causing diseases in seagrasses (Labyrinthula zosterae), , and turf grasses. Their abundance peaks in leaves, sediments, and , with studies reporting densities up to 5.6 × 10⁵ colony-forming units per gram in subtropical environments. Biotechnologically, genera like Schizochytrium and Aurantiochytrium are notable for producing high levels of omega-3 polyunsaturated fatty acids (PUFAs), such as (DHA) up to 53% of total fatty acids, making them promising for feeds and nutraceuticals. Despite their importance, the group remains understudied, with meta-analyses identifying dozens of novel environmental clades that highlight their global distribution and adaptive diversity.

Taxonomy and Classification

Higher Taxonomy

Labyrinthulomycetes belongs to the domain Eukaryota, within the supergroup SAR (Stramenopiles, Alveolates, and Rhizaria), phylum Stramenopiles (also known as Heterokonts), subphylum Sagenista, and class Labyrinthulomycetes. The class was originally established by J.A. von Arx in 1970 as part of a of fungi sporulating in pure culture, reflecting early perceptions of these organisms as fungus-like. It was subsequently emended by M.W. Dick in to incorporate their protistan nature and align with straminipilous systematics, emphasizing shared features with other heterotrophic stramenopiles. Historically, Labyrinthulomycetes were misclassified as slime molds within Myxomycota due to their colonial growth, ectoplasmic networks, and saprotrophic habits, which superficially resembled fungal or myxomycete structures. This placement persisted into the mid-20th century until ultrastructural studies in the 1970s and revealed diagnostic heterokont features, such as tubular cristae in mitochondria, Golgi-derived scales on cell walls, and biflagellate zoospores with heterokont flagellation (one anterior tinsel and one posterior smooth ). Molecular evidence from 18S rRNA gene phylogenies beginning in the late and early further confirmed their position among stramenopiles, distinguishing them from true fungi and myxomycetes by signatures like specific base pairings in ribosomal helices. In the current taxonomy outlined by Adl et al. (2019), Labyrinthulomycetes is recognized as a monophyletic class within the phylum Bigyra of Stramenopiles, forming a sister group to the class Eogyrea, which includes environmental clades and species like Pseudophyllomitus vesiculosus. This placement is supported by phylogenomic analyses integrating 18S rRNA and multigene data, resolving Bigyra as a basal heterotrophic lineage in Stramenopiles. Key diagnostic traits reinforcing this higher placement include their heterotrophic nutrition, production of biflagellate zoospores, and unique ectoplasmic elements (such as bothrosomes or sagenogens) for motility and nutrient uptake, which are characteristic of stramenopile osmotrophs.

Orders and Families

The internal classification of Labyrinthulomycetes recognizes five orders: Amphitremida, Amphifilida, Oblongichytriida, Labyrinthulida, and Thraustochytrida. These orders encompass diverse morphologies and life cycles, with Thraustochytrida including genera like Thraustochytrium and Schizochytrium, and Labyrinthulida featuring slime-net producers like Labyrinthula. Amphitremida includes phagotrophic forms such as Amphitrema, while Amphifilida and Oblongichytriida represent less-studied groups with amoeboid and zoospore-based forms, respectively. Nomenclature within Labyrinthulomycetes remains unsettled due to their ambiguous status as fungus-like protists, leading to dual regulatory systems: the International Code of Botanical Nomenclature (ICBN) for fungal affinities and the (ICZN) for protistan traits. Approximately 50–60 of Labyrinthulomycetes have been formally described, though environmental sequencing indicates substantial undescribed diversity, including over 200 operational taxonomic units recovered from marine sediments. Key genera illustrate this diversity: Labyrinthula, which produces ectoplasmic slime nets often associated with algal substrates; Thraustochytrium, a commonly found on organic ; and Diplophrys, a representative in freshwater environments.
OrderFamilyRepresentative Genera
AmphitremidaAmphitremidaeAmphitrema, Archerella
AmphifilidaDiplophryidaeDiplophrys, Quondamattia
OblongichytriidaOblongichytriaceaeOblongichytrium
LabyrinthulidaLabyrinthulaceaeLabyrinthula, Aplanochytrium
ThraustochytridaThraustochytriaceaeThraustochytrium, Schizochytrium, Aurantiochytrium
ThraustochytridaAplanochytriaceaeAplanochytrium

Morphology and Life Cycle

Cellular Structure

Labyrinthulomycetes cells are typically uninucleated and exhibit ovoid or spindle-shaped morphologies, with diameters ranging from 5 to 20 μm. These cells possess multilamellate cell walls composed primarily of , including , , , glucose, and , often arranged in thin scales characteristic of stramenopiles. The walls may also incorporate components, providing structural integrity and protection. A defining feature in certain life stages is the production of biflagellate zoospores, which bear an anterior flagellum with tripartite tubular hairs and a posterior smooth whiplash , aligning with . Internally, these cells contain prominent organelles such as tubular mitochondria and a bothrosome (or sagenogenetosome), a Golgi-derived structure responsible for ectoplasmic element production. droplets are abundant, particularly in thraustochytrids, serving as reservoirs rich in polyunsaturated fatty acids like (DHA). Ultrastructural variations occur between major groups. In thraustochytrids, cells are often spherical to ovoid with thick, scale-covered walls and are embedded within walled structures like sporangia, featuring a single bothrosome per cell. Labyrinthulids, in contrast, display elongated, cells that form chain-like colonies, with minimal or absent cell walls and multiple bothrosomes facilitating ectoplasmic networks. These differences underscore the diverse cellular adaptations within the class while maintaining core traits. Morphological diversity extends to other orders: Amphitremida includes amoeboid forms like Diplophrys with filose derived from ectoplasmic elements and no known stages; Amphifilida features oblong cells producing filaments; and Oblongichytriales comprises endoparasites of marine algae with zoosporic development, though details remain limited due to understudied taxa.

Ectoplasmic Network and Motility

Labyrinthulomycetes are distinguished by their production of an ectoplasmic network, an extracellular system of filamentous, wall-less extensions of the plasma membrane that facilitates both locomotion and nutrient acquisition. This network is generated by the bothrosome, a specialized, Golgi-derived approximately 200 nm in diameter, located at a cup-shaped of the plasma membrane. The bothrosome serves as the point of origin for the ectoplasmic elements, which branch out to form a slime track or net, allowing cells to interact with their environment without direct contact. In labyrinthulids, such as species of Labyrinthula, the ectoplasmic network forms tubular tracks within which multiple spindle-shaped cells collectively, often in a parasitic manner inside host tissues like seagrasses. These cells maintain equal distances via the network, which surrounds and interconnects colonies, enabling coordinated movement and host penetration. In contrast, thraustochytrids, including genera like Thraustochytrium and Schizochytrium, typically feature a single bothrosome per cell and produce ectoplasmic nets that function more as appressoria-like attachments to solid substrates, aiding saprotrophic nutrient uptake rather than extensive cellular within the net. Labyrinthulids generally possess multiple bothrosomes, reflecting differences in network structure and function between the two groups. Gliding motility in Labyrinthulomycetes occurs along the ectoplasmic filaments at speeds ranging from 5 to 175 μm/min, driven by an actomyosin-like mechanism rather than true . In labyrinthulids, force generation involves (approximately 96 kDa) interacting with F-actin in the trackways via bothrosomes, producing contractile forces exceeding 50 pN to propel cells forward; this process is sensitive to ATPase inhibitors but not to actin-disrupting agents like cytochalasin D. The ectoplasmic network anchors the cells to the substratum through a reticulopodial mesh rich in F-actin, facilitating directed movement without flagella in the vegetative stage. Surface tension along the filaments may contribute to extension in some cases, but the primary is actin-myosin based. The ectoplasmic network also plays a crucial role in nutrient absorption by extending toward organic substrates and secreting hydrolytic enzymes for . These enzymes, including cellulases, chitinases, amylases, lipases, phosphatases, and proteases, break down complex polymers such as , , and proteins into soluble forms that are then absorbed across the wall-less ectoplasm. In thraustochytrids, this appressorial attachment enhances enzymatic access to particulate matter, while in labyrinthulids, the network's penetration into hosts amplifies localized degradation. This osmotrophic strategy underscores the network's dual function in foraging and feeding efficiency.

Reproduction and Development

Reproduction in Labyrinthulomycetes is predominantly asexual, with sexual processes being rare and only recently documented in certain lineages. In the order Thraustochytrida, occurs through the formation of biflagellate zoospores within ; these structures develop from vegetative cells under specific conditions, producing 8 to 32 zoospores per Type I or up to 32 peripheral spores in Type II . Vegetative growth during the proliferating phase involves binary fission, where cells divide mitotically within the mother , often forming tetrads of daughter cells. In contrast, members of the order Labyrinthulida, such as Labyrinthula species, primarily reproduce asexually via binary fission of spindle-shaped vegetative cells or through sorogenesis, in which multiple cells aggregate into dense clusters called sori that subsequently release biflagellate zoospores. Other orders show variations: Amphitremida and Amphifilida lack documented zoospore stages, relying on amoeboid division and formation, while Oblongichytriales produce zoospores as , though life cycles are incompletely known. Sexual reproduction remains poorly understood and is not the dominant mode across the class, though has been observed in the thraustochytrid genus Aurantiochytrium. In A. acetophilum, occurs between paired motile cells, involving cytoplasmic and nuclear fusion to form zygote-like structures, marking the first documented instance of sexual processes in Thraustochytridaceae. Earlier inferences of sexuality in labyrinthulids, such as aggregating plasmodia in Labyrinthula, suggested potential sexual modes, but these lack confirmation of or fusion. Genetic recombination patterns in cultured strains have provided indirect support for occasional sexual exchange, though ultrastructural and molecular details are limited. The life cycle of Labyrinthulomycetes includes a proliferating vegetative phase characterized by binary fission and ectoplasmic network expansion for nutrient uptake, transitioning to reproductive stages under stress. Resting cysts form as survival structures during adverse conditions, featuring thick, stratified walls in thraustochytrids; these cysts excyst upon reintroduction of nutrients like , , and sulfates, resuming vegetative growth. Dispersal occurs primarily via biflagellate zoospores, which swim briefly (0.5–1 hour) before settling and germinating into new colonies, with transformation to vegetative cells observed within 2 days in some parasitic contexts. In labyrinthulids, sori serve a similar dispersive role, releasing zoospores from aggregated cells to initiate new infections or saprophytic growth. Environmental factors strongly influence reproductive transitions, with nutrient availability playing a key role in triggering sporulation and formation. In thraustochytrids, production and release are enhanced in media with specific compositions or under limitation, such as in seawater-based L1 medium at 32. Host stress or low levels similarly induce sorogenesis and sporulation in labyrinthulids, promoting development for dormancy during unfavorable conditions like or shifts. Overall, these triggers ensure adaptation to dynamic marine environments, with development from settlement to established colonies occurring rapidly to capitalize on transient resources.

Ecology and Distribution

Habitats and Distribution

Labyrinthulomycetes are predominantly found in marine environments, where they inhabit coastal sediments, the , and are often associated with , seagrasses, and mangroves. They are ubiquitous across global oceans, occurring from polar to tropical regions, and have been detected in every examined to date. Environmental surveys confirm their presence in diverse marine settings, including estuaries, lagoons, and open seas, with adaptations such as ectoplasmic facilitating their colonization of these substrates. Occurrences in freshwater habitats are rare compared to marine ones, primarily represented by the genus Diplophrys, which includes species isolated from ponds, rivers, and other inland waters. Some isolates have also been reported from soil environments, though these are infrequent, including in terrestrial environments such as turf grasses (e.g., Labyrinthula terrestris). Their abundance is notably high in organic-rich environments, such as eutrophic coastal waters, where surveys indicate concentrations exceeding 10^4 18S rRNA gene copies per liter, with peaks reaching up to 3.68 × 10^5 copies per liter in some areas. In these settings, Labyrinthulomycetes can achieve levels surpassing those of bacterioplankton, underscoring their prevalence in nutrient-enriched marine zones. Labyrinthulomycetes exhibit a pattern, reflecting their broad environmental tolerance. Diversity is highest in temperate and subtropical seas, followed by tropical regions, while polar areas show lower richness but consistent presence.

Ecological Roles and Interactions

Labyrinthulomycetes primarily function as saprotrophic decomposers in marine and coastal ecosystems, where thraustochytrids break down from sources such as leaves, seagrasses, and algal remains, facilitating the recycling of carbon and nutrients through extracellular secretion via their ectoplasmic network. This decomposition process is crucial for remineralizing in sediments and columns, with thraustochytrid often comparable to or exceeding that of during phytoplankton decay, contributing substantially to microbial community dynamics—reaching up to 30% of heterotrophic bacterial in nutrient-rich estuaries like the Pearl River. Their enzymatic activity targets recalcitrant substrates, enhancing nutrient availability for primary producers and influencing benthic oxygen and carbon fluxes. In terms of interactions, labyrinthulids such as Labyrinthula spp. exhibit parasitic behaviors, infecting algae and seagrasses; for instance, Labyrinthula zosterae causes seagrass wasting disease in eelgrass (Zostera marina), leading to tissue degradation via cell wall-lytic enzymes and historical population declines of up to 90% in affected regions. Some species also parasitize marine invertebrates or act as commensals and mutualists with living algae, potentially supporting mixotrophic lifestyles through symbiotic associations. These interactions can alter host microbiomes and contribute to biofilm formation on plant surfaces, influencing microbial ecology in coastal habitats. Trophically, Labyrinthulomycetes occupy a basal position in marine food webs, serving as prey for bacterivores, fungivores, , amoebae, and copepods, thereby transferring energy upward. Their production of omega-3 polyunsaturated fatty acids, particularly (DHA), is ecologically significant, providing an essential nutrient for like copepods that cannot synthesize it efficiently, with direct evidence of consumption in oceanic environments. This role positions them as a key link in supporting higher trophic levels, including fisheries. Environmentally, Labyrinthulomycetes thrive in organic-polluted, waters, where elevated nutrient loads boost their abundance and highlight their potential as indicators of anthropogenic enrichment. By promoting aggregation and decomposition, they enhance in sediments, playing a vital part in global marine carbon cycling and mitigating effects through efficient nutrient remineralization.

Evolution and Phylogeny

Phylogenetic Position

Labyrinthulomycetes form a monophyletic class within the Sagenista of the Bigyra in the s, supported by phylogenomic analyses using hundreds of genes that resolve their position as a cohesive group distinct from other heterotrophic lineages. Within Sagenista, Labyrinthulomycetes are the to Eogyrea, a comprising environmental sequences from marine clades such as MAST-4, MAST-6, MAST-7, MAST-8, MAST-9, and MAST-11, as confirmed by multi-gene phylogenies incorporating 18S rRNA, , and related markers that highlight their shared ancestry while underscoring the basal divergence of Bigyra from photosynthetic gyristean stramenopiles like ochrophytes. This positioning is further reinforced by broader stramenopile trees derived from 247 nuclear genes, which place Sagenista (including Labyrinthulomycetes and Eogyrea) as to , emphasizing the early radiation of heterotrophic lineages within the group. Divergence estimates from calibrated molecular phylogenies indicate that the early lineages, including the ancestor of Bigyra and thus Labyrinthulomycetes, originated approximately 1200–1600 million years ago, during the era following major environmental shifts that likely influenced diversification. This timeline aligns with fossil-calibrated analyses of evolution, positioning the Bigyra crown radiation postdating the initial stramenopile stem but predating the explosion of multicellular life. Molecular evidence for their phylogenetic placement includes shared synapomorphies such as the , a specialized structure for ectoplasmic net production that links Labyrinthulomycetes to ochrophytes through conserved cytoskeletal and secretory mechanisms, though it is absent in . Additionally, desaturase genes involved in polyunsaturated represent a genetic synapomorphy with ochrophytes, supporting their inclusion in a broader chromalveolate ancestry while distinguishing them from oomycete-specific metabolic pathways. These markers, analyzed via multi-locus approaches including and 18S rRNA, underscore the of Labyrinthulomycetes and their from , which lack these shared traits. Metagenomic studies have expanded the known phylogeny by revealing undescribed diversity, including novel lineages of Labyrinthulomycetes in anoxic marine sediments, where 18S rDNA sequences from uncultured protists form deep-branching clades outside described families like Thraustochytriaceae and Labyrinthulaceae. These environmental surveys, often from oxygen-depleted coastal and deep-sea habitats, indicate that the class harbors significantly greater phylogenetic breadth than currently cultured representatives, potentially including anaerobic-adapted forms that predate or parallel the radiation of aerobic stramenopiles. Recent studies as of 2025 have further revealed high phylogeographic diversity and novel clades through environmental sequencing in and estuarine systems, underscoring adaptive .

Evolutionary Origins and Adaptations

Labyrinthulomycetes are believed to have originated from a mixotrophic within the stramenopiles that possessed photosynthetic capabilities, with a secondary loss of plastids leading to their current heterotrophic lifestyle. This evolutionary transition is supported by vestigial features such as eyespot-like structures in the zoospores of labyrinthulids, which resemble the carotenoid-based eyespots found in photosynthetic stramenopiles like ochrophytes, suggesting retention from a plastid-bearing . Additionally, the presence of algal-like desaturase and elongase enzymes enables the synthesis of omega-3 polyunsaturated fatty acids (PUFAs), such as (DHA) and (EPA), which are biochemically linked to chloroplast-derived pathways in ancestral . Key adaptations in Labyrinthulomycetes include the development of an ectoplasmic network, a unique extracytoplasmic structure that facilitates osmotrophic nutrition by secreting hydrolytic enzymes to break down complex organic substrates in nutrient-poor marine environments. This network, produced via bothrosomes in the , initially evolved for substrate attachment and nutrient assimilation in non-gliding ancestors but later enabled ectoplasmic , allowing efficient exploration of detrital surfaces. Within the group, labyrinthulids exhibit a further shift toward parasitic lifestyles, colonizing algal and hosts, which contrasts with the predominantly saprotrophic thraustochytrids and reflects diversification in trophic strategies from free-living phagotrophy. The fossil record of Labyrinthulomycetes lacks direct evidence, as their soft-bodied, unicellular nature precludes preservation; however, their origins are inferred from broader traces in microfossils and molecular clock estimates placing heterokont diversification around 1200–1600 million years ago. Modern analogs, such as organic-walled microfossils from late assemblages, provide indirect support for early stramenopile-like protists in marine settings. Diversification of Labyrinthulomycetes was likely driven by marine anoxic events around 500 million years ago, which favored saprotrophic osmotrophy by increasing organic detritus availability in oxygen-limited sediments. More recent radiations correlate with the expansion of meadows, angiosperm-dominated habitats that provided new niches for parasitic labyrinthulids, enhancing their association with coastal ecosystems.

Genetic and Biochemical Features

Genome and Genetic Code

The nuclear genomes of Labyrinthulomycetes are compact, ranging from approximately 35 to 61 Mb in size, with relatively low densities that contribute to their streamlined structure. For instance, the of Aurantiochytrium limacinum ATCC MYA-1381 is 60.93 Mb and encodes 14,859 protein-coding , while Schizochytrium aggregatum ATCC 28209 has a 40.85 Mb with 10,612 ; these species typically exhibit 1.5 introns per , with median lengths of 345–494 . Recent assemblies, such as the 2023 chromosome-scale of Aurantiochytrium limacinum ATCC MYA-1381, reveal 26 linear chromosomes and a circular element, highlighting subtelomeric organization. The nuclear in Labyrinthulomycetes follows the standard universal code (NCBI translation table 1). Mitochondrial genomes in Labyrinthulomycetes are small, typically 30–40 kb in length, and are generally circular-mapping, encoding a core set of protein-coding genes, rRNAs, and tRNAs without introns. For example, the mitochondrial genome of Schizochytrium sp. TIO1101 is 31,494 bp and contains 33 protein-coding genes, 21 tRNA genes, and two rRNA genes. Unlike many stramenopiles, Labyrinthulomycetes lack plastids, but their nuclear genomes retain relics of red algal origin, reflecting the ancient secondary endosymbiosis that gave rise to plastids in the broader stramenopile lineage. The mitochondrial genetic code deviates from the standard, corresponding to NCBI translation table 23 (Thraustochytrium mitochondrial code), in which the codon TTA (UUA) functions as a stop codon rather than encoding leucine, a variant observed across thraustochytrids and labyrinthulids. Genome sequencing efforts have revealed extensive from , particularly for enzymes involved in specialized metabolic pathways; for example, polyketide synthase-like genes for polyunsaturated fatty acid (PUFA) in thraustochytrids show bacterial origins, enabling efficient DHA production.

Biochemical Pathways and Metabolites

Labyrinthulomycetes, particularly members of the thraustochytrid , exhibit specialized characterized by the accumulation of polyunsaturated fatty acids (PUFAs), including (DHA, 22:6 n-3) and (EPA, 20:5 n-3), which can constitute 30-70% of total in . These organisms primarily synthesize PUFAs through the aerobic front-end desaturase-elongase pathway, involving delta-4 and delta-5 desaturases that introduce double bonds proximal to the carboxyl end of fatty acyl chains, a mechanism akin to chloroplast-associated desaturation in photosynthetic eukaryotes. For instance, in Thraustochytrium sp., the delta-4 desaturase catalyzes the conversion of 22:5 n-3 to DHA, enabling efficient production under heterotrophic conditions. This pathway coexists with a polyketide synthase-like system in some strains, allowing flexible adaptation to availability. In addition to lipid synthesis, Labyrinthulomycetes secrete a suite of extracellular hydrolases that support saprotrophic lifestyles by breaking down complex . These include lipases for hydrolysis, proteases for protein degradation, and glycosidases such as chitinases and cellulases for polysaccharide breakdown. Notably, chitinases facilitate the decomposition of chitin-rich algal cell walls, as observed in parasitic interactions with marine , where activity peaks during host colonization. Such secretions are deployed via the ectoplasmic network, enhancing nutrient acquisition in organic-rich environments. Secondary metabolites in Labyrinthulomycetes encompass (e.g., and ), (e.g., and ), and associated antioxidants that protect cellular from . These compounds are biosynthesized via mevalonate and methylerythritol phosphate pathways, with serving as a key intermediate for production. Certain isolates, such as Thraustochytrium sp., exhibit antimicrobial activity attributed to lipases/phospholipases and carotenoid derivatives, inhibiting bacterial growth like Staphylococcus aureus with inhibition zones up to 78%. From a biotechnological perspective, engineered thraustochytrid strains, such as optimized Schizochytrium limacinum, achieve DHA contents exceeding 50% of total fatty acids within fractions comprising 50-70% of dry , yielding overall DHA levels up to 35% of under controlled . These high-purity omega-3 sources serve as sustainable alternatives to in feeds, supporting larval nutrition and reducing reliance on wild fisheries.

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