Thraustochytrids are single-celled saprotrophic eukaryotes (decomposers) that are widely distributed in marine ecosystems, and which secrete enzymes including, but not limited to amylases, proteases, phosphatases. They are most abundant in regions with high amounts of detritus and decaying plant material. They play an important ecological role in mangroves, where they aid in nutrient cycling by decomposing decaying matter. Additionally, they contribute significantly to the synthesis of omega-3 polyunsaturated fatty acids (PUFAs): docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA), which are essential fatty acids for the growth and reproduction of crustaceans. Thraustochytrids are members of the class Labyrinthulea, a group of protists that had previously been incorrectly categorized as fungi due to their similar appearance and lifestyle. With the advent of DNA sequencing technology, labyrinthulomycetes were appropriately placed with other stramenopiles and subsequently categorized as a group of Labyrinthulomycetes.
There are several characteristics which are unique to Thraustochytrids, including their cell wall made of extracellular non-cellulosic scales, zoospores with characteristic heterokont flagella, and a bothrosome-produced ectoplasmic net, which is used for extracellular digestion. Thraustochytrids are morphologically variable throughout their life cycle. They have a main vegetative asexual cycle, which can vary depending on the genus. While sexual reproduction has been observed in this group, it remains poorly understood.
Thraustochytrids are of particular biotechnical interest due to their high concentrations of docosahexaenoic acid (DHA), palmitic acid, carotenoids, and sterols, all of which have beneficial effects to human health. Thraustochytrids rely on a plethora of resources such as various sources of organic carbon (vitamins and sugars), and inorganic salts throughout their life cycle. Scientists have devised several potential uses for thraustochytrids stemming around increasing DHA, fatty acids, and squalene concentrations in vivo by either changing the genetic makeup or medium composition/conditioning. There have also been some breakthroughs which have resulted in gene transfers to plant species in order to make isolation of certain oils easier and cost effective. Thraustochytrids are currently cultured for use in fish feed and production of dietary supplements for humans and animals. In addition, scientists are currently researching new methodologies to convert waste water into useful products like squalene, which can then be utilized for the production of biofuel.
Morphology As labyrintulomycetes, thraustochytrids share distinct characteristics with other organisms in this group. These include, but are not limited to: biflagellate zoospores which have an anterior flagellum containing mastigonemes, a bothrosome-produced ectoplasmic net, and multilamellate cell walls with scales derived from Golgi bodies. Thraustochytrids are single-celled protists, characterized with only one sporangium (monocentric), an ectoplasmic net, and a multi-layered, non-cellulosic cell wall made of overlapping circular scales. Despite often being referred to as algae, they do not have a plastid, making them obligate heterotrophs.
At their vegetative state, thraustochytrids measure 4 to 20 μm in diameter and are globose or subglobose in shape. They have a multi-layered cell wall made of sulphated galactose. The singular sporangium of thraustochytrids is typically ovular or spherical in shape, and varies across genus. In the Botryochytrium genus, for example, the shape of the zoosporangium was compared to a grape. Thraustochytrids have biflagellate zoospores with heterokont flagella typical of other Stramenopiles. On the posterior end, the whiplash is short, and on the anterior end, a long tinsel flagellum protrudes.
Ultrastructure Within the granular cytoplasm lies single dictyosomes, centrioles, endoplasmic reticulum, mitochondria, and lipid bodies in some cases. Thraustochytrids contain many mitochondria, which are polymorphic and have tubular cristae. Made of sulphated polysaccharides, the cell wall of thraustochytrids are multilamellate and non-cellulosic. The cell wall is derived from the dictyosome cisternae during thallus development, where circular scales (vesicles) form on the basal membrane to merge. In thraustochytrids, the cell wall is rich in galactose and xylose.
Characteristic of thraustochytrids is their ectoplasmic net—which is an extension of the plasma membrane —emerging from the bothrosome (also known as the sagenogenetosome, or SAG). The cytoplasmic net is unilateral, motile, and resembles fine fibres when viewed under a scanning electron micrograph. Depending on the genus, they may be branched or unbranched, and are thought to originate from a single trunk or organelle. Ectoplasmic nets have the capacity to excrete hydrolytic enzymes (cellulases, amylases, lipases, phosphatases, and/or proteases) to digest organic material in the water, thus assuming the role of decomposition. In lab settings, the endoplasmic net of thraustochytrids has been shown the ability to penetrate the sporopollenin of pine pollen, which comprises a polymer that is highly resistant to microbial degradation. This experimental process is called pollen-baiting. Beyond decomposition, ectoplasmic nets also participate in providing adhesive function, as well as assimilation of digested organic material (absorption).
Life cycle The life cycle of thraustochytrids is generally complicated, differing from genus to genus, and typically consisting of multiple stages of cell types such as zoosporangia, multinucleated cells, mononucleated cells, and amoeboid cells.
Asexual reproduction All thraustochytrids undergo a main vegetative life cycle, beginning as a mononucleated cell that undergoes nuclear division to become multinucleated and maturing into a sporangia which release zoospores to begin the cycle again. Branching off of the main vegetative life cycle, additional paths can be taken based on the strain. Traustochytrids undergo cell division in two main ways: through a zoosporangium or through successive bipartition. These methods can occur in the same species and at different stages of the lifecycle. When cell division occurs through the formation of a zoosporangium, the nuclei divide within a single cell to create a multinucleate cell which becomes a zoosporangium following progressive cleavage. When cells divide through successive bipartition, the cell divides immediately after nuclear division, either by invagination of the plasma membrane or fusion with internal vesicular membranes. Thraustochytrids undergo open mitosis, meaning that the nuclear membrane breaks down during cell division, and then reforms following nuclear division.
Amoeboid loop Certain strains of thraustochytrid are able to enter an amoeboid loop from multiple vegetative life cycle stages, gaining an advantage of being able to move slowly across surfaces as either mononucleated or multinucleated amoeboid cells. Ulkenia, Schizochytrium, Hondaea, and Aurantiochytrium can undergo binary division to form a cluster of mononucleated cells which can then turn into amoeboid cells and enter an amoeboid loop. The amoeboid loop can also be entered from mononucleated cells directly turning into mononucleated amoeboid cells or multinucleated cells and sporangia directly turning into multinucleated amoeboid cells. Strains in the amoeboid loop eventually have to re-enter the main vegetative life cycle in order to produce zoospores.
Sexual reproduction Although the details of sexual reproduction are poorly understood, vegetative cells are thought to be diploid and undergo meiosis to form a sporangium, which releases gametes. While syngamy has been observed in Aurantiochytrium acetophilum, the fate of the zygote is relatively unknown, however, it is suspected that they enter the vegetative cycle as a mononucleated cell.
Taxonomy Thraustochytrids were first reported by F.K. Sparrow in 1934. Like other Labyrinthulomycetes, they were classified as fungi due to their ectoplasmic nets and ability to produce zoospores. However, the morphological plasticity of thraustochytrids prevents them from being accurately classified based on their appearance. It was not until 1973 that Sparrow reclassified them as oomycetes, indicating that they were stramenopiles and not fungi. In the years that followed, scientists began to perform concurrent morphological and molecular genetics studies to further explore the placement of thraustochytrids. Using ribosomal RNA as a phylogenetic marker, Cavalier-Smith et al. provided strong molecular evidence that indicated thraustochytrids were not closely related to fungi or oomycetes. Other studies supported these findings by highlighting morphological similarities between thraustochytrids and other labyrinthulomycetes. While the phylogeny of thraustochytrids is still relatively unresolved, they have been clearly defined taxonomically. Thraustochytrida is one of two orders in the class Labyrinthulea and the nomenclature in this group is highly variable due to its history of being considered fungi.
Ecology
Distribution Thraustochytrids have been found in various habitats such as tropical coasts in the Indian Ocean, Pacific Ocean, and the Northern Arabian sea; temperate and cold waters in Australia, Argentina, the Mediterranean Sea, and the North sea; and subantarctic, antarctic, and subarctic waters. Overall, thraustochytrids are widespread in marine waters. They can be found all the way through the photic, euphotic, and aphotic zones. The ideal salinity range for this protist is ~20‰-30‰; however they are euryhaline and can survive at a salinities as low as 12‰. Since they require a specific concentration of salt to survive, they are categorized as halophilic protists.
Living conditions Additionally, they require sodium to live and this cannot be substituted by potassium. They are found at a higher frequency in systems that have large amounts of detritus along with decaying plant material. Areas of note include mangroves, salt marshes, and river output zones. Thraustochytrids gain a significant amount of nutrients for growth from any form of decaying organic matter and, as a result, can thrive in areas with elevated pollution or rich in organic material. They can be found on materials either indigenous (autochthonous) or that has been transported there (allochthonous). They are not commonly found on living organisms, and if they are, it is sporadic and in low concentrations. The reasoning for this is suspected to be due to plants being able to release antimicrobial compounds to prevent them from being colonized by microorganisms. In the early stages of decomposition, there are low observed numbers of thraustochytrids as there are still materials that inhibit growth on the organism.
As decomposition progresses, thraustochytrids rapidly populate the substrate. In studies involving mangroves, the thraustochytrids on the leaves would produce the enzymes cellulase, amylase, xylanase, proteases, and pectinases, which suggest that they can play a role in the chemical processes taking place. There have been cases of thraustochytrids being cultured from algal surfaces but only in low numbers. Notably, in a case of culturing thraustochytrids on the brown alga Fucus serratus, they were found to be in low numbers potentially due to inhibitory material being secreted by the alga. A 1992 experiment found that thraustochytrids could not be cultured on the green algae Ulva fasciata and Valoniopsis pachynema. These two algae contain high amounts of phenolic compounds, which is believed to be the reason.
Parasitism