Thraustochytrid

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.

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