Tetraselmis

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Introduction and Description 

Tetraselmis is a green alga genus of eukaryotic marine and freshwater phytoplankton. Named for its four flagella arranged in two opposite pairs (“tetra” = four, “selmis” = an angler’s noose made of hair), this motile algae is cordiform (heart-shaped) with an invagination located where flagella emerge from the cell. It is covered in a thick wall composed of a series of fused plates made of carbohydrates, termed a “theca.” Tetraselmis usually possesses a single cup-shaped chloroplast enclosing a pyrenoid, or special sub-organelle used in the carbon concentration mechanism (CCM). The pyrenoid is surrounded by a wall of starch granules and mostly made up of the enzyme RuBisCO, which accepts carbon dioxide sourced from the atmosphere and transforms it into the first chemical step in the pathway used by algae to create the complex sugars they need for respiration. Thylakoids, or the structures inside chloroplasts that perform photosynthesis, weave between the pyrenoids of Tetraselmis to facilitate the transport of intermediate molecules. The position and shape of the pyrenoid within the chloroplast is thought to be taxonomically significant in Tetraselmis. 

Tetraselmis chuii under scanning electron microscopy (Credit: Steve Gschmeissner / Science Photo Library)

Evolution 

Tetraselmis species are chlorophytes, which are thought to have evolved during the Neoproterozoic era, one billion to 540 million years ago. During that period, unicellular eukaryotic algae known as prasinophytes diversified in the ocean and then radiated out into the lakes, rivers, estuaries, and near-water terrestrial habitats that chlorophytes inhabit to this day. The evolutionary position of Chlorodendrophyceae, the class to which Tetraselmis belongs, is still in flux and much debate exists regarding both it and the four genera it contains’ true place on the tree of life. 

Evolution of the Chlorophyta (Credit: Frederik Leliaert) 

Habitats, ecology and life history 

Tetraselmis has a particularly complex life cycle for chlorophytes, composed of three stages: flagellated, vegetative, and cyst. Reproduction only takes place in the vegetative phase, where cells divide via binary fission. Depending on the reproductive environment, daughter cells can be either flagellated or vegetative, the latter of which is most common. Cyst phases occur in harsh environments, when cells develop multiple cell walls and “hibernate” until conditions are more favorable. Because of their diverse life history, Tetraselmis are found in a wide range of environments, including terrestrial ones near sources of water. They can tolerate a broad spectrum of salinities and have even been found in hypersaline environments like salt marshes. 

Industrial and Commercial Uses 

Tetraselmis has been a staple genus for aquaculture and biotechnology for years due to its rapid growth rate and resilience. For the aquarium hobbyist industry, Tetraselmis has been used for years as a feedstock for brine shrimp and other invertebrates. It has a high lipid content, containing high levels EPA and DHA, and is rich in amino acids like alanine that can promote high growth rates. Its carbohydrate stores easily meet the metabolic demands of its predators, and it is a good source of several essential vitamins. For the biotech industry, the natural products (or chemicals secreted as byproducts of a cell’s natural activity) of Tetraselmis are of particular interest. It has been theorized as an extremely useful biofuel source (because of its high lipid content). Several species are capable of producing antibiotic compounds, which aquarium hobbyists may also benefit from when attempting to combat infections in their tanks. Because it is a photosynthetic organism, the photopigments it synthesizes (xanthophyll carotenoids) can be used in the industrial production of dyes and colorings. 

Elemental and biochemical composition of Tetraselmis sp. (Credit: Figure 1 of DOI 10.1039/C4RA11662C)

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References 

1. Arora, Mani. (2016). Tetraselmis: an Introduction. The Botanica. 66. 155-175. 
2. Fang, L., Leliaert, F., Zhang, Z.-H., Penny, D., & Zhong, B.-J. (2017). Evolution of the Chlorophyta: Insights from chloroplast phylogenomic analyses. Journal of Systematics and Evolution, 55(4), 322–332. https://doi.org/10.1111/jse.12248
3. https://www.algaebase.org/search/genus/detail/?genus_id=44099
4. https://www.algaebarn.com/blog/phytoplankton/tetraselmis-the-big-and-fatty-alga/

Key Words 

Flagella/flagellum, motile, cordiform, theca, pyrenoid, chlorplast, thylakoid, taxonomic, cyst, life history, hypersaline, carotenoid