18 Green Algae III

Trebouxiophyceans

Trebouxiophyceans occur most commonly in freshwater or terrestrial habitats. They include some familiar and widespread genera that are biogeochemically, technologically, or biotically significant. For example, Botryococcus is a common member of the freshwater phytoplankton whose ability to produce large amounts of lipid is unusual among the green algae. The lipid functions as a flotation device, which allows these relatively large colonies to remain suspended in well-illuminated surface waters. Fossil remains similar to modern Botryococcus indicate that this alga has been the source of significant fossil fuel deposits for hundreds of millions of years. Today, Botryococcus is being utilized in biotechnological efforts to produce renewable oil supplies.

Several genera of trebouxiophyceans function as photosynthetic symbionts in lichens. Trebouxia, for which the class is named, is the single-most-common algal component of lichens. Even so, Trebouxia occurs in many genotypes that lichen fungi readily trade, possibly as a way of adapting to changing conditions. In contrast to most trebouxiophytes, the genus Prototheca is colorless and obligately heterotrophic. It lives in soil, where it survives by absorbing organic compounds, but it can also infect dairy cattle, causing an incurable disease. Humans sometimes suffer Prototheca infections of the skin, which may require surgical treatment. Prototheca is related to what may be the strangest of all the algae, the parasitic genus Helicosporidium. This alga disperses as tough, drum-shaped spores that open only after entering the guts of insects and other invertebrate animals. In this location, Helicosporidium unfurls a barbed needle that stabs into gut cells, infecting the animal with parasitic algal cells that ultimately kill the animals. These examples illustrate that trebouxiophyceans can be more exciting than you might at first expect. This chapter surveys the general features of Trebouxiophyceae and provides examples of diversity.