Protists and microbial food webs: hybrid fuels for ecosystems
Protists are single cell eukaryotes (Eukarya) and include species that use sunlight (phototrophs) or organic carbon as an energy source (heterotrophs), or a mixture of the two (mixotrophs). Arctic waters contain a rich diversity of these organisms, including certain taxa that are highly adapted to low temperatures and that may be endemic to the region (Lovejoy et al. 2007). The phototrophs (including the phytoplankton in lakes, large rivers and seas) provide the biofuels at the base of aquatic food webs, while many of the heterotrophic forms act as intermediates in the microbial food web that allow some of the microbial carbon to flow to zooplankton and fish. Small colourless flagellates (heterotrophic nanoflagellates) and sometimes ciliates appear to be the primary grazers on the picoplankton, the smallest celled plankton (bacteria, cyanobacteria and picoeukaryotes).
Left: Flagellates in a microbial food web sample from Char Lake in the Canadian High Arctic. Top right:
Major components of the aquatic microbial food web, which also includes viruses (not shown). Allochthonous carbon refers to the organic matter that is derived from catchment soils and terrestrial vegetation. Bottom Right:
Diatoms from a subarctic lake. Diatoms include bottom-dwelling (benthic) species as well as taxa that live in the plankton (phytoplankton), and are a major group of phototrophic protists found in most aquatic environments. Their cell walls are composed of silica that provide excellent fossil records in lake and marine sediments and that are valuable indicators of past environmental conditions.
