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Microbial eukaryotes

Microbial eukaryotes are an evolutionary diverse group that gathers various eukaryotic organisms of often microscopic sizes, such as protist and fungi.

Protists, comprise a large variety of protozoa and micro-algae, like Amoebozoa (amoebae and slime molds), Excavates (trypanosomes, amitochondriate parasites), Archaeplastids (green algae and plants, red algae), Stramenopiles (brown algae and kelps), Alveolates (ciliates, apicomplexa, dinoflagellates), Rhizaria (foraminifera, radiolaria, cercozoans) and several smaller groups. They make up most of the diversity of eukaryotes (Walker et al. 2011; Adl et al. 2019). Most protists are unicellular, but some form multicellular colonies and the largest photosynthetic organisms in the Arctic are multicellular protists – algae. Protists typically range in size from 0.2 to 200 μm (some are larger) and are usually classified into the pico- (<2 μm), nano- (2–20 μm) and micro-sized fractions (20–200 μm). They are referred to as “pico-, nano- and micro- phytoplankton” or “-zooplankton”, according to whether the groups under study are photosynthetic or heterotrophic. 

Abundance and diversity of microbial eukaryotes in polar regions

Protists are found throughout the Arctic, in soils, lakes, streams, the marine water column, and in ice-associated (sympagic) communities; but they have mostly been described from marine environments, in the upper water column in coastal and oceanic areas, or in ice-associated communities (Poulin et al. 2011). They are important as primary producers, and as consumers of prokaryotes such as cyanobacteria. They are key to the “microbial loop”, the set of trophic interactions that operate below the level of classical food chains, from dissolved organic carbon through several size classes of unicellular organisms that are eaten by invertebrates (Azam et al. 1983; Fenchel 1988). Protists are very significant in the global carbon cycle: while marine photosynthetic protists (phytoplankton) make up less than 1% of photosynthetic biomass on the planet, together with cyanobacteria, they contribute ca. 45% of net annual primary production (Field et al. 1988; Falkowski et al. 2004). Arctic marine protists in the water column contribute about 98% of primary production in coastal regions, while in the mid-Arctic Ocean about 57% of primary production is in ice-associated protist communities (Gosselin et al. 1997).

Full diversity is yet to be discovered

The overall picture of ecological diversity of protists – the number and types of roles and functions in different habitats – is currently changing, due to major changes and developments in methods of detecting diversity, from light-microscopy-based to sequencing-based approaches. It is now clear that ecological studies of protists in the Arctic have only begun to scratch the surface (Poulin et al. 2011; Archambault et al. 2010), even though they make up a large proportion of the diversity in Arctic food chains. As the Arctic climate changes rapidly in the next few decades, changes in the communities of protists may have significant effects on food webs generally.  

Diatoms under the microscope.
Photo: Lise Øvreås
The phenomenon of watermelon snow caused by Chlamydomonas nivalis, due to their secondary red pigment astaxanthinl.
C. nivalis seen under the microscope.
Photos: Lise Øvreås
Green algae growing in a polluted creek in Bjørndalen, Svalbard.
Photo: Simen Salomonsen Hjelle

The Microbial Loop

Video: Ocean Portal

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