Roles of eukaryotic protists in the biosphere

Analysis of the various constituents of biotopes and their interactions shows that microbes, although invisible, play essential roles in the functioning of ecosystems. However, these studies place a great deal of emphasis on bacteria, and eukaryotic microorganisms are often widely overlooked. This trend seems to be increasing so much that in some recent books dealing with microbial diversity and their role in ecosystems, fungi, algae or protozoa are not even mentioned! However, eukaryotic microbes are present in almost all ecosystems, play crucial roles and profoundly modify the biosphere through their collective actions.

Eukaryotes are adapted to survive in many environments. In addition, their ability to encyst or form spores often allows them to resist very unfavorable conditions and to wait several years for the return of favorable conditions. For example, certain mesophilic fungi, that is to say, growing between 25°C and 35°C, produce spores capable of withstanding more than two hours at 100°C; in some species, they can even withstand 115°C! These fungi will be able to germinate, for example, after an episode of forest fires and proliferate on burnt plant debris. Nevertheless, eukaryotes are generally less resistant to heat than eubacteria and especially archaea. The maximum growth temperature is for the most thermophilic species around 60°C. Hyper-thermophilic media will therefore be devoid of eukaryotes.

Eukaryotes are found in all ecological niches, including underwater hot springs, where certain animals and protists are present as soon as the temperature drops below the fateful 60°C. They feed with the help of endosymbiotic bacteria or by consuming organic material debris. Their presence has also been detected in very hostile environments such as the “Rio Tinto” river of fire in Spain. This river has a pH equal to 2 and contains up to 20 g/l of iron. It is not human pollution that is responsible for these extreme conditions but the geology of the region; the Rio Tinto has therefore had these characteristics for thousands of years. The majority of the biomass present in this biotope, approximately 60% of the total biomass, is composed of eukaryotic algae, with in particular a diversity of Chlorophyta and Rhodophyta algae. Amplifications of ribosomal DNA extracted directly from biomass collected in the river also show the presence of non-photosynthetic eukaryotes. Almost all major groups of eukaryotes have been detected there. The invasion of this environment is probably recent, as comparisons show that some species are genetically close to species living in more welcoming environments. Eukaryotes are very present in the deep soils of continents and ocean floors, where their proliferation seems to be limited only by temperature. Our knowledge of these deep-sea eukaryotes is very patchy compared to that of prokaryotes. Other eukaryotes, such as fungi, are able to grow at temperatures below 0°C if water remains in liquid form, which occurs if it is loaded with organic matter, and thus can colonize arctic and Antarctic soils. Some may use environments poor in water such as desert soils, or rich in salts: fungi live in the Dead Sea!

Nevertheless, the major biotopes where eukaryotic protists are found are the classical mesophilic biotopes: marine and freshwater environments, soils or the interior of other organisms, animals, plants and other protists, where they live in parasitic, mutualistic or commensal relationships.

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