Mixotrophy among freshwater and marine protists

Freshwater and marine protists are often divided into phytoplankton and protozooplankton. However this dichotomy is fundamentally misleading as many protists combine photosynthesis and food uptake; they are mixotrophs. We now realize that many phytoflagellates are important grazers on bacteria, other protists and in some cases even metazoans. Food can serve as a source of carbon, nitrogen, and/or phosphorous during periods where inorganic nutrients are sparse for phytoflagellates. Many microzooplankton grazers, including dinoflagellates, ciliates, foraminiferans and radiolarians are mixotrophic owing to their retention of functional algal organelles or maintenance of algal endosymbionts. Mixotrophs are widespread in marine ecosystems. They can be found in warm, temperate, and cold seas and with stratification, fronts, and even upwelling zones. Modelling has indicated that mixotrophy has a profound impact on marine planktonic ecosystems and enhances primary production, biomass transfer to higher trophic levels, and the functioning of the biological carbon pump. Nevertheless, our understanding of this key trophic strategy in aquatic protists needs to be improved, in particular with the help of emerging molecular and microscopy techniques