Acquired phototrophy in aquatic protists

Abstract

Acquisition of phototrophy is widely distributed in the eukaryotic tree of life and can involve algal endosymbiosis or plastid retention from green or red origins. Species with acquired phototrophy are important components of diversity in aquatic ecosystems, but there are major differences in host and algal taxa involved and in niches of protists with acquired phototrophy in marine and freshwater ecosystems. Organisms that carry out acquired phototrophy are usually mixotrophs, but the degree to which they depend on phototrophy is variable. Evidence suggests that ‘excess carbon’ provided by acquired phototrophy has been important in supporting major evolutionary innovations that are crucial to the current ecological roles of these protists in aquatic ecosystems. Acquired phototrophy occurs primarily among radiolaria, foraminifera, ciliates and dinoflagellates, but is most ecologically important among the first three. Acquired phototrophy in foraminifera and radiolaria is crucial to their contributions to carbonate, silicate, strontium, and carbon flux in subtropical and tropical oceans. Planktonic ciliates with algal kleptoplastids are important in marine and fresh waters, whereas ciliates with green algal endosymbionts are mostly important in freshwaters. The phototrophic ciliate Myrionecta rubra can be a major primary producer in coastal ecosystems. Our knowledge of how acquired phototrophy influences trophic dynamics and biogeochemical cycles is rudimentary; we need to go beyond traditional concepts of ‘plant’ and ‘animal’ functions to progress in our understanding of aquatic microbial ecology. This is a rich area for exploration using a combination of classical and molecular techniques, laboratory and field research, and physiological and ecosystem modeling.