Acknowledging and incorporating mixed nutrition into aquatic protistan ecology, finally

We often instinctively think of life on our planet being composed of species capable of obtaining the organic matter and energy they need for growth from inorganic compounds (autotrophy) or species that utilize preformed organic matter produced by autotrophs to meet their energy and carbon needs (heterotrophy). This is particularly true for terrestrial, macroscopic life forms, where there exists an obvious dichotomy between most phototrophs (plants) and heterotrophs (animals, fungi). That perspective was also extrapolated to single-celled eukaryotic organisms (the protists) many years ago as one way of organizing the enormous diversity that exists among those species (Whittaker, 1969). That description was somewhat justified at the time by the fact that many protists exhibit either phototrophic nutrition (microalgae,) or heterotrophic nutrition (protozoa), and as a practical way to organize the enormous diversity of species within the kingdom Protista. However, recent phylogenies that have been proposed for the domain Eukarya (Burki, 2014) recognize that photosynthetic and heterotrophic ability are not always phylogenetically informative, and often not mutually exclusive behaviors. Many, perhaps even a dominant proportion of protists, it turns out, exhibit some combination of these nutritional modes (an ability generally referred to as mixotrophy). Mixotrophic protists occur throughout the eukaryotic tree of life although, functionally, most of these species can be grouped into one of three general behavioral categories (Mitra et al., 2016; Stoecker et al., 2016). Firstly, phagotrophic phytoflagellates are species that possess chloroplasts but also engulf and digest small prey such as bacteria and cyanobacteria by phagocytosis. We have long been aware that species within several algal classes exhibit this behavior (Porter, 1988). Secondly, kleptoplastidic protists are heterotrophic species that feed on algae, partially digest them, but retain their chloroplasts in a functional state (and occasionally other organelles from their prey). These species (e.g. many dinoflagellates and ciliates) possess ‘acquired phototrophy’ as a consequence of chloroplast retention. They can be generalists, consuming and acquiring chloroplasts from a variety of algal prey, or highly specialized in their preferred prey. Finally, numerous physicallyintimate, often-mutualistic associations exist between heterotrophic protistan species and intact photosynthetic algae or cyanobacteria that are generally now included under the broad definition of mixotrophy. These associations constitute efficient and productive relationships in which feeding and nutrient remineralization by the heterotrophic host support photosynthesis and growth of the symbionts which in turn contribute to host nutrition. These associations (holobionts) have been described for well over a century for the larger Rhizaria (e.g. foraminifera and radiolaria) (Haeckel, 1887) but they also exist among other protistan phyla such as the ciliates, including the ciliate-symbiont model system of ParameciumChlorella (Brown and Nielson, 1974). Given that mixotrophic nutrition and the organisms that conduct it have been known for quite some time, why is this subject only now receiving attention by a broad audience of aquatic microbiologists? One reason is that only recently have the three general categories of mixotrophy noted above been formally defined (Mitra et al., 2016). Additionally, new information now appearing in the literature indicates that we may have grossly underestimated the collective abundances of these species in aquatic ecosystems, and therefore poorly characterized their impacts on food web structure and function in the plankton. Evidence over the last few years has indicated substantial abundances of phagotrophic phytoflagellates in the ocean (Unrein et al., 2014). The ingestion of prey by these algae may provide them with a mechanism for obtaining vital nutrients for photosynthesis, relative to algae that do not possess phagotrophic ability. If so, mixotrophy bestows an ecological advantage in low-nutrient environments, and implicates those algae as a significant source of bacterial mortality in the plankton (overlapping with the ecological role attributed to small heterotrophic protists). Mixotrophic algae are also widespread in many freshwater ecosystems, although a clear generality across all freshwater ecosystems is not yet possible because of the vast number of *For correspondence. E-mail dcaron@usc.edu.