MIXOTROPHIC PROTISTS IN MARINE AND FRESH-WATER ECOSYSTEMS

Some protists from both marine and freshwater environments function at more than one trophic level by combining photosynthesis and particle ingestion. Photosynthetic algae from several taxa (most commonly chrysomonads and dinoflagellates) have been reported to ingest living prey or nonliving particles, presumably obtaining part of their carbon and/or nutrients from phagocytosis. Conversely, some ciliates and sarcodines sequester chloroplasts after ingestion of algal prey. Plastid retention or "chloroplast symbiosis" by protists was first demonstrated <20 years ago in a benthic foraminiferan. Although chloroplasts do not divide within these mixotrophic protists, they continue to function photosynthetically and may contribute to nutrition. Sarcodines and ciliates that harbor endosymbiotic algae could be considered mixotrophic but are not covered in detail here. The role of mixotrophy in the growth of protists and the impact of their grazing on prey populations have received increasing attention. Mixotrophic protists vary in their photosynthetic and ingestion capabilities, and thus, in the relative contribution of photosynthesis and phagotrophy to their nutrition. Abundant in both marine and freshwaters, they are potentially important predators of algae and bacteria in some systems. Mixotrophy may make a stronger link between the microbial and classic planktonic food webs by increasing trophic efficiency.