Small, aloricate ciliates as a major component of the marine heterotrophic nanoplankton1

Abstract

Small, aloricate ciliates with equivalent spherical cell diameters <20 µ m (biovolume <4,000 µ m 3 ) compose, on average, 4–57% of the total biomass of heterotrophic (apochlorotic) nanoplankton in diverse marine systems. Biomass production of nanoplanktonic ciliates in a southeastern U.S. estuary was also a significant part of the total production of heterotrophic microprotozoa. During summer in a salt marsh tidal creek, the production of small ciliates exceeded the production of heterotrophic microflagellates. Bacteria and coccoid cyanobacteria were frequently observed in the food vacuoles of the ciliates, while ingested nanoplanktonic algae were rarely seen. We suggest that small, aloricate ciliates can be an important component of the biomass of heterotrophic nanoplankton and deserve further attention as potential consumers of picoplanktonic cells in marine pelagic systems.