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

Small, aloricate ciliates with equivalent spherical cell diameters ~20 km (biovolume <4,000 pm3) 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. The smallest marine plankters are responsible for a large part of total production and respiration in the sea (Sieburth 1979; Pomeroy 1980; Williams 198 1). It appears that most of the marine picoplankton, cells of 0.2-2 pm including heterotrophic bacteria, cyanobacteria, and small eucaryotic phytoplankton (Sieburth et al. 1978; Johnson and Sieburth 1979, 1982), grow rapidly (Azam et al. 1983; Platt et al. 1983) and that the relatively low and constant in situ abundances of these organisms are maintained by predation (Bumey et al. 198 1, 1982; Da’ This work was supported by NSF grants OCE 8219866 and OCE 8 l-10707 and by grants from the Sapelo Island Research Foundation. Contribution 543 of the University of Georgia Marine Institute. vis et al. 1985). Heterotrophic nanoplanktonic organisms, apochlorotic cells of 2-20 pm, have been identified as the dominant consumers of picoplankton, especially of bacteria (Fenchel 1982; Sieburth and Davis 1982; Azam et al. 1983). Marine heterotrophic nanoplanktonic organisms have been considered to consist predominantly of phagotrophic microflagellates (Fenchel 1982; Sieburth and Davis 1982), while marine pelagic ciliates have been generally recognized as important only in the microzooplankton, organisms 20-200 pm in size which graze nanoplanktonic cells (Sorokin 198 1; Beers et al. 1980, 1982; Banse 1982). (Exceptions are Ibanez and Rassoulzadegan 1977; Johnson et al. 1982; Gast 1983, 1985.) Here we present evidence that small, aloricate ciliates with equivalent spherical cell diameters ~20 pm are present in both eutrophic and oligotrophic marine waters and at times make up a large fraction of the total heterotrophic nanoplankton biomass. Indirect evidence also indicates that these ciliates may utilize the same food resources as do heterotrophic microflagellates, i.e. bacteria and other picoplanktonic microorganisms. Ciliates in the nanoplanktonic size range have been reported in many parts of the world ocean, including coastal waters of southern California (Beers et al. 1980), the North Pacific central gyre (Beers et al. 1982), the northern Adriatic Sea (Revelante and Gilmartin 19 8 3), the Chesapeake Bay (Haas