Grazing by marine nanofiagellates on viruses and virus-sized particles: ingestion and digestion

We examined grazing of marine viruses and bacteria by natural assemblages and cultures of phagotrophic nanoflagellates. Ingestion rates were determined using fluorescently labelled viruses (FLVs) and bacteria (FLB), and 50 or 500 nm diameter fluorescent microspheres (FMs). Calculated clearance rates of viruses by natural nanoflagellate assemblages were about 4 % of those for bacter~a when the bacteria and viruses were present at natural concentrations. Different viruses were ingested at different rates w ~t h the smallest virus being ingested at the slowest rate. Further, we found differences in digestion times for the same flagellates grazing on different vlruses and for different flagellate assemblages grazing on the same viruses. FMs of 50 nm diameter were used as a control for egestion of undlgested particles. As rates of digestion were greater than those for ingestion both processes would occur simultaneously; hence, our estimates of grazing rate are likely conservative. Ingestion rates were positively correlated with the concentration of 50 nm FMs. Discriminat~on agalnst 50 nm FMs in favor of FLVs was also observed. Our calculations suggest that vlruses may be of nutritional significance for phagotrophic flagellates. When there are lob bacteria ml-l and 10' to 10' vvlruses ml-l, viruses may represent 0.2 to 9 % of the carbon, 0.3 to 14 ' % of the nitrogen and 0.6 to 28 "/a of the phosphorus that the flagellates obtain from ingestion of bacteria. This study demonstrates that both natural assemblages and cultures of phagotrophic nanoflagellates consume and digest a variety of marine vlruses, thereby deriving nutritional benefit and serving as a natural sink for marine viral particles. In addition, these results imply that some nanoflagellates are likely capable of consuming a wide spectrum of organlc particles in the colloidal size range.