GRAZING BY MARINE NANOFLAGELLATES 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 bacteria when the bacteria and viruses were present at natural concentrations. Different viruses were ingested at different rates with the smallest virus being ingested at the slowest rate. Further, we found differences in digestion times for the same flagellates grazing on different viruses and for different flagellate assemblages grazing on the same viruses. FMs of 50 nm diameter were used as a control for egestion of undigested 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. Discrimination against 50 nm FMs in favor of FLVs was also observed. Our calculations suggest that viruses may be of nutritional significance for phagotrophic flagellates. When there are 10(6) bacteria ml-1 and 10(7) to 10(8) Viruses ml-1, viruses may represent 0.2 to 9 % of the carbon, 0.3 to 14 % of the nitrogen and 0.6 to 28 % 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 viruses, 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 organic particles in the colloidal size range.