Effect of viruses on marine stramenopile (MAST) communities in an oligotrophic coastal marine system

Viruses are the most abundant biological entities in aquatic systems, infecting from bacteria to mammals. However, there has been little study so far on their impact on marine heterotrophic flagellates (HFs). For this reason, four experiments were carried out between April 2006 and February 2007 in the Blanes Bay Microbial Observatory (NW Mediterranean coast). We investigated whether viruses could affect HF communities, and specifically two uncultured marine stramenopile groups (MAST-4 and MAST-1C). For each experiment, four microcosms containing 12 L of 5-mm filtered seawater were prepared, two received active viruses and the other two received heat-inactivated viruses. Microcosms were then incubated for 48 h in order to measure changes in the abundance of the target groups. In three of the four experiments, both the growth rates of HF and MAST-4 cells and the percentage of MAST-4 cells with respect to HF after 48 h were higher in the heat-inactivated treatment compared with the active viruses treatment. These results indicate that viruses can negatively affect the HF community either directly via lysis of protists or indirectly via lysis of bacteria, and highlight the interactions between, virus, bacteria and protists.