Mojib, N.; Irigoien, X.

The size structure of phytoplankton communities plays a key role in the fate of carbon fixed by photosynthesis. Whether phytoplankton cells sink, enter the microbial loop, or are consumed by larger organisms is generally determined by their size. Grazing has been advanced as a factor determining size structure, but sources of mortality other than grazing, such as viruses also are recognized to be important. Based on the observation that cell size and genome size are related in phytoplankton, we hypothesize that viruses can also play a role in shaping the size structure of the phytoplankton community. Because cell size is related to genome size, we suggest that phytoplankton species with larger genomes will have a more developed immune system to defend against viral infection. As a first step to test this hypothesis, we screened the published transcriptomes of 125 phytoplankton species for expressed viral and immune-response related genes. We found a significant negative correlation between host-cell size and viral-gene diversity, and a positive correlation between host-cell size and the number of immune-response related genes. Our hypothesis supported by preliminary findings opens new pathways to explore whether we should consider viruses as an additional evolutionary driver for larger phytoplankton size, along with grazing and nutrients.