Diversity, seasonal dynamics and biotic interactions in marine phytoplankton and virus communities

The taxonomic composition of phytoplankton communities and relative abundance of taxa in oceanic ecosystems undergo continuous change. This community reorganization is of major importance to higher trophic levels and biogeochemical cycles. This succession is partly accounted for by hydrographical and meteorological variations that select for different traits in time and space, but also by biological processes, such as pathogens. Virus may terminate haptophyte blooms and, according to the “killing the winner” concept, possibly prevent blooms by keeping them under constant control. We assessed the haptophyte (algal host) and Phycodnaviridae/Mimiviridae (virus) taxonomic composition and relative abundance by high-throughput amplicon sequencing of monthly samples over 2 years from the outer Oslofjorden. Haptophyte richness and community composition exhibited high temporal variation and significant yearly periodicity, with highest richness in autumn (Sept-Oct) and lowest in the spring (April-May). Also, algal virus diversity showed seasonal variations with highest richness in September-October and lowest in August. Biotic interactions between haptophytes and viruses and among haptophytes were evaluated to provide new insight in the role of biotic processes for phytoplankton community composition and dynamics.