Diversity and seasonal dynamics of marine haptophytes and their viruses and the role of biotic interactions

Sammendrag

Members of the phytoplankton division Haptophyta are a major component in marine pico- and nanoplankton communities. They often occur in bloom concentrations that may have a major impact on the global carbon balance, on climate forcing and sometimes cause fish-kills. Because haptophytes are tiny and difficult to identify in the microscope little is known about their seasonal dynamics and ecological role at the species level. Virus may terminate haptophyte blooms and, according to the “killing the winner” concept, possibly prevent blooms by keeping them under constant control. The aim of this study within the HAPTODIV project was to reveal the diversity and dynamics of marine haptophytes to species level and how this relates to abiotic factors and viruses. We assessed the haptophyte and Phycodnaviridae/ Mimiviridae (algal viruses) taxonomic composition, OTU richness and relative abundance over time by high-throughput amplicon sequencing (metabarcoding) of monthly samples over 2 years from the outer Oslofjorden, Skagerrak, Southern Norway. Metabarcoding revealed higher haptophyte diversity (156 OTUs estimating species) than previously observed in the area with electron and light microscopy (85 spp), and most (84%) of this diversity remains to be cultured and sequenced. Many may represent novel lineages at the class to genus level, or species new for science. More than half of the OTUs (93) were affiliated with the order Prymnesiales such as members of e.g. Chrysochromulina and Prymnesium. Haptophyte richness and community composition showed high temporal variation and significant yearly periodicity correlating with light and temperature, with highest richness in autumn (Sep-Nov) and lowest in the spring (Apr-May). Our results indicated, however, that the abiotic factors only accounted for about 30% of the variation, and suggest that biotic factors explain some of the residual variation. Also algal virus diversity was high and showed temporal variations with highest richness in Sept-Oct. Biotic interactions between haptophytes and algal viruses were assessed by co-occurrence analyses. We detected 56 haptophyte-virus pairs with significant co-occurrence, of 49 000 possible pairs. The distribution patterns of these pairs may shed light on the role of virus infection on phytoplankton community composition and dynamics. We suggest that this approach may be a first step to test the “killing-the-winner” concept on haptophyte diversity and distribution.