Seasonality of marine virus communities

Background/Question/Methods

Viruses that infect cyanobacteria (cyanophages) are important components of marine food webs because they act as a top-down control on the major primary producers in the oceans. Despite this important ecological role, little is known about the dynamics and distribution of phage diversity in natural environments. To investigate the diversity of cyanophage communities over time, we isolated viruses using a host strain of Synechococcus (WH7803) every month for 28 months from several coastal locations in southern California. We then genetically characterized over 2000 of these isolates by amplifying and sequencing the g20 gene, a capsid assembly gene commonly used to characterize cyanophage taxonomic diversity. We also characterized the host range phenotypes of over 300 of the isolates by assaying their ability to infect four additional Synechococcus host strains.  

Results/Conclusions

We found that both the genetic composition and the host range phenotypes of the cyanophage isolates varied significantly by month, with marginal variation attributed to site. Specifically, we observed that the relative abundance of particular taxa (defined by sequence similarity) fluctuated dramatically in a seasonal pattern that repeated across years. Fall and winter months, regardless of year, were dominated by a single taxon. Spring and summer months were more diverse, exhibiting a more rapid turnover of taxa. Additionally, host range phenotypes also varied by season, however, this phenotypic diversity was not correlated with taxonomic identity. Isolates within a single taxon often differed in host range when they originated from different months within a year, suggesting that the ability of cyanophage to infect co-occuring host strains may be evolving rapidly and is not entirely constrained by taxonomic identity. Overall, cyanophage community composition, in terms of both genetic identity and host range phenotype, appears to be temporally dynamic yet seasonally recurring. These results suggest that seasonal factors such as host composition or temperature may be driving this temporal diversity. If so, altered conditions brought on by global change could in turn influence patterns of phage-induced mortality of cyanobacteria in marine ecosystems.