Associations between depth and micro-diversity within marine viral communities revealed through metagenomics
Viruses are extremely abundant and diverse biological entities that contribute to the functioning of marine ecosystems. Despite their recognized importance no studies have addressed trends of micro-diversity in marine viral communities across depth gradients. To fill this gap we obtained metagenomes from both the cellular and viral fractions of Mediterranean seawater samples spanning the epipelagic to the bathypelagic zone at 15, 45, 60 and 2000 meters deep. The majority of viral genomic sequences obtained were derived from bacteriophages of the order Caudovirales, and putative host assignments suggested that they infect some of the most abundant bacteria in marine ecosystems such as Pelagibacter, Puniceispirillum and Prochlorococcus. We evaluated micro-diversity patterns by measuring the accumulation of synonymous and non-synonymous mutations in viral genes. Our results demonstrated that the degree of micro-diversity differs among genes encoding metabolic, structural, and replication proteins and that the degree of micro-diversity increased with depth. These trends of micro-diversity were linked to the changes in environmental conditions observed throughout the depth gradient, such as energy availability, host densities and proportion of actively replicating viruses. These observations allowed us to generate hypotheses regarding the selective pressures acting upon marine viruses from the epipelagic to the bathypelagic zones.