High contribution of Pelagibacterales to bacterial community composition and activity in spring blooms off Kerguelen Island (Southern Ocean)

The ecology of Pelagibacterales (SAR11 clade), the most abundant bacterial group in the ocean, has been intensively studied in temperate and tropical ocean regions, but the distribution patterns of this clade remains largely unexplored in the Southern Ocean. Through amplicon sequencing of the 16S rRNA genes, we assessed the contribution of Pelagibacterales to bacterial community composition in the naturally iron fertilized region off Kerguelen Island (Southern Ocean). We investigated the upper 300 m water column at seven sites located in early spring phytoplankton blooms and at one site in HNLC waters. Despite pronounced vertical patterns of the bacterioplankton assemblages, the SAR11 clade had high relative abundances at all depths and sites, averaging 40% ({+/-}15%) of the total community relative abundance. Micro-autoradiography combined with CARD-FISH further revealed that the SAR11 clade contributed substantially (45-60% in surface waters) to bacterial biomass production (as determined by 3H leucine incorporation). A clear niche partitioning of the further resolved SAR11 subclades was observed with depth layers, but differences among sites were detectable for only a few subclades. Our study provides novel observations of the distribution and contribution to the marine carbon cycle of the SAR11 clade in the cold waters of the Southern Ocean.