Specialized Bacteroidetes dominate the Arctic Ocean during marine spring blooms

A metagenomic time series from Arctic seawater was obtained from the Canadian region of Dease Strait, to analyse the changes in bacterioplankton caused by the phytoplankton bloom that recurrently occurs in summer. This dataset documents the growth of bacterial clades specialised in the metabolism of plysaccharides, such as Bacteroidetes, along with the phytoplackton. These specialised taxa quickly displaced the microbial clades that dominate nutrient-poor waters during early spring, such as Archaea, Alpha- and Gammaproteobacteria. At the functional level, phyla Bacteroidetes, Planctomycetes and Verrucomicrobia showed higher contents of polysaccharide-degradation functions. Glycoside hydrolases revealed that the Bacteroidetes community shifted towards species with higher polysaccharide-degrading capabilities, targeting algal polysaccharides in summer. Regarding transporters, Bacteroidetes dominated SusC-TonB transporters and had an exclusive family of glycoside-binding proteins (SusD). These proteins were used to identify polysaccharide-utilisation loci that clustered transporters and polysaccharide-active enzymes, showing a higher level of specialisation towards polysaccharide use. Put together, all these genomic features point to the genetic adaptations that promote the dominance of Bacteroidetes during phytoplankton blooms. ### Competing Interest Statement The authors have declared no competing interest.