Dissolved organic matter assimilation by heterotrophic bacterial groups in the western Arctic Ocean

Abundance of the major bacterial groups and dissolved organic matter (DOM) assimila- tion in the western Arctic Ocean were determined using fluorescence in situ hybridization (FISH) and microautoradiography combined with FISH (Micro-FISH). Cytophaga-like bacteria (25 to 65%) and Alphaproteobacteria (17 to 40%) were the dominant bacterial groups, followed by Gammaproteo- bacteria (10 to 30%). In contrast, Betaproteobacteria and Actinobacteria were never abundant. While the distribution of Alphaproteobacteria was relatively uniform along a transect from the shelf to the basin, Cytophaga-like bacteria were more abundant on the shelf and shelf-break. Similarly, the contribution to DOM assimilation by Cytophaga-like bacteria was highest on the shelf and lowest in the basin. In contrast, Alphaproteobacteria contributed the most to DOM assimilation at the slope. About 80 to 99% of the variation in DOM assimilation was explained by bacterial group abundance. As a whole, the prokaryotic community was most active in assimilating free amino acids (50 to 60%), followed by diatom-derived extracellular polymers (30 to 40%) and protein (20 to 30%). In contrast, relatively few cells assimilated glucose (10 to 20%). This study revealed substantial variation in the abundance of major bacterial groups among the Arctic regions and in the assimilation of DOM components by these bacteria.