Comparison of free-living, suspended particle, and aggregate-associated bacterial and archaeal communities in the Laptev Sea

In cold oceans, the importance of microbes in degrading particulate organic matter (POM) and constraining carbon export has been questioned, given that a greater proportion of primary production reaches the seafloor at high latitudes than low latitudes. To learn more about POM-associated microbial communities in cold waters, we worked aboard the icebreaker 'Kapitan Dranitsyn' during the Nansen and Amundsen Basins Observational System (NABOS) 2005 cruise to the Laptev Sea, a river-impacted region in the Siberian Arctic, to sample 3 size fractions of particles operationally defined as sinking aggregates (> 60 mu m), smaller suspended particles (1 to 60 mu m), and free-living bacteria (0.22 to 1 mu m). Sample temperatures and depths ranged from -1.6 to 1.5 degrees C and 25 to 150 m, respectively. Analysis of associated microbial communities using 16S rRNA gene-based clone libraries and terminal restriction fragment length polymorphism (T-RFLP) indicated greatest diversity and richness in the smaller size fractions and significantly different communities in the aggregate fraction for both Bacteria and Archaea. The most abundant clones in the bacterial libraries were Gammaproteobacteria, followed by members of the Cytophaga-Flavobacterium-Bacteroides (CFB) group and Alphaproteobacteria; the archaeal libraries contained primarily Marine Group I Crenarchaeota. Canonical correspondence analysis indicated that bacterial communities, especially those associated with aggregates, were influenced by riverine input and temperature. Determinants of archaeal communities were less clear, but temperature appeared important. Distinctions in bacterial and archaeal community complexity observed among the different particle size classes suggest the importance of particle residence time in structuring associated microbial communities and defining their biogeochemical roles.