Spatial variations in composition of marine microbial communities and its causes have largely been disclosed in studies comprising rather large environmental and spatial differences. In the present study, we explored if a moderate but temporally permanent climatic division within a contiguous arctic shelf seafloor was traceable in the diversity patterns of its bacterial and archaeal communities. Soft bottom sediment samples were collected at 10 geographical locations, spanning spatial distances of up to 640 km, transecting the oceanic polar front in the Barents Sea. The northern sampling sites were generally colder, less saline, shallower, and showed higher concentrations of freshly sedimented phytopigments compared to the southern study locations. Sampling sites depicted low variation in relative abundances of taxa at class level, with persistent numerical dominance by lineages of Gamma- and Deltaproteobacteria (57–66% of bacterial sequence reads). The Archaea, which constituted 0.7–1.8% of 16S rRNA gene copy numbers in the sediment, were overwhelmingly (85.8%) affiliated with the Thaumarchaeota. Beta-diversity analyses showed the environmental variations throughout the sampling range to have a stronger impact on the structuring of both the bacterial and archaeal communities than spatial effects. While bacterial communities were significantly influenced by the combined effect of several weakly selective environmental differences, including temperature, archaeal communities appeared to be more uniquely structured by the level of freshly sedimented phytopigments.

Offshore drilling operations result in the generation of drill cuttings and localized smothering of the benthic habitats. This study explores bacterial community changes in the in the upper layers of the seafloor resulting from an exploratory drilling operation at 1400m water depth on the Barents Sea continental slope. Significant restructurings of the sediment microbiota were restricted to the sampling sites notably affected by the drilling waste discharge, i.e. at 30m and 50m distances from the drilling location, and to the upper 2cm of the seafloor. Three bacterial groups, the orders Clostridiales and Desulfuromonadales and the class Mollicutes, were almost exclusively confined to the upper two centimeters at 30m distance, thereby corroborating an observed increase in anaerobicity inflicted by the drilling waste deposition. The potential of these phylogenetic groups as microbial bioindicators of the spatial extent and persistence of drilling waste discharge should be further explored.

Spatial differences in microbial communities are observable even in habitats with moderate environmental variation, such as within the pelagic zone or seafloor of geographically finite regions of the oceans. Here we explore if biogeographical variations are manifested at this level also in the structure of viral assemblages by comparing DNA viromes from the Barents Sea upper seafloor, collected at five geographically separated locations. Twenty-seven to forty-four percent of the open reading frames showed significant similarity to genes of viral genomes in the Refseq database. The majority of the identified open reading frames, i.e. 86 to 95%, were affiliated with sequences of single-stranded DNA (ssDNA) viruses, but the ssDNA virus genetic material was likely strongly overrepresented due to the use of phi29 DNA polymerase for amplifying viral DNA. The majority of ssDNA virus sequences originated from the Microviridae family of phages and the eukaryotic Circular Rep-encoding ssDNA (CRESS-DNA) viruses. The sediment virus assemblages showed higher overall similarity to counterparts from deep sea sediment of the Pacific Ocean than to 2 e.g. Arctic Ocean pelagic viromes, supporting the presence of common compositional features in sediment viral assemblages across continental-scale geographical separations. The Barents Sea viromes grouped biogeographically in accordance with the south-north environmental division of this Arctic sea by the oceanic polar front, thereby mirroring a corresponding 16S rRNA gene based biogeographical division of the bacterial communities. However, compositional differences in the eukaryotic virus assemblages rather than the bacteriophages appeared as the primary basis for this spatial separation.

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