Community composition and extracellular enzyme activity of bacteria associated with suspended and sinking particles in contrasting arctic and sub-arctic marine environments

Heterotrophic bacteria contribute to controlling the oceans' biological carbon pump by solubilising organic particles and by using the dissolved organic matter for growth. To better understand how bacteria coloniseand degrade vertically exported organic matter, spatial and temporal variations in the bacterial community composition (BCC) and cell-specific extracellular enzyme activity were investigated in water column samples and in sinking material collected by short-term deployed sediment trapsin the Barents Sea (Arctic Ocean) and in a North-Norwegian fjord. Denaturing gradient gel electrophoresis showed that both the suspended and sinking BCC differed among locations (Arctic Water, Atlantic Water and fjord) and with time (summer vs. autumn in the fjord). Within stations, the BCC differed between shallow (20 to 50 m) and deep (120 or 200 m) suspended samples. The sediment trapsrevealed no depth-related difference but had high similarity to the shallow suspended samples, indicating that the exported community largely derived from suspended particles in the upper mixed layer and that little new colonisationor succession of the resident communitytook placeduring transfer to depth. Bacteria in the sedi- ment traps tended to have higher enzyme activities compared to suspended bacteria, likely reflecting a response to aggregation and different composition (carbon, nitrogen and chlorophyll) of suspended and sinking organic matter. While the present study demonstrates that particle export from the surface layer contributes to microbial diversity in the deep sea, future studies using high taxonomical resolution are required to further evaluate the importance of this dispersal pathway.