Abundance and production of bacteria and viruses in the Bering and Chukchi Seas

The distribution, abundance, and production of viruses and bacteria were investigated during an August to September 1992 cruise aboard the RV 'Alpha Helix' in the Bering and Chukchi Seas. Viruses were abundant in seawater samples at all stations (10(9) to 10(10) l(-1)) and exceeded the bacteria concentration by an order of magnitude on average. Virus-like particles and bacteria were also observed in the pore water of a sediment sample at 27 and 2.1 x 10(9) l(-1), respectively The concentrations of viruses and bacteria in pelagic samples were correlated (r = 0.83, n = 43). In a detailed depth profile from the deepest and northernmost station (72 degrees N), bacteria and viruses displayed subsurface maxima in the upper 100 m. Below 100 m, the concentrations declined, but were detectable even in the deepest-collected samples (402 m). Integrated bacterial biomass estimates were similar to results from a previous study in this area, but bacterial production measurements ranging from 0.3 to 0.45 g C m(-2) d(-1) were an order of magnitude higher. Production rates of bacterial viruses (also known as bacteriophages or simply phages) measured by radiolabeling ranged from 0.5 to 4.2 x 10(9) viruses l(-1) d(-1), which are similar to previous estimates for temperate coastal waters. The production measurements indicated turnover times ranging from 0.4 to 17 d for bacteria and maximum estimates of 1.2 to 15 d for bacterial viruses. Viral mortality of bacteria was estimated from the frequency of visibly infected cells (FVIC) and flagellate grazing was calculated from flagellate and bacterial abundances together with an assumed flagellate clearance rate. Overall, estimated viral lysis was roughly comparable to estimated grazing by flagellates as a source of bacterial mortality. Averaged over the water column, viral mortality of bacteria in the Chukchi Sea was estimated to be 23% of the bacterial production at 2 southern stations and approximately 10% at 2 northern stations. FVIC was correlated with bacterial production (r = 0.75, n = 18) and specific growth rate (r = 0.74, n = 18), but not with bacterial abundance (r = 0.22, n = 27). These data suggest viruses to be a ubiquitous and dynamic feature and a significant source of bacterial mortality in Arctic marine microbial communities. The implications of bacterial and viral production for C and N cycling in the Chukchi Sea are discussed.