The distribution, abundance, and production of viruses and bactena 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' to 10'' I-') 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 sed~ment sample at 27 and 2.1 X 10' l.', 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' N), bacteria and viruses displayed subsurface maxima in the upper 100 m. Below 100 m, the concentrations declined, but were detectable even in the deepestcollected 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-' 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 lO%iruses I-' d-l, which are similar to previous estimates for temperate coastal waters. The production measurements ind~cated 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 bactena 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 Arct~c marine microbial communities. The implications of bacterial and viral production for C and N cycling in the Chukchi Sea are discussed
Proteolytic activity and the role of bacteria in protein degradation in seawater were investigated during 2 cruises to the Santa Monica Basin. The rate of hydrolysis of L-leucyl-P-naphthylaniine was used as a measure of proteolytic activity, and was compared with bacterial numbers, production, and growth rates throughout the water column (905 m max. depth). Peak proteolytic activities in the upper 100 m of the water column were more than 10 times those of deeper waters, though the natural substrate concentrations (combined hydrolyzable amlno acids) in the Santa Monica Basin have prev~ously been found to vary by only 3 to 4-fold Sediment porewaters were assayed during one cruise and found to have 20 to 30 times more proteolytic activity than the peak water column samples from the same cruise, though the origin of this activity could not be determined. Most of the proteolytic activity in the water column was associated with particles the size of bacteria (0.2 to 0.8 pm) and, overall, total proteolytic activity was highly correlated with all bacterial variables tested. However, the average activities per cell were not related to average cell sizes, suggesting that proteolytic activity is not s~mply a function of bacterial biomass. In samples from one of the crulses, the average activities per cell covarled with bacterial growth rates (thymidme incorporation per cell), but this was not true for samples taken during the second cruise. Laboratory expenments using 'seawater cultures' indicated that neither changes in cell size nor growth rate alone could account for changes in the amount of proteolytic enzyme expressed by marine bacteria. These results suggest that proteolytic activity in the water column is primarily associated with bacteria and is likely to be controlled by a number of factors related to the bacterioplankton, including biomass and growth rates.
Bacterial urea production and decomposition were studied in samples from coastal waters in the Southern California Bight (the Bight), USA, and an estuarine system of the Mankyung and Dongjin rivers (MD estuary) in Korea. Bacterial urea production ranged from undetectable to 139 nM d-', and the mean value of bacterial urea production (58 nM d", n = 6) was equivalent to 35-91 % of the estimated phytoplankton N demand in the Bight. The rates of bacterial production of urea were 2 orders of magnitude higher than the bacterial urea decomposition rates. Consequently, bacteria were consistently net producers of urea in the euphotic zone. The concentration-dependence of urea decomposition showed the presence of a high affinity but low capacity system (K,+S,,: 26 to 33 nM, V,,,: 3 to 11 nM d-l). The low K, values indicate that in typical seawater samples, which have >l00 nM urea, the bacterial ureolysis system is always near-saturated. The significance of bacteria as urea producers should be incorporated into models of nitrogen regeneration in surface waters.
Stimulation of heterotrophic bacterial growth by inorganic nitrogen (nitrate and ammonium) was observed in natural assemblages of marine bacteria grown in continuous culture with unsupplemented sea water as primary medium. In the presence of nitrogenous supplements, bacterial numbers increased approximately 3-fold. These results indicate that re-evaluation of the role of heterotrophic bacterioplankton in the pelagic nitrogen cycle may b e necessary. INTRODUCTIONIn the classical view of the pelagic marine nitrogen cycle, heterotrophic bacteria have been portrayed as net mineralizers, producing ammonium from dissolved organic nitrogen (DON). Assimilation of dissolved inorganic nitrogen (DIN) as regenerated ammonium or upwelled nitrate is thought to be due mainly to phytoplankton. Recently, several lines of evidence have suggested that heterotrophic bacteria might be significant consumers of the inorganic N, in competition with phytoplankton (which are generally N-limited in the marine pelagial). Indirect evidence comes from an inability to balance a nitrogen budget without invoking DIN uptake by bacteria (Laws et al. 1985). Additional evidence comes from DIN uptake studies using specific inhibitors for prokaryotes and eukaryotes, respectively (Wheeler & Kirchman 1986) and 13NH4+ followed by size-fractionation (Fuhrman et al. 1988). Further, laboratory studies have demonstrated growth of marine bacteria using nitrate as a nitrogen source (Brown et al. 1972), and field and mesocosm studies suggest that bacteria may limit autotrophic production by competing for nitrate (Parker et al. 1975, Parsons et al. 1981.We studied DIN uptake by natural assemblages of bacteria grown in seawater cultures. We also asked whether DIN additions, made in the second stage of a ' Contribution No. 617
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