In order to better estimate bacterial biomass in marine environments, we developed a novel technique for direct measurement of carbon and nitrogen contents of natural bacterial assemblages. Bacterial cells were separated from phytoplankton and detritus with glass fiber and membrane filters (pore size, 0.8 μm) and then concentrated by tangential flow filtration. The concentrate was used for the determination of amounts of organic carbon and nitrogen by a high-temperature catalytic oxidation method, and after it was stained with 4′,6-diamidino-2-phenylindole, cell abundance was determined by epifluorescence microscopy. We found that the average contents of carbon and nitrogen for oceanic bacterial assemblages were 12.4 ± 6.3 and 2.1 ± 1.1 fg cell−1 (mean ± standard deviation; n = 6), respectively. Corresponding values for coastal bacterial assemblages were 30.2 ± 12.3 fg of C cell−1 and 5.8 ± 1.5 fg of N cell−1(n = 5), significantly higher than those for oceanic bacteria (two-tailed Student’s t test; P< 0.03). There was no significant difference (P > 0.2) in the bacterial C:N ratio (atom atom−1) between oceanic (6.8 ± 1.2) and coastal (5.9 ± 1.1) assemblages. Our estimates support the previous proposition that bacteria contribute substantially to total biomass in marine environments, but they also suggest that the use of a single conversion factor for diverse marine environments can lead to large errors in assessing the role of bacteria in food webs and biogeochemical cycles. The use of a factor, 20 fg of C cell−1, which has been widely adopted in recent studies may result in the overestimation (by as much as 330%) of bacterial biomass in open oceans and in the underestimation (by as much as 40%) of bacterial biomass in coastal environments.

Bacterial abundance and leucine incorporation rate were measured throughout the water column (depth, 4,000-6,000 m) at stations occupied in the equatorial, subtropical, and subarctic Pacific as well as in the Bering Sea during three cruises conducted between 1993 and 1997. In general, depth-dependent decreases of bacterial abundance and leucine incorporation in the bathypelagic layer (depth, Ͼ1,000 m) were well described by a power function with remarkably uniform exponents among distant locations: average exponents were Ϫ0.900 and Ϫ1.33 for abundance and leucine incorporation, respectively. Depth profiles of bacterial properties were complex at some subarctic stations, suggesting lateral transport of organic carbon by local eddies. Organic carbon fluxes from abyssal sediment to overlying water would explain increases in bacterial abundance and leucine incorporation in near-bottom layers. Biomass was twofold to fourfold and the production was threefold to sevenfold greater in subarctic than in subtropical regions. This latitudinal pattern was consistent with the basin-scale distribution of sinking fluxes of particulate organic carbon (POC) reported in the literature. Rates of bacterial carbon uptake accounted for 51% (range, 31-153) and 23% (14-58) of deep sinking POC fluxes in subarctic and subtropical regions, respectively. Average turnover time of deep bacterial assemblages was estimated to be 1-30 yr. These results suggest that deep bacterial biomass and production are generally coupled with sinking POC fluxes and that organic carbon is substantially transformed within bathypelagic environments via a sinking POC → dissolved organic carbon → bacteria pathway, as previously suggested in the mesopelagic zone.

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Submicrometer particles (SMP) are suggested to be a critical component for organic matter transitions in seawater, but little is known about variations and controls of SMP in coastal systems. We examined vertical and horizontal distributions of SMP (0.4–l µm in equivalent spherical diameter as measured by a resistive pulse particle counter) and biological variables (chlorophyll a concentration, abundance of bacteria, and heterotrophic nanoflagellates) in northwest Pacific coastal environments, The abundance and total volume of SMP in the upper 200 m varied in the range of 5 × 104−3 × 107 particles ml−1 and 4 × 103−3 × 106 µm3 ml−1, respectively. Over a large trophic gradient (Chl a, 0.02–4 µg liter−1), the total volume of SMP was strongly positively correlated with Chl a concentration (r = 0.90, P < 0.0001, n = 47) and with other microbial variables (r = 0.84–0.90) consistent with a hypothesis that SMP dynamics are closely related to microbial food‐web processes. Notably, size distribution of SMP in upper waters often exhibited a distinctive peak at a size range of 0.6–0.7 µm, which was most pronounced in productive nearshore waters and became less evident with depth and with distance from the shore. A sonication experiment revealed that the 0.6–0.7‐µm particles are primarily nonliving. We hypothesize that SMP, particularly the 0.6–0.7‐µm component, are directly produced by biological processes. Our data suggest that SMP are a highly reactive and abundant component of detrital colloids and play important roles in material cycles within coastal systems.

In order to test the hypothesis that bactenal membrane protein is more slowly degraded than soluble protein in seawater, we examined degradation by natural bacterioplankton of membrane and soluble proteins prepared from the marine bacterium Vjbno alginol~~ficus radiolabeled with 3H-or 14C-leucine. First order kinetic constants indicated that proteins in crude membrane extract are degraded at significantly slower rates ('4 to 'lb) than the soluble proteins. Proteins determined to be intimately associated with the membrane were not degraded during the initial 45 h, while a substantial fraction of soluble proteins was degraded during the same period. The data are consistent with a model in which membrane and cell wall materials severely restrict access of bacterial proteases to membrane proteins. After prolonged incubation, however, membrane proteins started to be degraded, suggesting that proteins protected by membrane components were made available for degradation after ectoenzymatic destruction of membrane components. Our data support the hypothesis that macromolecular organic complexes play a role in temporary storage of dissolved organic matter (DOM) in seawater and that complementary hydrolysis by different cctoenzymes produced by diverse bacterioplankton is important in determining rates and patterns of DOM degradation in the sea.