Most of the oceanic reservoir of dissolved organic matter (DOM) is of marine origin and is resistant to microbial oxidation, but little is known about the mechanisms of its formation. In a laboratory study, natural assemblages of marine bacteria rapidly (in <48 hours) utilized labile compounds (glucose, glutamate) and produced refractory DOM that persisted for more than a year. Only 10 to 15% of the bacterially derived DOM was identified as hydrolyzable amino acids and sugars, a feature consistent with marine DOM. These results suggest that microbial processes alter the molecular structure of DOM, making it resistant to further degradation and thereby preserving fixed carbon in the ocean.

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.

LETTERS TO NATURE membrane proteins) between the cisternae is thought to occur mainly at the dilated rims of the Oolgi stacks 22 • 23 • If Rab6p were involved in this process, it would be expected to be more concentrated at the edges of the cisternae. But the exact topology of intra-Oolgi traffic has been very difficult to examine in vivo and there is no strong evidence to support this model. Alternatively, Rab6p could be involved in an as-yet-unknown transport event between the Oolgi stacks.Finally, the observation of the polarized distribution of Rab6p in the Oolgi apparatus is of interest. This suggests that another OTP-binding protein may act in intra-Oolgi transport before Rab6p. Such a protein could be Rablp, the mammalian counterpart of yeast Yptlp (refs 5,12,13). Our results support the hypothesis that several small OTP-binding proteins localize to different intracellular compartments and have a pivotal role in maintaining the orderly flow of vesicular traffic in mammalian ~~-D

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