D-Gl~cose-6-[~H] uptake kinetics for marine microbial assemblages were multiphasic when the kinetics were determined over a broad concentration range (10-'M to I O -~ M). Maximum uptake velocity (V,,,,) and the sum of half saturation constant (K,) and ambient glucose concentration (S,), (K, + S,), increased gradually as the range of glucose concentration was increased. In all seawater samples examined the lowest (K, + SJ values were 2-5 x lO-'M; highest values were as high as 5.9 X 104 M. Simple diffusion into algae or bacteria cannot explain the non-linearity of kinetics curves. Removal of most algal cells did not change the kinetic pattern, and indirect estimates of diffusion into bacteria were too small to have greatly changed the uptake in 1 X 10 -1 X 10-' M range of glucose. The kinetic diversity of marine bacterial assemblages has in~plications in the cycling of dissolved organic matter in the varied concentration regimes presumably present in production microzones. Turnover times of the glucose pool at elevated concentrations were in the order of 10'-103 h, suggesting that episodic high concentrations of substrate would diffuse out of the production microzones before significant uptake. Sustained concentration gradients might attract high K, h~g h V, , , bacteria which might take up a significant fraction of substrate within the production zone. The observed kinetic diversity points to the need for a modified kinetic approach to account for the diversity of K, and V, , .

Rates of leucine lncorporahon have been suggested recently to be useful for estunating rates of protein synthesis and biomass product~on by bactena In natural water samples We examined 2 potential problems wlth this approach de novo synthesis of leucine and lntracellular protein turnover Rates of leucine and methionine biosynthesis were e s t~m a t e d from the incorporation of '4C-pyruvate and 35S0,', respectively Leucine inhibited '4C-pyruvate and 'H-glucose incorporahon and methionine inhibited "SO, lncorporation However rates of biosynthesis of leucine and lnethionine were still much higher than the maximum rate of exogenous amlno acld Incorporation This problem can be surmounted with emplncally determined conversion factors which relate rates of leucine incorporation to rates of proteln synthesis or biomass production The ratio of the emplncally determined factor to the theoretical factor is similar to the ratio of the rate of biosynthesis to the incorporation rate of exogenous leuclne The rate of intracellular proteln turnover as determined by the pulse-chase approach was large compared wlth net protein synthesis ~n only 1 out of 5 expcnments Leuclne lncorporation rates are at least an underestimate of rates of protein synthesis and In some environments may prove to b e a useful measure of bacterial biomass production Our results also Indicate that the supply of dissolved amino acids may affect the uptake and minerahzatlon of other dissolved compounds vla regulation of amino acid uptake and biosynthesls

Dissolved hurnic substances from 3 marine environments with varying vascular plant influence were shown to contain biologically labile components. Biomass of marine bacterioplankton increased in grazer-free incubations in the presence of humic substance supplements equivalent to 2-fold and 5-fold natural concentrations. Humic substances were present in highest concentration in seawater collected from a mangrove swamp site; these humics had the highest vascular plant influence (estimated at 6 6 % based on lignin phenol analysis) and yielded the least bacterial growth on a carbon basis. From 3 to 11 % of the humic substances from all 3 sites were utilized by marine bacterioplankton over a 5 d period. Results of this study suggest that some fraction of the humic substances pool can have a relatively short turnover time in seawater, and point out the importance of identifying which specific components of the &ssolved organic carbon pool are available for supporting marine bacterioplankton production.

As Spartina alter~~iflora lignocellulose degrades, it contributes soluble degradation products to the bulk dssolved organic carbon (DOC) pool in salt marsh environments. Experiments with radlolabeled S. alterniflora hgnocelluloses show that during the initial 6 mo of decomposition, DOC accounts for 50 to 60% of the total degradation products (DOC plus CO2) of the lignin fraction of hgnocellulose; by contrast, only 20 to 30 '10 of the polysaccharide portion of S alterniflora lignocellulose accumulates as DOC during decomposition. The differences in net accumulation are most llkely due to dfferential rates of microbial utilization of the soluble compounds denved from these 2 fractions and not to differential rates of formation. As a result, although lignin comprises only 7 % of undegraded S. alterniflora lignocellulose, it may contribute as much as 30 % of the carbon in lignocellulose-derived DOC. Soluble compounds denved from hgnin show evidence of significant chemical modification, such that only a small fraction of Iignin-derived carbon is present as recognizable lignin phenols The long residence time of lignin-derived carbon in salt marsh DOC pools, relative to that for polysaccharidederived carbon, demonstrates a mechanism by which lignin may serve as a source of aquatic humlc substances and contribute to the bulk DOC pool of salt marshes in greater proportion than expected from the ratio of lignin. polysaccharides in undegraded plant material. Based on quantification of lignin oxidation products, we estimate that S. alterniflora lignocellulose, both the lignin and polysaccharide components, contributes 44 "b of the carbon in the bulk DOC pool of a Georgia salt marsh creek.