Enzymatic analysis of riverine bacterioplankton production

Bacterial production is the entry point for detrital macronutrients into aquatic food webs. Many factors affect productivity, but the heterogeneity of detrital substrates and the diversity of microbial communities confound simple relationships between carbon supply and growth. WC tried to link the two by analyzing extracellular enzyme activities. Water samples were collected from three rivers and assayed for bacterial productivity and the activities of eight enzymes. Production varied among systems, peaking at 644, 170, and 68 pmol C liter-’ d-’ in the Ottawa (Ohio), Maumee (Ohio), and Hudson (New York) Rivers. V,,,,, values were generally correlated with productivity. The mean ratios of productivity per unit peptidase and esterase activity were similar among rivers, whereas carbohydrase and phosphatase ratios varied widely. The data were used to evaluate a model that relates productivity to carbon flow by using enzyme activities as indicators and assuming an optimum resource allocation relationship among C-, N-, and P-acquiring enzymes. The data supported the model, but predictive power was low. Bacterial productivity generally increased with inorganic nutrient availability, but high levels of productivity at any specific eutrophic state required sources of both saccharides and amino acids. In many aquatic ecosystems, a large fraction of macronutrient flow is through the microbial loop, a cycle that recovers detrital carbon and other nutrients and injects them back into the food web (Pomeroy 1974; Azam et al. 1983). Dissolved organic carbon (DOC) is assimilated into the microbial loop through heterotrophic bacterial production. Among ecosystems, bacterial production can be correlated with DOC concentration or, more indirectly, with primary production (Cole et al. 1988). Within an ecosystem the trophic basis of bacterial production is more difficult to discern (Findlay et al. 1991), partly because other physicochemical [UV radiation (Herndl et al. 1993), nutrients (Meyer-Reil 1987), temperature (Hudson et al. 1992; Shiah and Ducklow 1994)] and biological factors [grazing (Gonzalez et al. 1990; Bott and Kaplan 1990), growth efficiency (Kroer 1993; Biddanda et al. 1994)] play a role in regulating productivity. Also, because DOC is a heterogeneous mix of compounds, gross abundance does not equate with biodegradability. Additionally, DOC and production dynamics may be out of phase due to lags in bacterial response to changes in the DOC milieu (Scavia and Laird 1987). Various Chromatographic and spectrometric analyses have Acknowledgments This work was supported by grants from the Hudson River Foundation and the U.S. Dept. of Agriculture. Chemical analyses of Ottawa River water were conducted by Sarah Karpanty, Sara Mierzwiak, and Mark Robinson. Christine Foreman and William Sobczak assisted with the enzyme and productivity assays. been applied to DOC (Thurman 1986; Hedges et al. 1994; Moran and Hodson 1994) to trace its origin, composition, and fate, but connections to bacterial productivity have been difficult to demonstrate. Direct studies of bacterial productivity in relation to DOC have involved productivity assays in conjunction with analyses for specific labile DOC components, such as amino acids and sugars. At its highest resolution, this approach focuses on the kinetics of monomer generation and uptake, rather than on simple abundances. Both of these processes are mediated by ectoenzymes. The activity of enzymes involved in monomer generation is typically analyzed using artificial substrates linked to chromogenic or fluorogenic moieties (Hollibaugh and Azam 1983; Hoppe 1983; Someville and Billen 1983). The activities of membrane permeases, which collectively constitute assimilation, are usually studied with radiolabeled monomers (Wright and Hobbie 1966). Such research has shown that monomer generation is generally slower than assimilation; therefore, the rate-limiting step in bacterial macronutrient procurement is usually the generation of assimilable monomers from polymeric or condensed DOC components (Chrost 199 1). There is a gap between high-resolution biochemical models and larger scale spatiotemporal patterns of bacterial productivity (e.g. Kaplan and Bott 1983, 1985; Bott et al. 1984). If ectoenzymatic breakdown of complex organic molecules is the rate-limiting process in bacterial carbon acquisition, it may be possible to analyze productivity patterns in relation