Activity profiles of bacterioplankton in a eutrophic river

1. The significance of microbial diversity in processing dissolved organic matter (DOM) is largely unknown. We investigated the range of functional di versity in the bacterioplankton from a eutrophic river by profiling extrace llular enzyme activities (EEA) and substrate-induced respiration (SIR) patt erns. 2. The EEA profiles consisted of assays for 21 hydrolases, measured using f luorogenic substrates arrayed on 96-well microplates. Commercially availabl e BiOLOG(R) GN and ECO plates, which contained 120 different substrates, we re used for the SIR profiles. 3. The EEA data were more dynamic than the SIR. Five enzymes, leucine amino peptidase, alkaline phosphatase, alanine aminopeptidase, arginine aminopept idase and P-glucosidase, showed consistently high activity; ten others were ubiquitous at lower activity levels; the remainder were detected intermitt ently. The SIR data showed less temporal variability. With one exception (c itrate), the 20 substrates that generated the largest responses were all sa ccharides or their derivatives. 4. The EEA and SIR data did not generally correlate. Both methods were effe ctive for ordinating bacterioplankton although, unlike the SIR, the EEA ord ination followed a clear temporal trajectory. 5. Because the SIR profiles are based on a culture response, whilst the EEA profiles measure activity of the extant community, the latter appear to be more directly linked to the mechanics of DOM processing.