We investigated net growth rates of distinct bacterioplankton groups and heterotrophic nanoflagellate (HNF) communities in relation to phosphorus availability by analysing eight in situ manipulation experiments, conducted between 1997 and 2003, in the canyon-shaped Rímov reservoir (Czech Republic). Water samples were size-fractionated and incubated in dialysis bags at the sampling site or transplanted into an area of the reservoir, which differed in phosphorus limitation (range of soluble reactive phosphorus concentrations--SRP, 0.7-96 microg l-1). Using five different rRNA-targeted oligonucleotide probes, net growth rates of the probe-defined bacterial groups and HNF assemblages were estimated and related to SRP using Monod kinetics, yielding growth rate constants specific for each bacterial group. We found highly significant differences among their maximum growth rates while insignificant differences were detected in the saturation constants. However, the latter constants represent only tentative estimates mainly due to insufficient sensitivity of the method used at low in situ SRP concentrations. Interestingly, in these same experiments HNF assemblages grew significantly faster than any bacterial group studied except for a small, but abundant cluster of Betaproteobacteria (targeted by the R-BT065 probe). Potential ecological implications of different growth capabilities for possible life strategies of different bacterial phylogenetic lineages are discussed.

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The relationship between flow cytometry data and epifluorescence microscopy measurements was assessed in bacterioplankton samples from 80 lakes to estimate bacterial biovolume and cell size distribution. The total counts of 4,6-diamidino-2-phenylindole-stained cells estimated by both methods were significantly related, and the slope of their linear regression was not significantly different from 1, indicating that both methods produce very similar estimates of bacterial abundance. The relationships between side scatter (SSC) and 4,6-diamidino-2-phenylindole fluorescence and cell volume (microscopy values) were improved by binning of the data in three frequency classes for each, but further increases in the number of classes did not improve these relationships. Side scatter was the best cell volume predictor, and significant relationships were observed between the SSC classes and the smallest (R 2 ‫؍‬ 0.545, P < 0.001, n ‫؍‬ 80) and the largest (R 2 ‫؍‬ 0.544, P < 0.001, n ‫؍‬ 80) microscopy bacterial-size classes. Based on these relationships, a reliable bacterial biomass estimation was obtained from the SSC frequency classes. Our study indicates that flow cytometry can be used to properly estimate bacterioplankton biovolume, with an accuracy similar to those of more time-consuming microscopy methods.Planktonic bacteria are important members of aquatic ecosystems, and the calculation of their biovolume is relevant to our understanding of their roles within the microbial food web and in the cycling of organic matter and nutrients. Direct epifluorescence microscopy (EFM) counts of samples stained with nucleic acid fluorochromes such as acridine orange or 4Ј,6Ј-diamidino-2-phenylindole (DAPI) has for the last few decades been the standard method for determining bacterial abundance and biovolume in plankton samples (16,26,39). Based on these measurements, bacterial carbon biomass can be estimated by applying a general conversion factor (24, 28). However, despite improvements through the application of automated image analysis systems (29), microscopy counts are still time-consuming and require a considerable effort to obtain accurate measurements of bacterial cell volumes. In the early 1990s, flow cytometry (FC) was introduced as an alternative to EFM for estimating bacterial abundance in mixed natural assemblages (see references 13 and 30). FC has become a key tool in aquatic microbial ecology because it constitutes a rapid cell counting method and also makes it possible to process a high number of samples in a short time (33). Besides estimates of bacterial abundance, FC also provides information on single-cell parameters (e.g., light scatter values and specific channels of fluorescence) that can be useful for further discriminating distinct fractions of bacteria within mixed assemblages and thus for analyzing the heterogeneity of bacterial communities (3,11,38). The increase in commercially available fluorochromes, the use of molecular techniques, the application of cell sorting, and technological progress have i...

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As a first approach to the study of the dynamics of amino sugars in freshwater environments, we measured microbial uptake of N-acetylglucosamine and hydrolysis of 4-MUF-N-acetyl-&lucosaminide by extracellular enzymes in fishponds, reservoirs, backwaters, and lakes. Amino sugars were present in all localities and comprised up to several percent of both dissolved and particulate organic matter. Processes involving N-acetylglucosamine (extracellular enzymes, microbial uptake) were also detected in all localities; their potential rates accounted for -lO-100% of the rates of analogical processes involving glucose and were correlated with dissolved organic C and with general microbial activity. Two types of 4-MUF-@-N-acetylglucosaminidases (high and low affinity) were distinguished kinetically. The activity of the high-affinity enzyme was tightly correlated with variables coupled to bacterial activity. In fishponds, uptake of N-acetylglucosamine by diatoms and cyanobacteria was detected via autoradiography.

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The effect of substrate (glucose) shock on starved mixed cultures of aquatic bacteria under conditions resembling the natural aquatic situation has been studied. The prevalent organism in the system was a Pseudomonas sp., and short‐term loss of viability and long‐term loss of several biochemical properties of this strain were observed. Glucose uptake rate and cell size of the population increased immediately after the shock and synchronous growth occurred subsequently. The population which became established after the shock exhibited lower glucose uptake rates at low substrate concentrations than the starved population. The relevance to similar phenomena which occur when aquatic bacteria are transferred to cultivation media is discussed.