No abstract

Annual studies of kinetics of alkaline phosphatase (APA) activity and phosphorus availability for microplankton in the photic zone of an eutrophic lake are reported. The total APA activity of microplankton varied strongly. Vmax was highest during summer P depletion, and in autumn and winter total APA activity was low. The total APA specific activity of the microplankton was also highest (average 3.55 pmole PO4 (3-) ng ATP(-1) min(-1)) when ambient orthophosphate concentrations were very low. Both Vmax and specific APA activity were not dependent on the biomass of microplankton; they were strongly affected by P available for microplankton. A differential filtration technique was used for separation of microplankton into two size classes, i.e., algal, larger than 3μm, and bacterial fraction with size 0.2-3.0μm. The algal size fraction had lower specific APA activity (average 1.224 pmole PO4 (3-) ng ATP(-1) min(-1)) and higher KM values (38.8μmole × liter(-1)) than microorganisms which were smaller than 3μm (2.011 pmole PO4 (3-) ng ATP(-1) min(-1) and 25.4μmole liter(-1), respectively). The KM values of free, dissolved APA (36.8μmole liter(-1)) indicated that free APA was probably released by algae. Phytoplankton were major APA activity producers in the photic zone of the lake from March to November, and their activity constituted, on the average, 48.6% of the total APA activity in the water. Bacteria were the dominant APA activity producers in winter (41.3-44.9%); however, during other periods they contributed significantly (average 21.7%) to total APA activity. When surplus constituted less than 10% of particulate P in seston, phytoplankton produced high specific APA activity, and when surplus P was higher than 15%, the specific APA activity of phytoplankton size fraction rapidly decreased. APA of the bacterial size fraction of the seston was not affected by P concentrations. Orthophosphate was a competitive inhibitor of APA produced by microorganisms of the size fraction larger than 3.0μm, and increasing concentrations of inorganic phosphate caused an increase in KM values. The hypothetical metabolic-coupling between phytoplankton and bacterioplankton in the phosphorus cycle in conjunction with carbon metabolism in the lake is discussed.

No abstract

Kinetics of P-glucosidase activity (PGlcA) and bacterial activity (glucose uptake, thymidine incorporation) and cell numbers were measured in the euphotic zone and in the water column of PluBsee during spring phytoplankton bloom development and after its breakdown. Heterotrophic bacteria were the major producers of the enzyme. Activity of free &glucosidase, unassociated with microbial cells, was negligible. /3GlcA displayed a distinct temporal and spatial distribution pattern in lake water. @GlcA was low when algal populations grew actively, but during the algal bloom breakdown /3GlcA increased rapidly. The increase in PGlcA was proportional to the abundance of bacteria and to their heterotrophic uptake of glucose, as well as to bacterial production, measured by the thymidine incorporation method. In contrast with its response to pH, /3-glucosidase exhibited no obvious adaptation to ambient temperature of lake water. PGlcA produced by aquatic bacteria was under control of a repression-induction mechanism, and synthesis was derepressed when the level of directly assimilable hexoses (glucose or galactose) fell below a critical level. The tight relationship between the rates of PGlcA and glucose uptake indicated the existence of a specific, coupled hydrolysis-uptake system in lacustrine bacteria.During the past decade increasing numbers of ecological studies have considered the complexity of aquatic environments. One major outcome has been an accelerated interest in the role of bacteria and the mode by which organic matter is made available to them. Heterotrophic bacteria comprise the key level which affects whole-lake metabolism, i.e. nutrient cycles, organic matter transformation and mineralization, and energy flow. The measurements of bacterial activities in natural waters are therefore very important for understanding the pathways and dynamics of carbon flux between various trophic levels in lakes. The metabolic activity of microheterotrophs is also im-' Permanent address:

No abstract

The major aim of this study was to evaluate the relationships between the rates of microbial activities (phytoplankton primary production, bacterial secondary production, bacterial utilization of organic matter, enzymatic activities, protozoan grazing on bacteria), bacterial numbers, and dissolved organic carbon concentrations and the trophic state index (TSI) of lakes in the upper trophogenic water layer in the pelagial zone along the trophic gradient (from oligo/mesotrophy to hypereutrophy) in 19 lakes of the Mazurian Lake District (northeastern Poland). Multiple regression analysis (analysis of variance-ANOVA) on all collected data and the TSI along eutrophication gradient showed that all studied microbial processes and parameters were very tightly coupled to the trophic conditions of the studied lakes. All studied microbial processes involved in utilization and enzymatic degradation of organic matter were strongly positively dependent on the intensity and rates of photosynthetic organic matter production and exudation that markedly increased along the eutrophication gradient of lakes. V max of alkaline phosphatase, aminopeptidase, and nonspecific esterase showed significant correlation with the TSI of the studied lakes. Protozoans removed a significant portion of bacterial production, i.e., from ϳ20% to 75-85% of newly produced bacterial biomass was simultaneously consumed by protozoans along the eutrophication gradient. These observations suggest that the importance of protozoan grazing on bacteria on regulation of bacterial production depends on lake productivity. The general working hypothesis that the intensity of microbial processes of organic matter can be tightly coupled to increasing eutrophication was proven in these studies.

No abstract

This report presents results on relationships between the kinetics (Vmax and Km) of β-glucosidase (GLCase) and aminopeptidase (AMPase) activity, and dissolved organic carbon (DOC) and bacterial secondary production in freshwater mesocosms of differing degrees of eutrophication. These relationships varied in different mesocosms and depended on the trophic status of water and the exudation rates of organic carbon (EOC) by phytoplankton. Close coupling of bacterial production to Vmax of GLCase activity was observed only in nutrient-enriched mesocosms. The relationship between GLCase and DOC content was also significant in enriched water. There was no correlation between the Vmax, of GLCase and DOC and bacterial production in nutrient-impoverished and control (mesotrophic) enclosures. However, the Vmax of AMPase correlated well to DOC and bacterial production in these mesocosms. AMPase activity did not correlate with DOC and bacterial production in nutrient-impoverished mesocosms. There was no relationship between bacterial biomass and enzyme activity in all studied mesocosms. Comparison of the rates of phytoplankton production of EOC and rates of the bacterial organic carbon demand (BOCD) in nutrient-impoverished mesocosms showed that EOC flux constituted, on average, 90% of BOCD. However, in nutrient-enriched mesocosms EOC contributed only, on average, 27% to the BOCD; thus, in these mesocosms, bacteria were probably organic-carbon limited. It is hypothesized that to bypass substrate limitation, bacteria produced GLCase and AMPase. These enzymes had a high specific activity and high affinity to their substrates and efficiently hydrolyzed polysaccharides and proteins, thereby supplying microorganisms with readily utilizable products of enzyme catalysis.