Microbial production, utilization, and enzymatic degradation of organic matter in the upper trophogenic layer in the pelagial zone of lakes along a eutrophication gradient

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. Vmax 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 ;20% 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. The view of ecophysiology of aquatic microorganisms has changed dramatically in the last two decades. Several sophisticated biochemical and molecular approaches introduced to microbial ecophysiology (Overbeck and Chróst 1990; Kemp et al. 1994; Cooksey 1998) resulted in completely new ideas on the role and significance of microbial processes in the functioning of aquatic ecosystems (Azam and Worden 2004). Recent aquatic prokaryotic metagenome sequencing is providing a tremendous database for discovering metabolic capabilities and new ways to conceptualize and study microbial biodiversity (Venter et al. 2004). Environmental genomics is also revealing complex interactions of bacteria with other aquatic biota and ecosystem function. The results of these studies clearly show that bacteria are a major biological force in nutrient cycles and ecosystem structure. Nowadays, it is commonly accepted that heterotrophic bacteria play a substantial and dominating role in the cycling of organic and inorganic matter in pelagic waters (Azam and Smith 1991). Microorganisms influence ecosystems through molecular interactions. Because of their high abundance, 1 Corresponding author ([email protected]).