Assessment of the Structure and Activity of Bacterial Communities as a Tandem Tool for Estimating the Ecological Condition of Lakes in Poland

Abstract The assessment of the qualitative and quantitative structure of microbial communities is vital for recognizing the role that microorganisms play in aquatic ecosystems. We investigated microbial communities using high-throughput sequencing of the 16S rRNA gene, and the population activity, using fluorescent markers. Three lakes (oligotrophic, eutrophic, and hypertrophic) were studied in spring, summer, and autumn 2021 in the Zaborski Landscape Park, northern Poland. The results showed that dead bacterial cells dominated in the tested water samples. Their distribution was mainly related to the trophic status of the studied lakes. Bacteroidetes, Proteobacteria, Cyanobacteria, Verrucomicrobia , and Actinobacteria predominated in bacterioplankton. At lower taxonomic levels, certain groups were associated specifically with the eutrophic Lake Niedźwiedzie, mainly bacteria of the Flavobacterium and Rheinheimera genera, which in spring accounted for 14.4% and 11.95% of the population, respectively. Our analyses demonstrated that the structure of bacterial communities (β-diversity) was strongly correlated with the trophic status of the studied lakes. This study indicates that the environmental gradient has a strong influence on the functional and taxonomic structure of bacterioplankton. Our study demonstrates that the assessment of the activity and spatial structure of bacterial communities can be a useful tool for determining differences in the ecological state of waterbodies. Moreover, knowledge about the factors shaping the structure of microbial communities can be helpful in responding to any disturbances in the functioning of lake ecosystems.