The effects of inorganic nutrients (N, P) enrichment of mesotrophic lake water on changes in bacterial and protistan (heterotrophic nanoflagellates and ciliates) communities compositions were studied in the mesocosm experiment. Phosphorus (PO₄³⁻) and nitrogen (NH₄⁺) alone and in combination were added to three types of experimental mesocosms. Mesocosms results suggested that simultaneous addition of P and N stimulated phytoplankton growth and production rates of bacterial biomass its turnover rate. Strong positive correlations between chlorophyll a and bacterial secondary production rates suggested that bacteria were mainly controlled by organic substrates released in course of phytoplankton photosynthesis. Both nutrients increased distinctly protistan biomass and resulted in the shift in ciliate community composition from algivorous to large omnivorous species. The response of bacterial numbers and biomass to nutrients addition was less evident. However, intensive grazing caused their dynamic changes. Fluorescence in situ hybridization (FISH) revealed only small changes in bacterial taxonomic composition. There was an apparent shift in dominance from Cytophaga-Flavobacterium to the Alphaproteobacteria group in the mesocosm with simultaneous addition of P and N, which positively related to increased abundance of bacterivorous protists. Experiment demonstrated that inorganic N and P nutrients directly influenced the bottom-down control of microbial communities, which had a crucial effect on morphological diversity of bacteria.
Effects of mesotrophic lake water enrichment with organic phosphorus and nitrogen substrates (DNA and model protein, bovine serum albumin - BSA) on dynamics and diversity of natural microbial communities (bacteria, heterotrophic nanoflagellates, ciliates) were studied in mesocosm experiments. Simultaneous enrichment with DNA and BSA strongly increased the abundance and biomass of all studied groups of microorganisms and induced changes in their morphological and taxonomic structure. The increased participation of large heterotrophic nanoflagellates cells (larger than 10 μm) in their total numbers and shifts in taxonomic and trophic Structure of the ciliates, from algivorous to small bacterivorous, species were observed. Grazing caused changes in bacterial size distribution in all enriched mesocosms. Large (10-50 μm) filamentous bacteria significantly contributed to the total bacterial numbers and biomass. Pronounced increase in populations of (β- and γ-Proteobacteria was found in lake water enriched with organic P and N sources, whereas α-Pmteobacteria did not change markedly in the studied mesocosms. DNA additions stimulated the rates of bacterial secondary production. BSA shortened the rates of bacterial biomass turnover in lake water. Relatively high and constant (~ 30%) percentage contribution of active bacteria (MEM+) in two mesocosms enriched with DNA and DNA+BSA suggested the important role of nucleic acids as a source of phosphorus for bacterial growth, activity and production. Numerous and statistically significant correlations between bacteria and protists indicated the direct and selective predator-prey relationship.
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