Effect of nutrient enrichment on bacterioplankton biomass and community composition in mesocosms in the Archipelago Sea, northern Baltic

Effects of nutrient enrichment on the biomass and community composition of heterotrophic bacteria and picocyanobacteria were studied in large (42 m(3)) mesocosms in the brackish-water Archipelago Sea (Baltic Sea) in late summer 2000 using cell counts and denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction (PCR)-amplified 16S rRNA gene fragments. The identity of the major DNA bands was determined by sequencing. The obtained sequences were related to alpha- and gamma-proteobacteria, actinobacteria, verrucomicrobia and cyanobacteria. Nitrogen and phosphorus additions increased the biomasses of heterotrophic bacteria and picocyanobacteria and caused significant changes in their community composition judging from the DGGE banding patterns. Most verrucomicrobial bands had their highest relative intensity in the control treatment and their lowest in the higher nutrient addition treatment, whereas most Synechococcus-related bands had their lowest relative intensity in the lower nutrient addition treatment. The responses of proteobacteria and actinobacteria were more variable. The presence of both freshwater and marine sequences among the closest relatives to our sequences highlights the intermediate character of the Archipelago Sea between a freshwater and truly marine environment.