AME 43:233-241 (2006)  -  doi:10.3354/ame043233

ABSTRACT: Bacterial abundance and assimilation of 3H-leucine were examined using a combination of microautoradiography and fluorescent in situ hybridization (Micro-FISH) to determine the bacterioplankton community structure and estimate the contribution of major bacterial groups to total bacterial biomass production (leucine incorporation) from the Pearl River mouth to the open water area in the South China Sea. Alpha-, beta-, gamma-proteobacteria and the Cytophaga-Flavobacterium cluster differed substantially in their relative abundance and contribution to leucine assimilation along the salinity gradient. The contribution of major bacterial groups to leucine assimilation was closely associated with their relative abundance in the communities. Alpha-proteobacteria were the dominant group in the high-salinity water in terms of abundance and 3H-leucine assimilation, whereas beta-proteobacteria were more important in fresh water. At all stations, gamma-proteobacteria were a minor component, but the Cytophaga-Flavobacterium cluster was always a significant component in both marine and freshwater systems. The relative abundance of bacterial groups explained the variation in 3H-leucine assimilation by the major bacterial groups to a great extent. At the singlebacterial group level, a statistically significant correlation between abundance and leucine-uptake activity was observed for beta-proteobacteria, suggesting that the relative abundance of betaproteobacteria in bacterioplankton communities was controlled by growth-related processes. In general, the numerical dominance of the major phylogenetic groups and their contribution to total bacterial biomass production varied consistently along the salinity gradient in the South China Sea.

KEY WORDS: Proteobacteria · Cytophaga-Flavobacterium · Community · Leucine assimilation · Bacterial biomass production · Microautoradiography · Fluorescent in situ hybridization