Links of Extracellular Enzyme Activities, Microbial Metabolism, and Community Composition in the River-Impacted Coastal Waters

摘要

Extracellular enzymatic activity (EEA), production, respiration, and composition of the bacterial community were determined along environmental gradients in the river-impacted coastal waters in the South China Sea during summer in order to examine the links of EEA, metabolic activity, and composition of the bacterial community. The initial step of substrate utilization was coupled with metabolic activity of the bacterial community. The activity of leucine aminopeptidase was closely related to the level of phytoplankton biomass, suggesting that leucine-containing compounds primarily resulted from phytoplankton, which was likely partly responsible for the close link between bacterioplankton and phytoplankton in marine environments. A shift from the dominance of leucine aminopeptidase in EEA in eutrophic environments to chitobiases in pristine waters was attributed to bacterial preference for leucine-containing compounds and spatial variability in substrate supply. Spatial pattern of the enzymatic rates along an environmental gradient differed at the community and the cellular level. The enzymatic rates of the bacterial community decreased offshore, which were linked to substrate supply and the bacterial community composition. However, cell-specific enzymatic rates were modulated by not only the bacterial community composition but also the carbon partition between anabolic and catabolic processes as environmental conditions changed, both of which eventually altered bacterial abundance. Functional redundancy of bacterial groups existed despite shifts in the bacterial community composition, suggesting that the bacterial community might maximize their function of hydrolyzing substrates across marine environments. Our findings improved the understanding of the connection of extracellular enzymatic activity, metabolic activity, and community composition of bacterioplankton.