Bacterial diversity, community structure and potential growth rates along an estuarine salinity gradient

Very little is known about growth rates of individual bacterial taxa and how they respond to environmental flux. Here, we characterized bacterial community diversity, structure and the relative abundance of 16S rRNA and 16S rRNA genes (rDNA) along the salinity gradient in the Delaware Bay. Indices of diversity, evenness, structure and activity of the surface bacterial community significantly varied along the transect, reflecting active mixing between the freshwater and marine ends of the estuary. There was no positive correlation between relative abundances of 16S rRNA and rDNA for the entire bacterial community, suggesting that abundance of bacteria does not necessarily reflect growth rate. However, for almost half of individual taxa, 16S rRNA positively correlated with rDNA, indicating growth rate did follow abundance in these cases. Additionally, presumed growth rates of some taxa reflected differences in light, nutrient concentrations and other environmental factors along the estuarine gradient. These data suggest that physical and other bottom up factors control growth rates but not necessarily biomass of individual taxa in this highly variable estuarine environment.