Abstract
Assimilation of 3 HH‐thymidine and 3 HH‐leucine was examined at the single‐cell level using a combination of microautoradiography and fluorescent in situ hybridization (Micro‐FISH) to determine the contribution of various bacterial groups to bacterial production in aquatic systems. All of the major phylogenetic groups of bacteria examined along the salinity gradient of the Delaware estuary, including alpha‐, beta‐, and gamma‐proteobacteria and Cytophaga ‐like bacteria, assimilated 3 H‐thymidine and 3 H‐leucine. However, groups differed substantially in their contribution to the assimilation of these compounds. Alpha‐proteobacteria were the dominant substrate‐active bacteria at salinities of .9 PSU, whereas beta‐proteobacteria were more important in freshwater. At all salinities, Cytophaga ‐like bacteria comprised the second most important group, and gamma‐proteobacteria were overall the least important. Bacterial abundance explained about half of the variation in 3 H‐thymidine and 3 H‐leucine assimilation by the major bacterial groups. The sizes of silver grains of active bacteria indicate no difference in singlecell activity for the bacterial groups, suggesting that the average growth rates of the groups we examined were similar. However, activity per cell was distributed differently in the phylogenetic groups. Our study suggests that estimates of bacterial production measured using 3 H‐thymidine and 3 H‐leucine include bacteria in all of the major phylogenetic groups found in aquatic systems and that growth rates within bacterial groups vary substantially.