Bacterial production and growth efficiencies: Direct measurements on riverine aggregates

Heterotrophic bacteria transform organic matter by respiration and production of new biomass. Because there are only a limited number of studies on the respiration of bacteria attached to particulate organic matter, their role in the carbon cycle of aquatic systems is not well known. In this study, we combine radiotracer with microsensor techniques to measure bacterial production and respiration rates on the same aggregate and to directly determine the growth efficiency of the microbial community attached to aggregates. Aggregates of defined age were formed after incubation of water samples of the river Weser, Northern Germany, in roller tanks and their bacterial community was analyzed by in situ hybridization. The growth efficiency was 0.45 +/- 0.04 (SE) on 1-3-d-old aggregates, anti it was independent of the growth rate (mu). There was no correlation between respiration and the particulate organic carbon (POC) or particulate organic nitrogen (PON) content of the same aggregate. Bacterial growth efficiencies on aggregates decreased after 5 d of incubation, as bacterial production decreased and respiration increased. On 7- and 14-d-old aggregates, the growth efficiency was 0.23 +/- 0.06 and 0.04 +/- 0.01, respectively, and proportional to I-e The bacterial production was thus apparently substrate limited. Respiration was then correlated with both POC and PON content of the same aggregates. The changes in bacterial production and respiration occurred with concurrent changes in the bacterial community. The percentage of members of the alpha- and beta-subclass of Proteobacteria decreased from 13% and 33.7% to 2.6% and 9.0%, respectively, whereas those of the gamma-subclass of filamentous Proteobacteria and Cytophaga increased from 31.9% to 50.4% and from 8.5% to 24.9%, respectively, during the 14 d of incubation. These results demonstrate that bacterial production and respiration on aggregates are dependent on the bacterial community and the substrate composition of aggregate. High growth efficiencies of aggregate-associated bacteria, especially during the first days of colonization, suggest that aggregates are spots of high bacterial growth where a rapid and efficient transfer of organic matter into bacterial biomass takes place.