Bacterial production and growth efficiencies: Direct measurements on riverine aggregates

Heterotrophic bacteria transform organic matter by respiration and producti on of new biomass. Because there are only a limited number of studies on th e respiration of bacteria attached to particulate organic matter, their rol e in the carbon cycle of aquatic systems is not well known. In this study, we combine radiotracer with microsensor techniques to measure bacterial pro duction and respiration rates on the same aggregate and to directly determi ne 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 com munity was analyzed by in situ hybridization. The growth efficiency was 0.4 5 +/- 0.04 (SE) on 1-3-d-old aggregates, anti it was independent of the gro wth rate (mu). There was no correlation between respiration and the particu late organic carbon (POC) or particulate organic nitrogen (PON) content of the same aggregate. Bacterial growth efficiencies on aggregates decreased a fter 5 d of incubation, as bacterial production decreased and respiration i ncreased. 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 bacteria l production was thus apparently substrate limited. Respiration was then co rrelated 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%, r espectively, whereas those of the gamma-subclass of filamentous Proteobacte ria and Cytophaga increased from 31.9% to 50.4% and from 8.5% to 24.9%, res pectively, during the 14 d of incubation. These results demonstrate that ba cterial production and respiration on aggregates are dependent on the bacte rial community and the substrate composition of aggregate. High growth effi ciencies of aggregate-associated bacteria, especially during the first days of colonization, suggest that aggregates are spots of high bacterial growt h where a rapid and efficient transfer of organic matter into bacterial bio mass takes place.