Bacterial growth and grazing on diatom aggregates: Respiratory carbon turnover as a function of aggregate size and sinking velocity

Bacterial growth, respiration, particulate organic carbon (POC), and partic ulate organic nitrogen (PON) were measured directly on differentially sized diatom aggregates incubated individually in suspension in order to study t he coupling between these parameters under controlled conditions. After 3 d of incubation, bacteria, flagellates, and ciliates were present on aggrega tes in the ratio of 1,100:30:1. Bacterial generation times ranged from 0.4 to 2 d, but these short generation times did not result in an increase of b acterial abundance because bacteria were grazed approximately at similar ra tes. The entire microbial community respired 2.90 carbon units for each car bon unit incorporated by the bacteria. Bacterial production, community resp iration, POC, and PON increased with increasing aggregate size, and respira tion was proportional to POC and PON content. The POC specific respiration rate on aggregates was 0.083 d(-1), and 40% of the initial POC content was respired after 6 d. From simple calculations combining carbon-specific resp iration rates and aggregate sinking velocities, it is concluded that a tigh t coupling between POC and microbial respiration may control the carbon flu xes of sinking diatom aggregates in the ocean.