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

Abstract

Bacterial growth, respiration, particulate organic carbon (POC), and particulate organic nitrogen (PON) were measured directly on differentially sized diatom aggregates incubated individually in suspension in order to study the coupling between these parameters under controlled conditions. After 3 d of incubation, bacteria, flagellates, and ciliates were present on aggregates 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 bacterial abundance because bacteria were grazed approximately at similar rates. The entire microbial community respired 2.90 carbon units for each carbon unit incorporated by the bacteria. Bacterial production, community respiration, POC, and PON increased with increasing aggregate size, and respiration 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 respiration rates and aggregate sinking velocities, it is concluded that a tight coupling between POC and microbial respiration may control the carbon fluxes of sinking diatom aggregates in the ocean.