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Photosynthesis and respiration were measured in 1 to 6 mm large aggregates (marine snow) collected in the Southern Californian Bight, USA. The aggregates were freely sinking in a vertical flow system with an upward flow velocity which opposed the sinking velocity of individual aggregates during the measurements. The aggregates were net heterotrophic communities at light intensities 500 μE m 2 S 1 . Bacterial densities on aggregates were >2000-fold higher than in the surrounding water. Cytophaga was highly abundant in the aggregate-associated bacterial community as identified by in situ hybridization techniques. Both the respiration rate per aggregate volume and the bacterial densities decreased with increasing aggregate size. The respiration rates normalized to the number of bacteria in single aggregates were 7.4 to 70 fmol C cell 1 d -1 , The aggregate community respired 433 to 984 ng C d -1 per aggregate in darkness, which yielded a turnover time of 8 to 9 d for the total organic carbon in aggregates. Thus, marine snow is not only a vehicle for vertical flux of organic matter; the aggregates are also hotspots of microbial respiration which cause a fast and efficient respiratory turnover of particulate organic carbon in the sea.

marine ecology. progress series

Photosynthesis, respiration, and carbon turnover in sinking marine snow from surface waters of Southern California Bight:implications for the carbon cycle in the ocean