PRODUCTION OF HETEROTROPHIC BACTERIA INHABITING MACROSCOPIC ORGANIC AGGREGATES (MARINE SNOW) FROM SURFACE WATERS

Macroscopic detrital aggregates, known as marine snow, are a ubiquitous and abundant component of the marine pelagic zone. Descriptions of microbial communities occurring at densities 2-5 orders of magnitude higher on these particles than in the surrounding seawater have led to the suggestion that marine snow may be a site of intense heterotrophic activity. We tested this hypothesis using incorporation of [3H]thymidine into macromolecules as a measure of bacterial growth occurring on marine snow from oceanic waters in the North Atlantic and from neritic waters off southern California [USA]. Abundances of marine snow ranged from 0.1 to 4.3 aggregates liter-1. However, only 0.1-4% of the bacteria in the water occurred in association with marine snow. Mean thymidine incorporation per cell on aggregates was generally equal to or lower than that of bacteria found free-living in the surrounding seawater, indicating that attached bacteria were not growing more rapidly than free-living bacteria. Bacteria inhabiting aggregates were up to 25 times larger than free-living forms. Thus, the contribution of these attached bacteria to total bacterial production in surface waters was low but occasionally significant, ranging from 3 to 26% and averaging 8 .+-. 7%.