Production of heterotrophic bacteria inhabiting macroscopic organic aggregates (marine snow) from surface waters1

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Abstract
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.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%

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