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. Abundances of marine snow ranged from 0.1 to 4.3 aggregates liter-‘. However, only O.l-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%. Flocculent, amorphous aggregates, ranging from 0.5 mm to many centimeters in longest dimension, are a ubiquitous and often abundant component of epipelagic marine waters. Although most microscopic particles in seawater are generally sparsely colonized by microbes (Wiebe and Pomeroy 1972; Hobbie et al. 1972; Azam and Hodson 1977), these macroscopic particles, known as marine snow, are inhabited by a rich detrital community of living phytoplankton, cyanobacteria, protozoa, and bacteria at densities two to five orders of magnitude higher than populations found freely dispersed in the surrounding seawater (Silver et al. 1978; Silver and Alldredge 198 1; L This research was supported by National Science Foundation grants OCE 80-2444 1, OCE 8 11299 1, and OCE 83-08615, by Ocean Industry Grant 4473, and by the Woods Hole Oceanographic Institution Education Program. Caron et al. 1982). Macroscopic aggregates are enrichment sites for organic matter (Alldredge 1979) and nutrients (Shanks and Trent 1979) and serve as food sources for various zooplankton and fish (Alldredge 1972, 1979). Moreover, they may contribute significantly to the flux of organic matter to the deep ocean (Shanks and Trent 1980; Fellows et al. 198 1; Joint and Morris 1982). Descriptions of the dense populations of microorganisms inhabiting marine snow have led to suggestions that these particles may be disproportionately important as sites for the biologically mediated transformation of organic matter in the pelagic zone (Karl 19 8 2). Macroscopic aggregates are known to be important sites for the incorporation of new carbon into neritic waters and the dense populations of phototrophs inhabiting marine snow can collectively photosynthesize at rates up to two orders of magnitude higher than the phototrophs in