Bacterioplankton Roles in Cycling of Organic Matter: The Microbial Food Web

Abstract

More than a decade has passed since the realization that bacteria are quantitatively important consumers of organic carbon in marine food webs. The basic information on the significance of the microbial food web was put forth eloquently by Pomeroy (1974), who pieced together data from a variety of sources that all indicated a major role of small heterotrophs consuming dissolved and particulate material. However, these ideas did not gain wide recognition until the high abundance of marine bacteria was shown by epifluorescence microscopy (Ferguson and Rublee, 1976; Hobbie et al., 1977), and the bacterial heterotrophic production was shown to be large (i.e., 10–30%) compared to primary production (Hagstrőm et al., 1979; Fuhrman and Azam, 1980; 1982). With reasonable estimates of bacterial growth efficiency (i.e., near 50%), it became clear that heterotrophic bacteria consume an amount of carbon equivalent to approximately 20–60% of total primary production. Williams (1981) reached this conclusion when he synthesized the extant results on bacterial biomass and production. He also showed that “normal” well-known processes and mechanisms could lead to as much as 60% of the primary production becoming dissolved organic carbon (DOC), and subsequently, being taken up by bacteria. Azam et al. (1983) formalized the concept of the microbial loop by which significant quantities of organic matter are produced or processed through prokaryotic and very small eukaryotic organisms, eventually feeding into the larger macrozooplankton.