Microbial food webs and the export of biogenic carbon in oceans

Microbial food webs, which comprise phototrophic picoplankton (or ultraplankton), heterotrophic bacteria and protozoa, are ubiquitous in marine waters. Members of the microbial food web may play various roles in the export of biogenic carbon. For example, it has been postulated that high bacterial activity may sometimes prevent a significant fraction of the production by large phytoplankton from reaching metazoan consumers. Phototrophit or heterotrophic small plankton cells may be readily exported (i.e. biological CO2 pump) if incorporated into large particles, through either endosymbiosis with larger cells, or development on material accumulated in hydrodynamic traps, or inclusion into marine snow, or grazing by large planktonic microphages (e.g. salps) and incorporation in their sometimes fast-sinking faecal pellets. Another aspect is the export of carbonate by members of the microbial food web (e.g. coccolithophores, foraminifera), which influences the carbon dioxide balance of the ocean (i.e. carbonate pump). Finally, carbon bound into refractory dissolved organic matter is chemically sequestered in the upper ocean, before being exported to depth. The activity of microbial food webs may therefore influence in several ways the export (and sequestration) of biogenic carbon in oceans.