Marine bacteria and biogeochemical cycling of iron in the oceans

Prokaryotic microbes play a critical role in oceanic Fe cycling. They contain most of the biogenic Fe in offshore waters and are responsible for a large portion of the Fe uptake by the plankton community. In the subarctic North Pacific, surface populations of heterotrophic species assimilate more than 50% of the dissolved Fe and thus compete directly with phytoplankton for this limiting resource. In oligotrophic tropical and subtropical waters, photosynthetic bacteria become more important in Fe cycling as the number of unicellular cyanobacteria increases and the nitrogen-fixing Trichodesmium, which contains most of the biogenic Fe in the mixed layer, becomes abundant. Like their terrestrial counterparts, heterotrophic and phototrophic marine bacteria produce Fe-binding siderophores that are involved in Fe acquisition. Evidence exists that bacteria may modify Fe chemistry in the sea through the production of these ligands and thereby play a significant role in regulating production of eukaryotic phytoplankton.