Marine bacteria and biogeochemical cycling of iron in the oceans

Prokaryotic microbes play a critical role in oceanic Fe cycling. They conta in most of the biogenic Fe in offshore waters and are responsible for a lar ge portion of the Fe uptake by the plankton community. In the subarctic Nor th Pacific, surface populations of heterotrophic species assimilate more th an 50% of the dissolved Fe and thus compete directly with phytoplankton for this limiting resource. In oligotrophic tropical and subtropical waters, p hotosynthetic 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 abundan t. Like their terrestrial counterparts, heterotrophic and phototrophic mari ne bacteria produce Fe-binding siderophores that are involved in Fe acquisi tion. Evidence exists that bacteria may modify Fe chemistry in the sea thro ugh the production of these ligands and thereby play a significant role in regulating production of eukaryotic phytoplankton. (C) 1999 Federation of E uropean Microbiological Societies. Published by Elsevier Science B.V. All r ights reserved.