The role of microorganisms in iron reduction in marine sediments

Iron-reducing microbial activity may occur in marine sediments in zones where it is not outcompeted by abiotic reaction of three-valent (ferric) iron with sulfide. Because near neutral pH ferric iron prevails in solid form, microorganisms have developed different strategies for extracellular electron transfer. Microbial pili, nanowires, and electron shuttles have been observed in two microorganisms, Shewanella and Geobacter , to transfer electrons to iron oxide mineral surfaces. Alternatively, iron may be chelated and transported into the cell. However, other than in these two model microorganisms, the pathways of electron transfer are poorly understood. Previous diversity studies showed that a number of phylogenetic groups of both Bacteria and Archaea, capable of using iron as an electron acceptor, occur in marine sediments, some of which are versatile, capable of reducing various electron acceptors. These groups seem to be associated with zones of iron reduction, such as suboxic zones and sulfide-free methanogenic zones, but their pathways of iron reduction are largely unknown. We highlight the necessity to further elucidate electron transport pathways including extracellular systems, using molecular genetic studies, to understand their ecological role in natural systems such as in marine sediments.