Multiple roles of siderophores in free-living nitrogen-fixing bacteria.

Free-living nitrogen-fixing bacteria in soils need to tightly regulate their uptake of metals in order to acquire essential metals (such as the nitrogenase metal cofactors Fe, Mo and V) while excluding toxic ones (such as W). They need to do this in a soil environment where metal speciation, and thus metal bioavailability, is dependent on a variety of factors such as organic matter content, mineralogical composition, and pH. Azotobacter vinelandii, a ubiquitous gram-negative soil diazotroph, excretes in its external medium catechol compounds, previously identified as siderophores, that bind a variety of metals in addition to iron. At low concentrations, complexes of essential metals (Fe, Mo, V) with siderophores are taken up by the bacteria through specialized transport systems. The specificity and regulation of these transport systems are such that siderophore binding of excess Mo, V or W effectively detoxifies these metals at high concentrations. In the topsoil (leaf litter layer), where metals are primarily bound to plant-derived organic matter, siderophores extract essential metals from natural ligands and deliver them to the bacteria. This process appears to be a key component of a mutualistic relationship between trees and soil diazotrophs, where tree-produced leaf litter provides a living environment rich in organic matter and micronutrients for nitrogen-fixing bacteria, which in turn supply new nitrogen to the ecosystem.