Intellectual merit: N2 fixation is effected exclusively by bacteria and archea using the enzyme nitrogenase, which contains Fe and Mo as metal cofactors in its most active form, and Fe and V, or Fe-only in its alternative forms. The aim of this project is to elucidate how the mechanisms and kinetics of bacterial acquisition of Fe, Mo and V from soils may limit or control N2 fixation rates. N2-fixing bacteria produce siderophores (or ?iron carriers?) to bind iron in the external medium and take up the resulting Fe-siderophore complexes. Our recent work has shown that the siderophores are actually ?metallophores? used in the uptake of Mo and V, along with Fe. The biological acquisition of the various nitrogenase metal cofactors thus depends on their binding by bacterial metallophores. This project is organized around two major hypotheses: 1. Free-living N2-fixing bacteria excrete metallophores that are particularly efficient at capturing the metal (Mo, V or Fe) that is limiting N2 fixation; 2. Because of both competition with other metals for metallophore binding and scarcity in soils, Mo is inherently difficult to acquire and free-living N2-fixing bacteria often use alternative nitrogenases, particularly the V-nitrogenase, to fix N2. These hypotheses will be tested through a combination of laboratory and field experiments. Field studies will focus on sites where Mo may be limiting and employ molecular biological techniques to identify alternative nitrogenases.
Broader Impacts: One postdoctoral researcher, one graduate student and several undergraduates will work on this project and be mentored by the PIs. Students from the local junior colleges will participate in the summer field research. The PIs will participate in the Quest summer program for school teachers.
Nitrogen fixation is an important mechanism for ecosystems to self-fertilize by removing the limitations of nitrogen as a limiting nutrient. However, nitrogen fixation can only be performed by bacteria and archea micro-organisms, not the plants that are typically limited by nitrogen availability. It is well known that specific conditions, and in particular the availability of trace metals, limit nitrogen fixation in aquatic systems. However, it has been assumed that metals, which should be abundant in soils, do not limit nitrogen fixation in terrestrial ecosystems. The overall goal of this project is to understand the mechanisms of metal limitation of N2 fixation in soils. This project has identified for the first time that iron, and sometimes molybdenum, is limiting to asymbiotic nitrogren fixation in forest litter. This limitation appears to be a function of landscape position, and possibly of metal concentrations in litter. It may be that this N-fixation is significant enough to promote litter decomposition rates. This is a collaborative project being led by Anne Kraepiel and Francois Morel at Princeton University (NSF award #1024553). Collaborators and their institutions include Susan Brantley (Penn State University, NSF award #1024559) and Anthony Aufdenkampe (Stroud Water Research Center, this award, #1024545). The role of the Stroud Water Research Center in carrying out the overall project objective is to assist with field measurements within the study sites of the Christina River Basin Critical Zone Observatory (CRB-CZO).
