CO2 Reduction by the Iron Nitrogenase Competes with N2 Fixation Under Physiological Conditions

Abstract

Nitrogenases are the only known enzymes that reduce molecular nitrogen (N2) to ammonia. Recent findings have demonstrated that nitrogenases also reduce the greenhouse gas carbon dioxide (CO2), suggesting CO2 to be a competitor of N2. Intriguingly, nitrogenase isoforms (i.e., molybdenum (Mo), vanadium and iron (Fe) nitrogenase) differ significantly in their ability to reduce CO2, but the mechanisms underlying these differences remain elusive. Here, we study the competing reduction of CO2 and N2 by the two nitrogenases of Rhodobacter capsulatus, the Mo and Fe nitrogenase. Analyzing their full CO2 reduction product spectrum in vitro, we find the Fe nitrogenase almost three-fold more efficient in CO2 reduction than the Mo isoform. Furthermore, the in vitro competition experiments reveal the Fe nitrogenase to be profoundly less selective for the reduction of N2 than the Mo nitrogenase. We observe the same effects in vivo, where adding CO2 drastically increases the doubling times of diazotrophically grown R. capsulatus strains that rely on the Fe nitrogenase. The Fe nitrogenase-dependent R. capsulatus strains reduce CO2 to methane under physiological conditions, highlighting the potential of the Fe nitrogenase for the biotechnological conversion of CO2 into value-added compounds. Furthermore, both products are secreted into the surrounding, potentially influencing the composition of microbial communities in Mo-deficient environments.