Carbon monoxide production is not enhanced by nitrogenase activity

A diverse group of nitrogen-fixing bacteria and two heme degraders were grown with and without fixed nitrogen sources under oxic and suboxic conditions, with and without addition of heme-containing compounds. Several of the strains produced carbon monoxide (CO) under one or more of these conditions, but nitrogenase activity did not stimulate rates of production relative to controls. Although nitrogenase can reduce CO(2) to CO in vitro in the absence of N(2), this process likely contributes minimally to CO production in soils under in situ conditions. In contrast, myoglobin or hematin addition under oxic conditions significantly stimulated CO production by the heme degraders. However, estimates of CO production from microbial heme turnover suggest that this too is likely to be only a small source of CO in soils in situ.