Dissimilatory nitrate reduction to ammonium is a two step process, reducing NO to NO then NO to NH, though the reaction may begin with NO directly. Each step is mediated by a different enzyme, the first step of dissimilatory nitrate reduction to ammonium is usually mediated by a periplasmic nitrate reductase. The second step (respiratory NO reduction to NH) is mediated by cytochrome c nitrite reductase, occurring at the periplasmic membrane surface. Despite DNRA not producing nitrous oxide (NO) as an intermediate during nitrate reduction (as denitrification does), NO may still be released as a byproduct, thus DNRA may also act as a sink of fixed, bioavailable nitrogen. DNRA's production of NO may be enhanced at higher pH levels.