The ability of some aerobic microorganisms to reduce nitrogen oxides, especially nitrate, to gaseous products enables them to grow in oxygen-limiting conditions by using the oxides as electron acceptors. Denitrifying respiration is not as efficient as oxygen respiration so growth is slower. The ability to denitrify is found in a wide variety of unrelated bacteria and even in some fungi, and biochemical mechanisms are now moderately well understood. Major factors regulating denitrification are the availability of the nitrogen oxides, the availability of reductant (mostly organic carbon compounds but inorganic compounds may be used), and decreased oxygen concentration. These three factors are in turn governed by many other factors such as water content, pH, porosity, and the presence of inhibitory compounds, which may act to cause accumulation of ionic (nitrite) or gaseous (nitric oxide, nitrous oxide) intermediates. The prediction of rates of denitrification and release of intermediates such as nitrous oxide in particular environments is proving to be difficult.