Anaerobic refers a region having no oxygen present (with the oxidation reduction potential <-50 mV.), or an anaerobic organism meaning it has a metabolism that is based on something else other than oxygen.
Within a reef system, denitrification (conversion of nitrate to nitrogen gas) occurs in the anaerobic regions (includes interior regions of liverock, bottom layers of sand bed and inside a denitrator i.e. surfaces not directly in contact with the water). However, if the oxidation reduction potential is too low, <-50mV, then sulfate reduction (generating hydrogen sulphide), acid and methane formation can take over.
The presence of oxygen is referred to as aerobic.
- (http://dx.doi.org/10.1016/j.bej.2003.10.003): Hibiya, K., Nagai, J., Tsuneda, S., and Hirata, A., Simple prediction of oxygen penetration depth in biofilms for wastewater treatment, Biochemical Engineering Journal, 19(1), (2004), 61-68.