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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.

Oxygen will penetrate through approximately 0.5 millimeters of a stationary biofilm.[1]



  1. ( 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.