Methanogens. These are a very broad group of bacteria that contribute to the formation of methane as part of their metabolism. This process is anaerobic (not O2 involved), in fact O2 can be an inhibitor for the normal growth and metabolism of these microorganisms. In the oil and gas industry they have started to play a role in cathodic depolarization which has the potential to cause corrosion rates similar to SRB. There are two groups of methanogens based on the source of carbon they used. Acetoclastic methanogens use acetate and hydrogenotrophic methanogens use CO2 as a carbon source and as a final electron acceptor (they “breathe” a form of carbon instead of oxygen). Most of these organisms oxidize organic matter but, some hydrogenotrophic methanogens can cause direct steel corrosion by direct iron oxidation (Skovhus et al. 2017). On the other hand, acetoclastic methanogens can live in close proximity of SRB and benefit from their products. Some acetoclastic bacteria include members of the Methanosaeta genus. Hydrogenotrophic methanogens include members of the Methanogenium, Methanoplanus, and Methanocalculus genera, among others.