Methanogens are strict anaerobes which share a complex biochemistry for methane synthesis as part of their energy metabolism. Methanogenic bacteria are abundant in habitats where electron acceptors such as O2, NO3−, Fe3and SO42are limiting. Common habitats for methanogens are anaerobic digestors, anoxic sediments, flooded soils, and gastrointestinal tracts. Methanogens are generally absent from the water column of unstratified lakes and rivers because convection currents rapidly aerate the deep waters. However, the diffusion of O2 between the layers of stratified lakes is often too slow to maintain oxic conditions in the lower layers. The methanogens are widely distributed in nature, but confined to strictly anaerobic environments. In addition to aquatic sediments (ponds, marshes, swamps, rice soils, lakes, and oceans), other methanogenic habitats include the intestinal tracts of man and animals (especially the rumen of herbivores), sewage digesters, landfills, heart wood of living trees, hot springs, decomposing algal mats, oil wells, and mid-ocean ridges. In these habitats, the methanogens occupy the terminal niche in the transfer of electrons generated by the anaerobic degradation of organic matter.
