Methanogens and methanogenesis in hypersaline environments

Methanogenesis in hypersaline environments is determined by redox potential and permanency of anaerobic conditions, and by the concentration of other terminal electron acceptors, particularly sulfate, because sulfate-reducing bacteria have a greater affinity than methanogens for competitive substrates like hydrogen and acetate. Hypersalinity, however, is not an obstacle to methanogenesis; in many cases it provides higher concentrations of noncompetitive substrates like methylamines, which derive from compatible solutes such as glycine-betaine that is synthesized by many microbes inhabiting hypersaline environments. Also, depletion of sulfate, as may occur in deeper sediments, allows increased methanogenesis. On the other hand, increasing salinity requires methanogens to synthesize or take up more compatible solutes at a significant energetic cost. Aceticlastic and hydrogenotrophic methanogens, with their lower energetic yields, are therefore more susceptible than methylotrophic methanogenesis, which further explains the predominance of methylotrophic methanogens like Methanohalophilus spp. in hypersaline environments. There are often deviations from the picture outlined above, which are sometimes difficult to explain. Identifying the role of uncultivated Euryarchaeota in hypersaline environments, elucidating the effects of different ions (which have differential stress effects and potential as electron acceptors) and understanding the effects of salinity on all microbes involved in methane cycling, will help us to understand and predict methane production in hypersaline environments.