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 affin
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K. N. Timmis (ed.), Handbook of Hydrocarbon and Lipid Microbiology, DOI 10.1007/978-3-540-77587-4_53, # Springer-Verlag Berlin Heidelberg, 2010
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Methanogens and Methanogenesis in Hypersaline Environments
Abstract: 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.
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Introduction
Hypersaline environments are simply defined as those with a greater concentration of salts than seawater. Such environments are many and varied, in terms of their overall salinity and predominant ions. Coastal environments, both man-made and natural are subject to desiccation, resulting in a wide variety of habitats from small, ephemeral salt pans within temperate salt marshes to large, permanently hypersaline sabkhas in sub-tropical regions (Hovorka, 1987). Similarly, inland salt lakes can be as large as the Great Salt Lake or a tiny spring. Salt deposits, often several hundred meters in thickness, lie beneath about a quarter of the Earth’s landmass, and contain brines from a cubic micrometer in volume to many cubic meters. Hypersaline environments are widespread and were more prevalent in former geological times, for example much of northern Europe was covered by the salt-saturated Zechstein Sea during the Permo-Triassic period (Zharkov, 1981), and the Mediterranean Sea was desiccated more recently, with halite precipitation starting between 5.6 and 5.55 million years ago (Hsu¨ et al., 1973). Deep-sea, anoxic, hypersaline brines, derived from dissolution of such ancient evaporites, form large la
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