| Abstrakt: |
Methanogens are represented across a diverse range of aquatic ecosystems, including habitats that are driven principally by geochemically-supplied substrates (e.g., hot springs and hydrothermal vents, volcanically-influenced settings, and, potentially, the deep crustal subsurface) and those dependent on substrates derived from organic matter (e.g., aquatic sediments, wetlands, agricultural or natural soils subject to inundation, sewage digesters, and the anoxic portions of animal digestive tracts). Similarly, they are tolerant to a broad range of physicochemical conditions, including temperatures from 0 to 122°C, pH values of 3–10, salinities from 0 to halite saturation, and pressures of at least 75 MPa. In general, methanogen distribution is constrained by ecological interactions (both stimulatory and competitive) and/or physicochemical environmental factors that act at biochemical or bioenergetic levels. In addition to physicochemical ˵extremes″ (principally temperature, salinity, and pH), the environmental distribution of methanogens is constrained to a large degree by energy availability and by environmental distributions of oxygen (biochemical inhibition) and the seawater anion sulfate (competitive effects that act at a bioenergetic level). Although methanogen tolerances to individual extremes are documented in culture, and the corresponding biochemical adaptations understood to varying degrees, the natural environment frequently presents combinations of extremes, combined with energy limitation, that may serve to limit methanogen distribution to less than the optimally tolerated range. Little is understood about the compound effects of such extremes, nor the commonalities among them that will ultimately form the basis for predictive models of environmental population distribution. Future work that targets these questions, through a combination of culture work, in situ studies, and theoretical (conceptual and quantitative) models, represents the way forward in better understanding the physiological ecology of methanogens. [ABSTRACT FROM AUTHOR] |
