Methanogens Through Geological Time And Space: Impact On Planetary Evolution and Significance For Life Beyond Earth
Methanogens, also known as methanogenic archaea, are among the most ancient and widespread microorganisms, despite their particular requirements for growth. These oxygen-sensitive microorganisms have impacted climate and biogeochemical cycles throughout Earth’s history, although their specific roles in the long-term carbon cycle remain little explored.
Methanogens evolved early during Earth’s history, likely during the Archaean Eon, in layered benthic microbial communities called microbial mats. These ancient mats, when lithified, form microbialites that represent some of the earliest evidence of life in the fossil record dating back > 3.5 Gy.
Contemporary microbial mats experience a wide range of fluctuating conditions, including dramatic diel shifts in oxygen, sulfide, redox, temperature, salinity and pH. Methanogens are an integral part of marine and freshwater microbial mats and have been identified in the oxic zone of these sedimentary ecosystems; however, their adaptations to apparently unfavorable conditions and their role in long-term CO2 sequestration through precipitation of carbonate are unclear.
Furthermore, the importance and coevolution of methanogens and microbial mats may explain the global role these organisms had on Earth’s major climate events during the Archean and Proterozoic eons, notably in the ending of icehouse periods and recovery of mats following mass extinctions – often in conditions with low or no oxygen. In addition to an important role in the evolution of our planet, methanogens may also produce biosignatures that are relevant for astrobiology research [and space exploration]. This review will discuss the diversity, physiology, and ecology of methanogens in order to clarify their role in biogeochemical processes through geologic time.
Methane in the carbon cycle during the Archean and Proterozoic Eons. (A) Atmospheric CH4 and O2 concentrations from ~4.0 Gy to 0.5 Gy, adapted from 57 The role of CH4 in the iron-rich oceans of the Archean, when there was a large flux of CH4 from methanogenesis (MA) in the oceans; sinks of methane included photochemical breakdown in the upper atmosphere and possible anaerobic oxidation of methane (AOM). The CH4 production exceeded removal, which resulted in a strong greenhouse effect and warming characteristic of the Archean atmosphere. Arrows show the cycling of CH4, including the flux of CH4 to the atmosphere by MA and the removal and decomposition of CH4. (C) CH4 and the carbon cycle in the Paleo-Proterozoic. Oxygen produced by cyanobacterial photosynthesis emerges as a novel oxidant for microbial removal (sink) of CH4 from the atmosphere. (D) Atmospheric CH4 is further reduced during the Meso-Neo-Proterozoic due to increased removal by oxic and anoxic methane oxidation, i.e., reverse methanogenesis coupled to sulfate and/or arsenate reduction, as well as through photochemical reaction of CH4 with O3 (through OH.). (E) CH4 production by syntrophic microbial processes through carbon degradation in sediments during the Archean and Meso-Neo-Proterozoic Eons. H2, and organic products (such as CH3COOH), from fermentation (FMT) could have supported MA in sediments. Anaerobic oxidation of methane (AOM) by sulfate reduction (SRF; shown here, but possibly also respiration using other electron donors such as nitrate, selenate, or arsenate (see Section 1.3.8) could have contributed to the removal of CH4 in sediments and promoted mineral formation (such as (arseno)pyrite). Active MA combined with other microbial processes [196] can drive carbonate mineral precipitation (such as CaCO3) in sediments and microbial mats. — preprints.org
Methanogens Through Geological Time and Space: Impact on Planetary Evolution and Significance for Life Beyond Earth, preprints.org
Astrobiology,
