"Has the moon been moving away from planet earth from the beginning? Also, does it keep moving away faster or has the rate always been about the same?"
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Acetylene as a Substrate for the Development of Anaerobic Microbial Ecosystems on Primordial Earth: Implications for Microbial Life on Planets and Satellites with Jovian/Titan-like Atmospheres.
PI: Oremland, Ronald
Acetylene is present in substantial quantities in the hydrogen-rich, reducing atmospheres of Jovian planets, as well as that of Saturn’s moon Titan. In these cases, acetylene arises as a reactive intermediate initiated by the photolysis of the abundant methane also present therein. In contrast, acetylene is only an anthropogenic trace component of the Earth’s current atmosphere. Nonetheless, a hydrogen- and methane-rich primordial atmosphere has been hypothesized for that of the early Earth, in part as a means of retaining heat (greenhouse warming) to counter the effects of the sun’s lower luminosity ~ 4 Ga years ago. If this was the case, the Earth’s early anoxic atmosphere would also have been correspondingly rich in acetylene. This poses a biological paradox because acetylene is a potent inhibitor of a number of anaerobic microbial processes thought to have been critical in the development of the primordial microbial ecosystems and the biogeochemical cycles of the early Earth. These include (but are not limited to) methanogenesis, anaerobic methane oxidation, nitrogen fixation, and hydrogen oxidation. Curiously, anaerobic fermentation of acetylene was discovered 25 years ago as a serendipitous consequence of the use of this gas to block N 2 O reductase activity in sediment denitrification assays. One microorganism, Pelobacter acetylenicus, has been isolated and shown capable of fermentative growth on acetylene. This it achieves by virtue of its unique low-potential tungsten-containing enzyme, acetylene hydratase, which results in the exothermic formation acetaldehyde. Acetaldehyde subsequently dismutates to ethanol and acetate (plus some hydrogen), substrates that can readily couple with either sulfate reduction or methanogenesis. However, acetylene hydratase is specific for acetylene and will not react with any other compounds, including a number of triply bonded analogs. Thus an additional biological paradox is posed, namely why does acetylene hydratase exist when this gas has no current biological sources and is not present in any reasonable abundance in the Earth’s present troposphere? We hypothesize that microbes which possessed acetylene hydratase played a key role in the evolution of the Earth’s early biosphere by exploiting a readily fermentable source of carbon (“acetylenic-manna”) from the atmosphere (as well as from geothermal and other sources), and in doing so formed protective niches that allowed for other anaerobic processes to flourish within. We hypothesize that the ability to ferment acetylene has been retained by the microbial gene-pool as an evolutionary advantage to exploit the acetylenic-manna during episodes of global anoxia in the Earth’s past. In this proposal we will survey a number of diverse anaerobic sediments and soils for their ability to consume acetylene, and will determine the lower substrate affinities of these materials for the gas. We will also employ both classic culturing and modern culture-independent techniques to determine the microbial diversity of acetylene fermenting prokaryotes in nature. The work proposed here will contribute to our understanding of the potential nature of Archaean-era microbial metabolisms and the role of acetylene in the evolution of the biosphere and Earth’s early atmosphere. Furthermore, the presence of acetylene in the atmosphere of a planet(oids) could be construed, under certain circumstances, as evidence for an extraterrestrial anaerobic microbial ecosystem. Thus, it is relevant to several of NASA’s dominant themes, including the search for signs of extant (or extinct) life outside of the Earth’s biological envelope.
