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Soil sulfur metabolism surprise
Soil bacteria have a range of metabolic pathways that contribute to acquiring and recycling nutrients and carbon. Curiously, some of these organisms give off ethylene gas when starved for sulfur under anaerobic conditions. North et al. traced the source of ethylene to a small, sulfur-containing organic molecule produced by certain reactions in cells. Growing cells in sulfur-limiting conditions enabled them to identify the enzymes involved in sulfur salvage, and the concomitant ethylene production, through this pathway. Methane and ethane were also observed as products when appropriate substrates were provided. The key genes involved are distantly related to nitrogenase and several other reductase enzymes found in bacteria and archaea. The involvement of such nitrogenase-like genes in sulfur metabolism highlights the potential of unexplored diversity in this family of enzymes and raises many mechanistic and evolutionary questions that are now ripe for exploration.
Science, this issue p. 1094
Abstract
Bacterial production of gaseous hydrocarbons such as ethylene and methane affects soil environments and atmospheric climate. We demonstrate that biogenic methane and ethylene from terrestrial and freshwater bacteria are directly produced by a previously unknown methionine biosynthesis pathway. This pathway, present in numerous species, uses a nitrogenase-like reductase that is distinct from known nitrogenases and nitrogenase-like reductases and specifically functions in C–S bond breakage to reduce ubiquitous and appreciable volatile organic sulfur compounds such as dimethyl sulfide and (2-methylthio)ethanol. Liberated methanethiol serves as the immediate precursor to methionine, while ethylene or methane is released into the environment. Anaerobic ethylene production by this pathway apparently explains the long-standing observation of ethylene accumulation in oxygen-depleted soils. Methane production reveals an additional bacterial pathway distinct from archaeal methanogenesis.
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