Anaerobic oxidation of methane in marine sediments, of central importance for the global methane cycle, is a collaborative process performed by consortia of methane-oxidizing archaea and sulfate-reducing bacteria. The biochemical basis of this syntrophic relationship is not fully understood. It has been suggested that exchange of a diffusible metabolite between the cooperating microbes is essential, but two groups reporting in this issue of Nature challenge this idea. Victoria Orphan and colleagues examined the biosynthetic activity at the single-cell level in microbial consortia prepared from sediment sampled from an active methane seep at Hydrate Ridge North in the Northwest Pacific. They find that cell activities are independent of the distance between syntrophic partners, which is inconsistent with a model involving the diffusion of intermediates over short distances. Instead, direct electron transfer between archaea and bacteria, mediated by large multi-haem cytochromes produced by ANME-2 archaea, is a central mechanism of their interaction. Gunter Wegener et al. show that interspecies exchange of electrons in microbial samples derived from hydrothermal vent sediments from Guaymas Basin in the Gulf of California is most probably through direct transfer of electrons by means of ‘nanowires’ connecting the two partners. These authors propose that electron transfer is mediated by pili-like structures and outer-membrane multi-haem cytochromes.
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