Ever since the symbiosis between anaerobic methanotrophic (ANME) archaea and sulfate reducing bacteria (SRB) was discovered a couple of decades ago, the inner workings of this close partnership has remained a mystery. What became clear relatively quickly was that the ANME appeared to oxidize methane in an energetically anemic process, and the SRB generated an energetic windfall by reducing sulfate, presumably with the electrons from their ANME neighbors. However, the precise nature of that electron-rich go-between remained unclear - diffusable molecules like hydrogen, formate, or acetate were all ruled out. A recent study out of the Orphan lab at Caltech (COI alert: my PhD home) suggests that ANME release reducing power through direct electron transfer - it's a compelling marriage of anabolic nanoSIMS data, modeling, and metagenomics analysis.
Full paper [here], and abstract below:
Multicellular assemblages of microorganisms are ubiquitous in nature, and the proximity afforded by aggregation is thought to permit intercellular metabolic coupling that can accommodate otherwise unfavourable reactions. Consortia of methane-oxidizing archaea and sulphate-reducing bacteria are a well-known environmental example of microbial co-aggregation; however, the coupling mechanisms between these paired organisms is not well understood, despite the attention given them because of the global significance of anaerobic methane oxidation. Here we examined the influence of interspecies spatial positioning as it relates to biosynthetic activity within structurally diverse uncultured methane-oxidizing consortia by measuring stable isotope incorporation for individual archaeal and bacterial cells to constrain their potential metabolic interactions. In contrast to conventional models of syntrophy based on the passage of molecular intermediates, cellular activities were found to be independent of both species intermixing and distance between syntrophic partners within consortia. A generalized model of electric conductivity between co-associated archaea and bacteria best fit the empirical data. Combined with the detection of large multi-haem cytochromes in the genomes of methanotrophic archaea and the demonstration of redox-dependent staining of the matrix between cells in consortia, these results provide evidence for syntrophic coupling through direct electron transfer.