Marine archaea couple methane oxidation to sulfate reduction in a newly identified pathway that produces zero-valent sulfur species (Nature, DOI: 10.1038/nature11656). The finding upends scientists’ understanding of how microorganisms consume methane before the greenhouse gas can be released from seafloor sediments and escape to the atmosphere.
Undersea fossil reservoirs commonly discharge methane at the seafloor, yet relatively little of the gas reaches the ocean or atmosphere. Instead, microorganisms living in marine sediment consume it, but the processes are poorly understood. Research over the past few decades suggested that anaerobic methanotrophic archaea and sulfate-reducing bacteria might team up to chew up the methane. The proposed mechanism involved the archaea oxidizing methane. H2 or an organic compound shuttled electrons to the bacteria, which reduced sulfate.
The new work overturns that model, at least for one group of archaea. A team led by postdoctoral researcher Jana Milucka and professor Marcel M. M. Kuypers of Germany’s Max Planck Institute for Marine Microbiology studied microbial species cultured from mud volcano sediment in the Mediterranean Sea. They found that the archaea both oxidize methane to carbon dioxide and reduce sulfate to zero-valent elemental sulfur species, which they think probably take on colloidal form.
In the reducing, sulfide-rich environment of the seafloor, some of that colloidal sulfur likely gets converted to HS2–, the researchers think. Seafloor bacteria then consume the HS2– and transform it to more sulfide and sulfate.
Milucka and colleagues are now working to identify the enzymes and intermediates involved in archaeal sulfate-to-S0 reduction. “That’s the most exciting question now,” Milucka says. She adds that the team believes other seafloor archaea use the same chemistry too.
The work provides needed insight into the important global-scale problem of how archaea and bacteria manage to consume methane gas, says David L. Valentine, a professor of earth science at the University of California, Santa Barbara.