A microbe that breathes rocks has entered the record books. Scientists at the University of Vienna and collaborators have uncovered a new metabolism, where bacteria oxidize toxic sulfide and reduce iron minerals for energy. The team reports that these “MISO bacteria” turn dangerous hydrogen sulfide into sulfate faster than chemistry alone, potentially slowing the spread of oxygen-free dead zones in oceans and wetlands. The study, published in Nature, suggests these organisms may account for up to 7 percent of global sulfide removal in marine sediments.
A new twist in Earth’s sulfur and iron cycles
The cycles of carbon, nitrogen, sulfur, and iron keep Earth habitable. For decades, geochemists assumed the reaction between sulfide and iron oxides was purely chemical, forming black iron monosulfide. This work shows otherwise: life is in charge. Using the cultivated bacterium Desulfurivibrio alkaliphilus, researchers demonstrated a biological shortcut in which sulfide is directly oxidized to sulfate while iron(III) is reduced. That means the bacteria bypass the expected chemical intermediates entirely.
“We show that this environmentally important redox reaction is not solely chemical,” said Alexander Loy of the University of Vienna.
In the lab, the microbes grew on iron oxide while consuming sulfide or FeS. Rates of sulfide oxidation outpaced abiotic controls, especially at environmentally realistic concentrations. When supplied with carbonate labeled with 13C, the bacteria fixed it into their biomass, proving they live autotrophically. They are, in essence, rock-breathing, sulfide-eating, carbon-fixing machines.
A metabolism hiding in plain sight
Genomic surveys indicate that bacteria and archaea across 37 phyla may carry the genes to run MISO metabolism. That includes environments from marine sediments to freshwater aquifers to engineered microbial fuel cells. The genetic toolkit involves cytochromes and pili for moving electrons from sulfide to insoluble iron minerals, echoing strategies used by famous metal-respiring bacteria like Geobacter and Shewanella. The common theme: an elaborate extracellular electron transfer system, tuned for solid minerals instead of dissolved oxygen.
That metabolic ingenuity suggests MISO may be more than a curiosity. Iron oxides are abundant in rivers, soils, and coastal seas. Hydrogen sulfide, the gas that smells like rotten eggs, accumulates wherever organic matter rots without oxygen. Put the two together, and these microbes have a niche. A niche that chemistry alone cannot fill as quickly.
“MISO bacteria remove toxic sulfide and may help prevent the expansion of so-called dead zones in aquatic environments,” said Marc Mussmann, co-lead of the study.
Ecological and climate implications
Dead zones, stripped of oxygen by microbial decomposition, are growing in coastal seas worldwide. Sulfide buildup can make conditions even worse, toxic to animals and plants alike. By scrubbing sulfide and regenerating sulfate, MISO bacteria could buffer against runaway anoxia. Globally, the authors estimate, their activity could represent 1 to 7 percent of sulfide re-oxidation in ocean sediments. A small fraction, maybe. But big when you consider planetary scale fluxes.
The discovery also forces a rethink of Earth’s ancient cycles. Sulfur transformations powered some of the earliest life. That microbes today can breathe iron while oxidizing sulfide points to a metabolism that may have deep evolutionary roots. It also reminds us that biology often outpaces chemistry in subtle ways, reshaping the planet’s chemistry grain by grain.
Kicker
Microbes do not just survive in oxygen-free muck. They engineer it. The MISO discovery highlights how even in the dark, bacteria can clean poison from water, fuel their growth, and leave their fingerprint on Earth’s cycles. Tiny rock-breathers, quietly running the planet.
Nature (2025). DOI: 10.1038/s41586-025-09467-0