Experimental Evidence for a Microbial Origin of Reduction Spots in Red Beds on Earth-and Mars?

The Perseverance rover recently discovered sedimentary rocks reddened by ferric oxides and peppered with bleached spots lacking these oxides. Some of these spots are associated with phosphate, iron sulfide minerals, and organic matter and are regarded as "potential biosignatures," suggestive of microbial iron- and sulfate-reduction and organic matter oxidation. Similar millimeter-centimeter-scale "reduction spots" occur in many ancient red beds on Earth. Although terrestrial reduction spots are widely considered biogenic, the available evidence is not decisive, and the proposed microbial mechanism of spot formation has not been tested experimentally. Here, we report a successful laboratory demonstration of bleached spot formation in ferruginous sediment. Millimeter-centimeter-scale rounded bleached spots appeared within weeks on the underside of anaerobic sand-ferrihydrite slurries inoculated with microbial communities from the reducing zones of Winogradsky columns, originally seeded with soil and pondwater. The spatial and temporal distribution of observed bleaching events, which did not occur in sterile controls, is best explained by a microbially induced process, and DNA sequencing confirms that bacteria of iron-reducing genera (e.g., Paradesulfitobacterium) are abundant in the bleached areas. These results strongly support the longstanding hypothesis that microbial colonies can indeed generate visibly bleached reduction spots in ferruginous sediments and rocks. Further experiments are needed to establish whether and how nonbiological processes can mimic these features and to search for features that disambiguate biogenic and abiogenic reduction spots.