Survival of dormant (hyper)thermophilic sulfate reducing microorganisms at high temperatures over geologic timescales

Seepage of warm fluids from subsurface petroleum reservoirs and ocean crust releases a constant flux of thermophilic microorganisms into the cold deep sea, where some of them form spores and settle on the sea floor. Previous studies showed that spores from sulfate reducing Firmicutes can be revived upon heating to temperatures >50°C. As these experiments were restricted to surface sediments, the long-term survival of these spores remained unclear. We measured biological sulfate reduction rates (SRR) using highly sensitive radiotracer incubations in samples from a drill core from Guaymas Basin, a marginal ocean basin off the Pacific coast of Mexico with active sea floor spreading and a very high geothermal gradient of 950°C km^-1. The samples covered in-situ temperatures between 6 and 61°C and ages up to ~200 ky. Samples were incubated under in-situ pressure (25 MPa) and temperature and under increasingly higher temperatures, up to 50°C above in-situ values. Maximum incubation temperature was 111°C.

Samples from all depths revealed SRR increasing with temperature, SRR was detectable up to maximum incubation temperature, albeit at extremely low rates. The SRR data reveal two broad peaks of activity at 45 to 60°C and 75 to 95°C, respectively, with a distinct gap in between. We interpret this gap as a shift in microbial community composition from mesophiles/thermophiles to hyperthermophiles. As we can exclude any delivery of spores through advective transport from below or laterally, we postulate that these sulfate reducing microorganism were embedded at the time of deposition and survived over hundreds of thousands of years, even at elevated temperatures.