Microbial carbon sequestration in a subsurface aquifer contaminated by geologic CO2

Sequestration of CO2 in the subsurface is a strategy under serious consideration to limit accumulation of anthropogenic CO2 in the atmosphere. Possible negative impacts include perturbation of biogeochemical cycles and groundwater contamination due to CO2 leakage. If sequestration stimulates growth of autotrophic microorganisms, recapture of CO2 into organic carbon would be an important feedback that could lower sequestration risks. Currently, little is known about the capacity of organisms resident in such environments to fix CO2. Here we investigated the CO2 fixation capacity of microorganisms in groundwater from the CO2–driven Crystal Geyser, located near Green River, Utah (USA). We reconstructed and analyzed 227 near-complete microbial genomes, representing 150 distinct species. At least 25% of the studied organisms are predicted to be autotrophs, and these represent a substantial portion the community. The most common carbon fixation pathways were the Calvin Benson Basham Cycle and Wood-Ljungdhal pathway, which incorporate readily available CO2 rather than bicarbonate. Unexpectedly, one organism from a novel phylum sibling to the candidate phyla radiation has genes of the reverse TCA cycle, which it may use to fix CO2. Autotrophic metabolism is likely supported by iron and sulfur cycling and by H2 produced by fermentation. These resources may be provided as trace components in the CO2 stream or could be liberated from minerals by carbonic acid-promoted dissolution. We predict that elevated CO2 levels will select for microbial communities enriched in organisms that can recapture CO2 into biomass, decreasing negative side effects of CO2 leakage from geological repositories.