Microbial CO2 fixation potential in a tar-oil-contaminated porous aquifer

CO2 fixation is one of the most important processes on the Earth's surface, but our current understanding of the occurrence and importance of chemolithoautotrophy in the terrestrial subsurface is poor. Groundwater ecosystems, especially at organically polluted sites, have all the requirements for autotrophic growth processes, and CO2 fixation is thus suggested to contribute significantly to carbon flux in these environments. We explored the potential for autotrophic CO2 fixation in microbial communities of a petroleum hydrocarbon-contaminated aquifer by detection of functional marker genes (cbbL, cbbM), encoding different forms of the key enzyme RubisCO of the CalvinBensonBassham cycle. Quantification of (red-like) cbbL genes revealed highest numbers at the upper fringe of the contaminant plume and the capillary fringe where reduced sulphur and iron species are regularly oxidized in the course of groundwater table changes. Functional gene sequences retrieved from this area were most closely related to sequences of different thiobacilli. Moreover, several cultures could be enriched from fresh aquifer material, all of which are able to grow under chemolithoautotrophic conditions. A novel, nitrate-reducing, thiosulfate-oxidizing bacterial strain, recently described as Thiobacillus thiophilus D24TNT sp. nov., was shown to carry and transcribe RubisCO large-subunit genes of form I and II. Enzyme tests proved the actual activity of RubisCO in this strain.