DO SUPERPLUMES SELECT FOR HYPERTHERMOPHILES IN THE DEEP SUBSURFACE?

Samples were collected from South African Witwatersrand Basin Au mines across a variety of geological formations, hydrological regimes, temperatures and depths (up to 3.3 km below land surface). The microbial communities found in the subsurface fissure water reflect the source of the water, the principle terminal electron acceptor and the hydrogeological history.

Bacterial populations indigenous to this system are composed of meso/thermophilic Desulfotomaculum-like gram positives and several novel, deep branching lineages. Sulfur isotopic data confirm that bacterial reduction of sulfate is a dominant electron acceptor. DNA extracts from fissure water bearing isotopic evidence of a deeper geothermal source were dominated by archaeal lineages related to known hyperthermophiles (mostly Pyrococcus sp.). The predominant archaeal clone type from the highest temperature water (60°C) was 99% similar to Pyrococcus abysii. Methanogens have not been identified in the fissure water, although CH4 is a dominant gas species. Carbon isotopic analyses confirm that the CH4 is nonbiogenic in origin, but may be functioning as an electron donor. Another electron donor is H2, which is a major gas constituent in the U-rich Au mines of the northern Wits basin.

36Cl analyses constrain the minimum age of some fissure water to be between 1-3 myr. The maximum age of in situ microbial communities has been constrained by apatite fission track analyses. Modeling of a sample collected at 3.7 km indicates that the region cooled below 110-120°C at 70-90 Ma. Other apatite fission track results indicate that this cooling trend follows a 90 Ma thermotectonic event that increased heat flow, initiated uplift and erosion. The response of the South African crust to the superplume may also be reflected in the deep subsurface microbial communities. Because the rock temperatures at 3.3 km were at 100°C at 70 Ma, a thermal bottleneck existed that only hyperthermophilic microorganisms could have survived. The meso/thermophilic microorganisms at these depths represent more recent immigrants. To date the immigrants that have been characterized are quite novel and include a metal-reducing bacterium Thermus multireducens and an extreme alkaliphilic anaerobe Alkaliphilus transvaalensis.