Ecogeochemistry of the subsurface food web at pH 0-2.5 in Iron Mountain, California, USA

Pyrite oxidation in the underground mining environment of Iron Mountain, Ca lifornia, has created the most acidic pH values ever reported in aquatic sy stems. Sulfate values as high as 120 000 mg l(-1) and iron as high as 27 60 0 mg l(-1) have been measured in the mine water, which also carries abundan t other dissolved metals including Al, Zn, Cu, Cd, Mn, Sb and Pb. Extreme a cidity and high metal concentrations apparently do not preclude the presenc e of an underground acidophilic food web, which has developed with bacteria l biomass at the base and heliozoans as top predators. Slimes, oil-like fil ms, flexible and inflexible stalactites, sediments, water and precipitates were found to have distinctive communities. A variety of filamentous and no n-filamentous bacteria grew in slimes in water having pH values < 1.0. Fung al hyphae colonize stalactites dripping pH 1.0 water; they may help to form these drip structures. Motile hypotrichous ciliates and bdelloid rotifers are particularly abundant in slimes having a pH of 1.5. Holdfasts of the ir on bacterium Leptothrix discophora attach to biofilms covering pools of sta nding water having a pH of 2.5 in the mine. The mine is not a closed enviro nment - people, forced air flow and massive flushing during high intensity rainfall provide intermittent contact between the surface and underground h abitats, so the mine ecosystem probably is not a restricted one.