ACCELERATION OF PYRITE OXIDATION BY LITHOTROPHIC MICROORGANISMS AT NEUTRAL PH

Pyrite is the most abundant sulfide mineral in Earth’s crust and its oxidation via abiotic and biotic pathways plays an important role in the biogeochemical cycling of Fe and S and also has significant effects on environmental quality parameters such as contaminant mobility and the generation of acid rock drainage (ARD). While the mechanism and occurrence of microbially accelerated pyrite oxidation under acidic conditions is well established, much less is known about the occurrence and mechanism of microbial pyrite oxidation at neutral pH despite the fact that pyrite bearing soil and sedimentary environments are capable of maintaining a circumneutral pH and the initial stages of (ARD) involve circumneutral pH reactions. Our recent work has begun to catalog the extent to which natural environments host microorganisms with the capacity to accelerate pyrite oxidation at neutral pH, with environments ranging from surficial freshwater bodies to subsurface sediments and most recently a deep shale hosted pyrite weathering front at Susquehanna-Shale Hills Critical Zone Observatory (SSHCZO). Culture-based investigations utilizing natural inocula obtained from the inferred depth of the pyrite oxidation front at SSHCZO (c.a. 26 m) demonstrate that in the presence of live cells, sulfate generation can be up to 2.5 times greater than abiotic controls. 16S and shotgun metagenomic analysis of pyrite oxidizing cultures reveal that reactors amended with pyrite are dominated by organisms of the genus Thiobacillus. Metabolically, this organism appears to grow lithoautotrophically via the oxidation of sulfur as evidenced by the presence genes encoding the enzyme RuBisCo and the sulfur oxidation gene soxY. Notably, Thiobacillus was also the dominant genus observed in reactors using freshwater sediment as an inocula. Metagenomic characterization remains ongoing, however our evidence suggests a role for sulfur oxidizing bacteria in circumneutral pyrite oxidation. While we have demonstrated that neutral pH pyrite oxidation, while once a poorly documented phenomenon, is likely to be ubiquitous in nature, further work is necessary to address the underlying mechanism(s) by which the phenomenon occurs.