Thermodynamic constraints on microbially mediated processes in lakes of the McMurdo Dry Valleys, Antarctica

Recent research has shown that the distribution of biogenic nitrogen and sulfur compounds ( in particular, N2O, DMS and dissolved DMSO) observed in five perennially ice-covered lakes of the McMurdo Dry Valleys, Antarctica, appear to lack obvious biogeochemical explanations. This study examined the hypothesis that the distribution of these compounds resulted from thermodynamic constraints on microbially mediated processes. The thermodynamic favorableness of a number of ecologically important redox reactions in these lakes was assessed using a simplified Nernst equation and mathematically modified Eh measurements. Our qualitative analysis revealed that the relative dominance of denitrification and nitrification as formation and loss processes for N2O was a complex pattern that was related to the redox conditions present. The results indicate that nitrification was initially the sole pathway for the formation of N2O with denitrification being thermodynamically unfavorable. As the redox conditions became less oxic, both denitrification and nitrification were possible. Once complete anoxia occurred, the importance of nitrification decreased considerably and the loss of N2O via denitrification became increasingly important. With respect to DMSO, the results indicated that DMSO was thermodynamically unfavorable as an electron acceptor for the respiration of organic material by bacteria in well-oxygenated environments, giving a partial explanation for the relatively elevated levels of DMSO found in many aquatic environments. Overall, the results of this study showed that subtle variations in the redox conditions present provided a plausible explanation for the unusual distributions of biogenic sulfur and nitrogen compounds observed in the lakes of the McMurdo Dry Valleys.