PH-DEPENDENT INTERACTION BETWEEN IRON REDUCERS AND METHANOGENS AND ITS IMPACT ON CARBON BUDGETS

Microbial iron reduction is very effective at trapping carbon in aqueous systems by generating carbonate alkalinity. Methanogenesis, in contrast, produces little alkalinity but large quantities of methane gas, which is poorly soluble and can be lost from aquatic systems. Shifts in the balance between these microbial reactions in response to environmental change, therefore, can significantly impact carbon storage in aqueous systems. In this study, we used semi-continuous bioreactors to examine how interactions between iron reducers and methanogens respond to changes in pH and the impact of those responses on carbon budgets. The bioreactors each contained a slurry of marsh sediment amended with goethite (1 mmol) and acetate (0.25 mM). During the 91-day incubation, one set received acidic media (pH 6) while the other received media with basic pH (7.5). We also included a set of acetate-deficient bioreactors to serve as controls. The extent of iron reduction was higher in acidic reactors: 21 times more iron reduction occurred at acidic pH than basic pH. In contrast, the extent of methanogenesis was higher in alkaline reactors: 1.6 times more methanogenesis occurred at basic than acidic pH conditions. Thus, methanogenesis increased relative to iron reduction as pH increased. Reflecting differences in the balance between each reaction, the carbonate alkalinity content of acidic reactors was nearly double the value observed in the alkaline reactors by the mid-point of the experiment. Thus, carbon was more effectively trapped within the aqueous phase of the acidic reactors than the alkaline reactions. Analysis of the geochemical composition of groundwater across the United States reveals a similar relationship between pH and the balance between iron and methane. Together, these observations imply that environmental changes that influence pH can affect carbon storage by altering interactions of methanogens and iron reducers.