Broad‐Scale Evidence That pH Influences the Balance Between Microbial Iron and Sulfate Reduction

Abstract

Article Impact Statement: Broad-scale trends in groundwater chemistry indicate a shifting balance between microbial iron and sulfate reduction with pH AbstractUnderstanding basic controls on aquifer microbiology is essential to managing water resources and predicting impacts of future environmental change. Previous theoretical and laboratory studies indicate that pH can influence interactions between microorganisms that reduce ferric iron and sulfate. Here we test the environmental relevance of this relationship by examining broad-scale geochemical data from anoxic aquifers. We isolated data from the U.S.Geological Survey National Water Information System for 19 principal aquifer systems. We then removed samples with chemical compositions inconsistent with iron-and sulfate-reducing environments and evaluated the relationships between pH and other geochemical parameters 2 using Spearman's rho rank correlation tests. Overall, iron concentration and the iron-sulfide concentration ratio of groundwater share a statistically significant negative correlation with pH (P <0.0001). These relationships indicate that the significance of iron reduction relative to sulfate reduction tends to increase with decreasing pH. Moreover, thermodynamic calculations show that, as the pH of groundwater decreases, iron reduction becomes increasingly favorable relative to sulfate reduction. Hence, the relative significance of each microbial reaction may vary in response to thermodynamic controls on microbial activity. Our findings demonstrate that trends in groundwater geochemistry across different regional aquifer systems are consistent with pH as a control on interactions between microbial iron and sulfate reduction. Environmental changes that perturb groundwater pH can affect water quality by altering the balance between these microbial reactions.

pH as a Primary Control in Environmental Microbiology: 1. Thermodynamic Perspective

“…At pH 5.7, iron reduction consumed at least 90% of acetate while sulfate reduction consumed a negligible amount (<1%). In agreement with these findings, furthermore, Kirk et al (2016) found that broad-scale patterns in groundwater geochemistry in U.S. aquifers are also consistent with an increase in the significance of iron reduction relative to sulfate reduction as pH decreases.…”

Influence of pH on the balance between methanogenesis and iron reduction

“…However, as described by Kirk et al. (), we can overcome this uncertainty by assuming an arbitrary amount of electron donor is present and then calculating the difference between the amount of usable energy for methanogenesis and iron reduction (). If the reactions are both written in terms of the same electron donor stoichiometry, then the reaction energy yields respond uniformly to changes in electron donor availability.…”

Thermodynamic and Kinetic Response of Microbial Reactions to High CO2

“…According to Kirk et al (2016b), in 18 principal aquifer systems of the United States, Ca 2+ has an average concentration of 1.4 mM, and four aquifer systems have average pH values near 8.0. Thus, the simulation assumes that groundwater in the two hypothetical aquifers has pH of 8 and contains 10 mM Na + , 10 mM Cl − , and 2.0 mM Ca 2+ .…”

“…In aquifers, there are few concentration measurements for lactate, propionate, butyrate, methanol, and ethanol. On the other hand, the concentrations of ferrous iron, sulfate, sulfide, and bicarbonate vary over orders of magnitude (Kirk et al, 2016b). Using the changes also simplifies the discussion of ferric mineral reduction.…”

“…The simulation considers the partial pressure from near 0 to 30 atm, the range commonly encountered during groundwater sampling. For example, 95% of the wells in the principal aquifers of the United States analyzed by Kirk et al (2016b) have depths less than 300 m. At this depth, CO 2 can build up to a maximum partial pressure of 30 atm.…”