• Energy Research

pH influences the occurrence and distribution of microorganisms. Microbes typically live over a range of 3–4 pH units and are described as acidophiles, neutrophiles, and alkaliphiles, depending on the optimal pH for growth. Their growth rates vary with pH along bell- or triangle-shaped curves, which reflect pH limits of cell structural integrity and the interference of pH with cell metabolism. We propose that pH can also affect the thermodynamics and kinetics of microbial respiration, which then help shape the composition and function of microbial communities. Here we use geochemical reaction modeling to examine how environmental pH controls the energy yields of common redox reactions in anoxic environments, including syntrophic oxidation, iron reduction, sulfate reduction, and methanogenesis. The results reveal that environmental pH changes energy yields both directly and indirectly. The direct change applies to reactions that consume or produce protons whereas the indirect effect, which applies to all redox reactions, comes from the regulation of chemical speciation by pH. The results also show that energy yields respond strongly to pH variation, which may modulate microbial interactions and help give rise to the pH limits of microbial metabolisms. These results underscore the importance of pH as a control on microbial metabolisms and provide insight into potential impacts of pH variation on the composition and activity of microbial communities. In a companion paper, we continue to explore how the kinetics of microbial metabolisms responds to pH variations, and how these responses control the outcome of microbial interactions, including the activity and membership of microbial consortia.

Citation: Kirk, MF, Altman, SJ, Santillan, EFU, Bennett, PC (2016) Interplay between microorganisms and geochemistry in geological carbon storage. International Journal of Greenhouse Gas Control 47, 386-395.

Study region: Regional precipitation gradient across Kansas, USA. Study focus: As precipitation increases, baseflow and surface runoff generally increase, but it is unclear whether they increase proportionally and how proportions respond to climate and land use changes. This study examined variation in streamflow components of perennial streams across the study region and its relationships with watershed properties. We evaluated streamflow components with hydrograph separation and used Spearman’s rank correlation tests and principal component analysis (PCA) to assess spatial trends (28 sites) and Mann-Kendall and Sen’s Slope tests to assess temporal relationships (9 sites, 1960–2018). New hydrological insights for region: Runoff and baseflow both increase eastward with precipitation but the increase is greater for runoff. As such, baseflow index (BFI, baseflow/streamflow) decreases with increasing precipitation, potentially reflecting the limits of infiltration on recharge/runoff partitioning. Spatial patterns in variables that influence infiltration (land use and soil texture) also vary with precipitation, consistent with long-term influences of climate on landscapes. Since 1960, the watersheds included in our temporal analysis experienced small, mainly insignificant increases in precipitation and temperature and large, significant increases in irrigation. During this time, BFI increased significantly only in semi-arid, agriculture-dominated catchments overlaying higher permeability deposits. These findings underscore the importance of watershed characteristics as controls on current spatial patterns in streamflow and BFI and also the sensitivity of streamflow and BFI to climate and land use changes over time.

Geological carbon sequestration captures CO2 from industrial sources and stores the CO2 in subsurface reservoirs, a viable strategy for mitigating global climate change. In assessing the environmental impact of the strategy, a key question is how microbial reactions respond to the elevated CO2 concentration. This study uses biogeochemical modeling to explore the influence of CO2 on the thermodynamics and kinetics of common microbial reactions in subsurface environments, including syntrophic oxidation, iron reduction, sulfate reduction, and methanogenesis. The results show that increasing CO2 levels decreases groundwater pH and modulates chemical speciation of weak acids in groundwater, which in turn affect microbial reactions in different ways and to different extents. Specifically, a thermodynamic analysis shows that increasing CO2 partial pressure lowers the energy available from syntrophic oxidation and acetoclastic methanogenesis, but raises the available energy of microbial iron reduction, hydrogenotrophic sulfate reduction and methanogenesis. Kinetic modeling suggests that high CO2 has the potential of inhibiting microbial sulfate reduction while promoting iron reduction. These results are consistent with the observations of previous laboratory and field studies, and highlight the complexity in microbiological responses to elevated CO2 abundance, and the potential power of biogeochemical modeling in evaluating and quantifying these responses.

Abstract Microbial biomass can clog porous media and ultimately affect both structural and mineral trapping of CO 2 in geological carbon storage reservoirs. Whether biomass can remain intact following a sudden decrease in groundwater pH, a geochemical change associated with CO 2 injection, is unclear. We examined this question using twelve biologically-active and three control column-reactor experiments. Cell abundance and distribution was monitored using confocal microscopy, plating, and direct counting. Hydraulic conductivity (K) was monitored using pressure sensors. Growth occurred for four days at neutral pH. During that time, K within the clogged portion of the reactors decreased from 0.013 to 0.0006 cm s −1 on average, a 1.47 log reduction. Next, the pH of the inflowing aqueous medium was lowered to pH 4 in six experiments and pH 5.7 in six experiments. As a result, K increased in five of the pH 4 experiments and two of the pH 5.7 experiments. Despite this increase, however, the columns remained largely clogged. Compared to pre-inoculation K values, log reductions averaged 1.13 and 1.44 in pH 4 and pH 5.7 experiments, respectively. Our findings show that biomass can largely remain intact following acidification and continue to reduce K, even when considerable cell stress and death occurs.