Terminal microbial metabolisms in the deep subsurface under conditions relevant to CO2 sequestration and enhancing methanogenesis from coal

Abstract

Microbial life in the deep subsurface may extend several kilometers below the surface, and by some estimates surpasses the mass of all life on the surface of the Earth. Microbes living in the subsurface play important roles in the global carbon cycle, through the breakdown of organic matter and methane formation. The goal in this thesis is to describe the microbial dynamics of two deep subsurface environments, and relate how porewater geochemistry and human-induced changes to those environments affect microbial carbon cycling and community structure. The environments of interest in this thesis are 1) deeply buried coal seams, and 2) sandstone reservoirs. The goals in this thesis include the characterization of microbial community structure and metabolic function in methanogenic coal microcosm experiments in response to imposed geochemical gradients, and the characterization of a microbial community in response to supercritical CO2 injection. Acetate was found to be a key intermediate in methanogenic coal microcosms. Experiments with variable concentrations of sulfate showed the same methanogenic potential as molybdate inhibited experiments, showing that sulfate concentration is not a determining factor in methanogenesis from coal. Experiments amended with acetate showed a decreasing proportion of acetate consumed by methanogens as acetate concentrations increased, suggesting that in situ acetate concentrations determine the relative importance of methanogenesis and sulfate reduction in coal seams. The growth of sulfate reducers and methanogens was monitored using phospholipid fatty acids (PLFA) and quantitative polymerase chain reaction (qPCR) for the functional genes for methanogens (mcrA) and sulfate reducers (dsrA). Bioindicators for SRM, including 10Me16:0, and cy17:0 correlated positively with acetate amendment, which also corresponded with an increase in dsrA and mcrA copy numbers in experiments without molybdate, showing that sulfate reducers and methanogens could metabolize acetate simultaneously in coal bed aquifers with low acetate concentrations. Experiments with variable pCO2 and urea amendment showed that methanogenesis from coal was relatively insensitive to pCO2 at pressures up to 2.5 atm, but urea concentrations above 2.5 g/L caused a cessation of methanogenic activity, most likely due to a pH effect. Methanogenic potential in coal microcosms was also shown to be a function of the metabolic activity of fermenters making acetate prior to the start of each experiment. The consortium of organisms used for experiments with coal was also utilized for experiments with supercritical CO2 (scCO2) at 40 degrees C and 10 MPa. Pre-incubation of sandstone cores caused a 30-60% reduction in permeability of Berea sandstone cores, which was not recovered following scCO2 injection. The injection of a microbial enrichment into sterile sandstone cores also reduced the measured permeability up to 83%. scCO2 did not have a significant impact on the bacterial or archaeal microbial community, as measured by PLFA, phosphoether lipid, and most probable number (MPN) analyses of injection system effluent as well as extracts of the solid cores post-experiment, suggesting that either contact time with scCO2, or heterogeneous flow are major factors in microbial susceptibility to scCO2.