THE SEARCH FOR LIFE IN THE DEEP SUBSURFACE: MEMBRANE LIPID BIOSIGNATURES

Microbial communities were investigated in the extreme environments of deep South African and Canadian mines. Microbial membrane lipids were used as biosignatures to estimate viable biomass, respiratory potential, community structure and metabolic status. The membrane lipid components analyzed included phospholipid fatty acids (PLFA) and quinones. Fracture water, service water, biofilms, air, carbon leader rock, and quartzite rock samples were collected from ten mines. PLFA and quinone profiles were assessed in conjunction with a variety of physical and chemical parameters. PLFA-based biomass estimates varied by > 9 orders of magnitude (one-hundred to one-hundred-billion cells per g or L). On average, biofilms had the highest biomass followed by service water, and then fracture water. Quinone-based biomass estimates were higher for samples containing Archaea, as determined by SSU rRNA gene analysis. This higher estimate is due to the quinone assay detecting archaeal menaquinones while the PLFA assay does not detect archaeal ether-linked phospholipids. When PLFA community composition was examined, air samples were dominated by normal saturates and polyunsaturates, whereas service water was dominated by monounsaturates, indicating dominance by eukaryotic microorganisms and Gram-negative bacteria, respectively. In contrast, fracture water samples exhibited diverse microbial community profiles. In some samples the ratio of cyclopropyl to monounsaturated fatty acid indicated that the microbial communities may be constrained by conditions such as limited nutrients. The quinone profiles for samples exposed to oxygen showed a higher abundance of ubiquinones, while anoxic samples had higher abundances of menaquinones. The ratio of ubiquinone to menaquinone indicates that many of the fracture waters have been exposed to anoxic conditions for a long time. When the PLFA profiles were combined with the physical and chemical data for redundancy ordination analysis, temperature, pH, Eh, and concentrations of chloride, sulfate and nitrate were indicated as parameters that influenced changes in the microbial community composition. Continued investigations into the deep terrestrial subsurface will provide a better understanding of the microbial communities and their biogeochemical processes.