ISOTOPIC, AND MICRO-BIOGEOCHEMICAL CONTROLS ON THE CYCLING OF METALS AND METALLOIDS IN LOW-TEMPERATURE AQUEOUS SYSTEMS

This research is conducted to advance our understanding on the plausible role that microbes play in the biogeochemical interactions that take place in surface and shallow aqueous systems. The relationship between microbial activity and metals and metalloids cycling is being targeted for environmental remediation and water quality assessment. We used geochemical, stable isotope, and membrane lipid analyses to investigate the geochemical changes and microbial community structure associated with metals and nutrients cycling in surface and shallow environments. The study area focused on the artesian springs discharging the down-dip section of the karstic Edwards Aquifer in south central Texas having distinct freshwater and brackish water quality in the vicinity of its updip-downdip interface. Water samples were collected along the artesian well-head stretch to the mouth of the aquifer. The geochemical measurements in the water samples showed a decrease in the alkalinity concentrations over distance with enrichment in carbon isotopic signatures (δ¹³C). The sulfate concentration decreased over distance with a slight enrichment in sulfur isotope (δ³⁴S) for the first 20 m and then a slight decrease in δ³⁴S values throughout. The Sr concentration showed a slight decrease over distance, but increases were recorded within the pond, and the TDS concentrations increased over distance albeit slightly. The intact phospholipid biomarkers consisted of branched, monounsaturated, saturated, polyunsaturated, hydroxy, and cyclopropyl fatty acids. The combination of lipid biomarkers in both samples indicated a potentially diverse assemblage of both aerobic and anaerobic bacteria, stress and toxicity related biomarkers, and sulfate-reducing bacteria. The stress-indicative lipid biomarkers provide evidence of physiological adaptation of microbial community to stress associated with changing environmental conditions. A better understanding and utilization of these micro-biogeochemical processes will help open up the possibility of improving biocide performance in aquifer settings and microbial colonization.