Volatile organic acids and microbial processes in the Yegua formation, east-central Texas

Geochemical and microbiological evidence indicates that viable microorganis ms produce and consume volatile organic acids (VOA) in the Yegua formation. Acetic and propionic acid concentrations in mudstones range from 200 to 12 70 and 20 to 38 nmol.gdw(-1) respectively, whereas concentrations in sands are 50-200 and less than 20 nmol.gdw(-1). VOA concentrations in sediments a nd in laboratory incubations suggest net production of VOAs by microorganis ms in mudstones, and net consumption of VOAs by SO4 reducing bacteria (SRB) in sands. Notably, SRB activity is mostly confined to aquifer sands. Vertical diffusion and advection were modeled to estimate acetic acid trans port from aquitard to aquifer. Assuming that SRB completely respire the ace tic acid transported into the aquifer (3.2 mu mol.l(-1).m.a(-1)), the CO2 p roduction rate in the aquifer sands is 5.3 mu mol.l(-1).a(-1). This slow mi neralization rate of in situ organic matter is within the range for deep aq uifers, and probably accounts for the long-term survival of microorganisms in oligotrophic environments. Finally, the microbial communities in Yegua s ediments appear to exhibit a loose commensalism, with microorganisms in aqu itards providing VOAs for respiratory processes (i.e., SO4 reduction) in aq uifers. (C) 2000 Elsevier Science Ltd. All rights reserved.