The abundance of aerobic microorganisms, the influence of sediment properties and groundwater geochemistry on microbial processes, and the community structure of bacteria were investigated in sediments of a Gulf Coast aquifer. Five boreholes were drilled into the sands, silts, clays, and lignite of the Eocene Yegua formation in east-central Texas, and monitoring wells were installed in all water-bearing sands. Numbers of microorganisms ranged from 10^6 to 10^8 cells per gram dry weight (gdw-1), and viable counts ranged from 0 to 10^6 cells gdw-1. Numbers of S- and Fe-oxidizing bacteria decreased with depth and ranged from 0 to 10^2 cells gdw–1. Low pH (3.8) and high sulfate (28 mM) and Fe2+ (0.3 mM) contents argue for pyrite oxidation in shallow sediments. S- and Fe-oxidizing bacteria were readily detected in these sediments and likely play a role in pyrite oxidation. In consistent fashion, there was a relative paucity of pyrite in shallow sediments and a low 34S/32SSulfate ratio (0.2‰), reflecting contributions from 34S-depleted sulfides, in shallow regions. This pyrite oxidation potentially supplies sulfate to the deeper aquifers.

PCR amplification of 16S rDNA genes from sediment and water samples, using primer sets specific for the domain Bacteria and others specific for organisms of the genus Thiobacillus, yielded amplification that corresponded well with sediment geochemistry. Amplified DNA fragments of the V3 region of 16s rDNA were resolved by denaturing gradient gel electrophoresis (DGGE), and the resulting profiles were reproducible and specific for communities from different depths. Bacterial diversity estimated from the number and intensity of specific fragments in DGGE profiles decreased with depth. DNA amplified with primers specific for T. ferrooxidans and T. thiooxidans showed the presence of T. ferrooxidans to be most abundant above the oxidizing-to-reducing transition zone. Organisms isolated from the Yegua formation were phylogenetically most closely related to the species T. ferrooxidans and T. thiooxidans based on comparative molecular sequence (16s rDNA) analysis. Overall, sulfur cycling in shallow sediments and the transport of sulfate represents an important mechanism for commensal interaction among subsurface microorganisms by providing electron donors for chemoautotrophic bacteria and electron acceptors for heterotrophic bacteria.