BIOGEOCHEMICAL SULFUR, IRON, AND CARBON CYCLING IN SUBSURFACE SEDIMENTS: ITS ROLE IN MICROBIAL SURVIVAL

GROSSMAN, ETHAN L.; AMMERMAN, JAMES W.; TEXAS A&M UNIVERSITY; SUFLITA, JOSEPH M.; UNIVERSITY OF OKLAHOMA

DOE SUBSURFACE SCIENCE PROGRAM (ORIGINS SUBPROGRAM)

Keywords: bacteria, sulfate reduction, pyrite oxidation, sulfur oxidizers, iron oxidizers, microbial survival, sulfur cycle, carbon cycle, aquifer, groundwater, geochemistry, stable isotopes

Objective: To determine the factors controlling microbial activity and survival in the subsurface; specifically, to determine whether microbial communities in aquitards and in aquifer microenvironments provide electron donors and/or acceptors that enhance microbial survival in aquifers

Approach: This study utilizes field and laboratory approaches and integrates subsurface microbiology, geochemistry, and hydrogeology. We have drilled eight boreholes in the Eocene Yegua formation at four localities on the Texas A&M campus using a hollow-stem auger. The drilling pattern forms a "T", with well clusters parallel and perpendicular to dip direction. Four boreholes were sampled for sediments and screened in the deepest sand, and four were drilled to install wells in shallower sands. Boreholes range in depth from 8 to 31 m, with screened intervals ranging from 6 to 31 m.
Sediments are collected aseptically and assayed for a variety of microorganisms and metabolic capabilities including viable aerobic and anaerobic heterotrophs, sulfate reducing bacteria (SRB), SRB activity, and Fe- and S- oxidizers. In addition, DNA is being recovered and amplified by polymerase chain reaction, and experiments are being performed to evaluate the biodegradability of natural-occurring carbon sources amended to subsurface sediments. Sediment geochemistry-organic carbon content, 13C/12C, sulfur chemistry, 34S/32S, and "bioavailable" Fe(III)-provides information on the interrelation between microorganisms and their environment, and slug tests and groundwater analyses (major ions, pH, dissolved oxygen, H2S, Fe, 34S/32S, 13C/12C, 14C, and tritium) help define chemical environments, flow patterns, and groundwater ages.

Results to Date:
Pyrite oxidation in shallow sediments (~6-8 m) is indicated by low pH (<5), high iron oxide and sulfate concentrations, and low 34S/32S ratios in groundwater sulfate. Sulfur and iron oxidizing bacteria enumerated from unsaturated zone sediments have concentrations of up to 104 cells/gram sediment, and are able to use pyrite as a sole energy source. Imaging of radiolabeled sulfide production from intact cores indicates high spatial variability in sulfate reducing activity, even at the centimeter scale. Greater in situ rates of sulfate reduction (SR) are measured near clay-sand contacts and lignite clasts, suggesting that activity is limited by availability of electron donors. 35SO4 reduction assays indicate high SR rates in shallow (0-8 m) and deep (17-28 m) sediments. Concurrent SR and sulfur-oxidizing activity at shallow depth is strong evidence for internal sulfur cycling in an aerobic aquifer.
Deeper sandy aquifers (17-30 m) exhibit high SR activity and SRB numbers (104 cells/mL). Sulfate is completely consumed in one month in unamended incubations of these sediments even though they are low in organic matter. Calculations of vertical sulfate transport rates suggest that shallow sulfide oxidation provides sulfate for deeper SR. We propose that sulfur cycling provides a survival mechanism for subsurface bacteria.

Collaborations: Ellyn Murphy (Pacific Northwest Laboratory)-origin of dissolved organic carbon in Yegua groundwaters; James McKinley (Pacific Northwest Laboratory)-isotopic heterogeneity in the Cerro Negro aquifer system: a multi-layer sampler study; T. C. Onstott (Princeton University) and F. Colwell (Idaho National Engineering Laboratory)-origin and potential consumption of gaseous hydrocarbons from Piceance Basin, Colorado (Parachute Site; project supplement)