NS44A-01 INVITED
Ocean Microbial Fuel Cell: Power Source and Research Tool for Studying Marine Biogeochemistry
Ocean microbial fuel cells (OMFCs) are devices capable of producing modest levels of electrical power. The cells are ultimately driven by the oxidation of marine organic matter at the anode and reduction of dissolved oxygen at the cathode, but microbial transformations and electrochemically active intermediates play important roles in the overall process of electricity generation. By separating the factors that affect the performance of OMFCs into components of an equivalent circuit and manipulating these factors in laboratory and field experiments, we are gaining new insight into how specific redox reactions, sources of organic matter, and mass transport at small and intermediate scales may enrich environments with certain groups of microorganisms that in turn regulate anaerobic organic matter degradation. This talk will illustrate these relationships with the results from at least four experiments in which either fresh plankton, or substrates within continental margin sediments, fuelled the OMFCs. In each example, reduced sulfur compounds were found to be major electron carriers to the fuel cell anode. These intermediates came from a variety of sources including sulfide generated from sulfate reduction in mixed solutions surrounding the electrode, sulfide generated distally but transported by pore-water diffusion and advection, iron monosulfides and pyrite present is a sediment matrix centimeters from the electrode, and sulfide or polysulfide produced within an electrode biofilm. To illustrate a practical application of an OMFC, we are currently constructing a benthic cell that will power a sonic receiver in a network of underwater sensors. The form of this OMFC resembles a benthic chamber with a footprint of one square meter. It should be capable of supplying electrical power and regulating its output for years to decades.
NS44A-02 INVITED
Novel applications for biogeophysics: Prospects for detecting key subseafloor geomicrobiological processes or habitats
The new subdiscipline of biogeophysics has focused mostly on the geophysical signatures of microbial processes in contaminated subsurface environments usually undergoing remediation. However, the use of biogeophysics to examine the biogeochemistry of marine sediments has not yet been well-integrated into conceptual models that describe subseafloor processes. Current examples of geophysical measurements that have been used to detect geomicrobiological processes or infer their location in the seafloor include sound surveillance system (SOSUS)-derived data that detect seafloor eruptive events, deep and shallow cross-sectional seismic surveys that determine the presence of hydraulically conductive zones or gas-bearing sediments (e.g., bottom-simulating reflectors or bubble-rich strata), and thermal profiles. One possible area for innovative biogeophysical characterization of the seafloor involves determining the depth of the sulfate-methane interface (SMI) in locations where sulfate diffuses from the seawater and methane emanates from subsurface strata. The SMI demarcates a stratum where microbially-driven anaerobic methane oxidation (AMO) is dependent upon methane as an electron donor and sulfate as an electron acceptor. AMO is carried out by a recently defined, unique consortium of microbes that metabolically temper the flux of methane into the overlying seawater. The depth of the SMI is, respectively, shallow or deep according to whether a high or low rate of methane flux occurs from the deep sediments. Presently, the SMI can only be determined by direct measurements of methane and sulfate concentrations in the interstitial waters or by molecular biological techniques that target the microbes responsible for creating the SMI. Both methods require collection and considerable analysis of sediment samples. Therefore, detection of the SMI by non-destructive methods would be advantageous. As a key biogeochemical threshold in marine sediments, the depth of the SMI defines methane charge in marine sediments, whether it is from dissolved methane or from methane hydrates. As such, a biogeophysical strategy for determining SMI depth would represent an important contribution to assessing methane charge with respect to climate change, sediment stability, or potential energy resources.
NS44A-03 INVITED
Investigating Transient Heterogeneity in a Bioactive Hydrocarbon Plume Using GPR and PVPs
Aquifer heterogeneity can place severe restrictions on deterministic modeling of contaminant fate and transport.
One response to this is to collect geostatistical data so that multiple realizations of the aquifer can be simulated
and probabilities of various transport scenarios estimated. However, both the deterministic and stochastic
modeling approaches generally assume that the heterogeneities do not change in time. This assumption
comes into question in a growing body of literature, mainly from laboratory studies, suggesting that heterogeneity
in aquifers can be transient. In chemically active systems such as reactive barriers, or bioactive systems such as
contaminant plumes, time dependent perturbations in flow have been documented. To date there are limited
data investigating transient heterogeneity in contaminant plumes in the field. This research investigates the
biological cases where the perturbations occur due to biomass growth, gas production, and chemical precipitate
formation.
A controlled gasoline release was conducted at one end of a sheet-pile alleyway in which groundwater was
flowing at a constant rate of about 10 cm/day. Several meters downgradient of the release, the aquifer was
instrumented with ORC wells, 5 PVP multilevel stands, and 6 GPR access tubes that were installed to a depth of
about 5 m surrounding a section of the aquifer measuring about 2 m (along the flow direction) by 4 m (across the
flow direction). Changes in the aquifer character following oxygen addition was monitored by repeatedly
measuring groundwater velocites at 20 points across the plume, and by using GPR to obtain several
tomographic images of the aquifer through time. The data suggest that indeed the aquifer responded to the
oxygen additions with changes to the flow system. 1) assess the GPR response to microbial activity in a tank
constructed to simulate the field aquifer, 2) assess the PVP responses in the same tank, and 3) correlate the
above responses to microbial population changes in the simulated aquifer.
http:www.people.ku.edu/~jfdevlin/Research.html
NS44A-04
Inverting Residual Self-Potential Data for Redox Potentials of Contaminant Plumes
Self-potential (SP) data can be separated into a streaming potential component that is associated with pore water flow and a redox potential component, which is sensitive to differences in the redox potentials of organic-rich contaminant plumes and the surroundings. This work presents the first inversion method that uses residual SP (i.e., corrected for the streaming potential component) to invert for the redox potentials of contaminant plumes. We consider a two-layered electrical conductivity structure, where the boundary corresponds to the water table. We assume that the electrical dipole sources are associated with microbial breakdown of contaminants at the water table. This geobattery model is hypothesized to exist (1) because the water table is associated with a strong redox gradient between highly reducing conditions within the contaminated groundwater (due to biodegradation and oxygen depletion) and the oxidized vadose zone, and (2) because the microbial biofilms and precipitation of metallic particles can provide an electron conductor to complete the circuit required for the geobattery. The inverse method was applied to residual SP estimated from SP measurements collected at the ground surface in the vicinity of the Entressen landfill, South of France. The estimated redox potentials correlate well with in situ measurements (correlation coefficient is 0.93) and the estimated amplitudes of the redox potentials are similar to those measured in situ. A sensitivity analysis reveals that meaningful estimates of the redox potential can be derived even if the electrical conductivity structure is only known within an order of magnitude. These results provide further evidence that the SP method can be useful to monitor the spreading of contaminants around landfills and to evaluate the efficiency of remediation programs.
NS44A-05
Geoelectrical Signatures Of Microbial Stimulated Mineralization
Bioremediation techniques are commonly utilized to address soil and groundwater contamination due to acid- mine drainage, industrial sources, and government nuclear weapon programs. One critical component of these efforts is the real time, spatially accurate monitoring of the remediation processes. For this reason non-invasive high resolution geophysical methods have been employed in the recent years to elucidate system transformations occurring during bioremediation. In our study, we performed laboratory column experiments to investigate the geoelectrical response of microbe-mediated iron sulfide (FeS) precipitation accompanying stimulated sulfate-reduction; a bioremediation technique currently utilized for the sequestration of heavy metals in the subsurface. In order to monitor the biomineralization process, we used two geoelectrical methods - induced polarization (IP) and self-potential (SP) - in conjunction with conventional geochemical measurements. The IP data showed significant anomalies associated with ongoing FeS mineralization accompanying microbial activity. The magnitude of the IP response can be considered a proxy for the mass of minerals accumulating in the pore space and may provide insight into the aggregation state of the mineralization. Additionally, strong SP anomalies developed during the mineralization as a result of the continuous redox state changes following the microbial induced mineral formation. Visibly black precipitates accumulated with the column indicating FeS precipitation, and high H2S content confirmed the observed geochemical and geophysical data. Overall, the results suggest that the IP and SP methods can be used to monitor the progress of the microbial induced mineralization process associated with the precipitation of insoluble metal sulfides, and indirectly monitor the microbial activity within the subsurface. These methods can be valuable tools to increase the efficiency of bioremediation techniques.
NS44A-06
Electrical Signatures Associated with Abiotic and In Vitro Dissimilatory Iron Reduction
Several researchers have described anomalous electrical signatures associated with bacterial activity in anoxic zones in aquifers containing organic contaminants. It is thought that these signals can be attributed to (bio)geochemical changes caused by the oxidation of organic contaminants and the reduction of associated species like iron oxides. We report laboratory observations of changes in electrical conductivity (EC) that can be attributed to specific (bio)geochemical reactions involving reductive dissolution of iron oxides enzymatically and nonenzymatically. Abiotic reduction of ferrihydrite by ascorbic acid in batch experiments causes a cumulative 20- 40% increase in measured conductivity, (EC increases by ~300 μ S/cm). This change can be attributed to a decrease in conductivity (Δ EC) from increasing proton activity (Δ pH = 3.25 --> 5.07, Δ EC = -200 μ S/cm) and an increase in dissolved Fe(II) (Δ [Fe] = 2.2 - 3.3 mM, Δ EC = 400 -700 μ S/cm). Conductivity is presumably unaffected by Fe(II) sorbed to the ferrihydrite. Rates calculated from this method are comparable to literature rates for similar experiments. In a similar in vitro system, total membrane fractions from Shewanella oneidensis MR-1 were used to reduce ferrihydrite in the presence of formate and HEPES buffer. A 10 - 15% increase in conductivity was observed in the batch experiment (Δ EC = ~280 μ S/cm). This Δ EC is attributed to an increase in the concentration of de-protonated HEPES as well as carbonate ion as formate is oxidized. Fe(II) released in this system is quickly sorbed onto the ferrihydrite surface and is not thought to change conductivity. Despite the sorption of iron in these in vitro experiments, conductivity changes measurably and documents the rate of the reaction. Accessory changes like buffer de- protonation play an important role in interpreting the electrical signals caused by dissimilatory iron reduction. In order to accurately interpret field data it is necessary to anticipate these changes and attempt to monitor them chemically. Through this work we hope to link chemical changes caused by bacterial activity in the lab to electrical anomalies measured in the field. By quantifying the changes in conductivity, we will investigate rates and distributions of bacterial activity at the field scale.