Camelia Rotaru

Potential for Direct Interspecies Electron Transfer in Methanogenic Wastewater Digester Aggregates

ABSTRACT Mechanisms for electron transfer within microbial aggregates derived from an upflow anaerobic sludge blanket reactor converting brewery waste to methane were investigated in order to better understand the function of methanogenic consortia. The aggregates were electrically conductive, with conductivities 3-fold higher than the conductivities previously reported for dual-species aggregates of Geobacter species in which the two species appeared to exchange electrons via interspecies electron transfer. The temperature dependence response of the aggregate conductance was characteristic of the organic metallic-like conductance previously described for the conductive pili of Geobacter sulfurreducens and was inconsistent with electron conduction through minerals. Studies in which aggregates were incubated with high concentrations of potential electron donors demonstrated that the aggregates had no significant capacity for conversion of hydrogen to methane. The aggregates converted formate to methane but at rates too low to account for the rates at which that the aggregates syntrophically metabolized ethanol, an important component of the reactor influent. Geobacter species comprised 25% of 16S rRNA gene sequences recovered from the aggregates, suggesting that Geobacter species may have contributed to some but probably not all of the aggregate conductivity. Microorganisms most closely related to the acetate-utilizing Methanosaeta concilii accounted for more than 90% of the sequences that could be assigned to methane producers, consistent with the poor capacity for hydrogen and formate utilization. These results demonstrate for the first time that methanogenic wastewater aggregates can be electrically conductive and suggest that direct interspecies electron transfer could be an important mechanism for electron exchange in some methanogenic systems. IMPORTANCE The conversion of waste organic matter to methane is an important bioenergy strategy, and a similar microbial metabolism of complex organic matter in anaerobic soils and sediments plays an important role in the global carbon cycle. Studies with laboratory cultures have demonstrated that hydrogen or formate can serve as an electron shuttle between the microorganisms degrading organic compounds and methanogens. However, the importance of hydrogen and formate as intermediates in the conversion of organic matter to methane in natural communities is less clear. The possibility that microorganisms within some natural methanogenic aggregates may directly exchange electrons, rather than producing hydrogen or formate as an intermediary electron carrier, is a significant paradigm shift with implications for the modeling and design of anaerobic wastewater reactors and for understanding how methanogenic communities will respond to environmental perturbations. The conversion of waste organic matter to methane is an important bioenergy strategy, and a similar microbial metabolism of complex organic matter in anaerobic soils and sediments plays an important role in the global carbon cycle. Studies with laboratory cultures have demonstrated that hydrogen or formate can serve as an electron shuttle between the microorganisms degrading organic compounds and methanogens. However, the importance of hydrogen and formate as intermediates in the conversion of organic matter to methane in natural communities is less clear. The possibility that microorganisms within some natural methanogenic aggregates may directly exchange electrons, rather than producing hydrogen or formate as an intermediary electron carrier, is a significant paradigm shift with implications for the modeling and design of anaerobic wastewater reactors and for understanding how methanogenic communities will respond to environmental perturbations.

Chloride Dispersion across Silt Deposits in a Glaciated Bedrock River Valley

Soil and groundwater from the Neponset River floodplain deposit that receive high concentrations of deicing agents from nearby highways were investigated. The silty sand floodplain is separated by a silty aquitard from the underlying aquifer that serves as a public water supply. We made a transport-based assessment of the capacity of the aquitard to protect the underlying aquifer. One hundred seventeen soil samples and 469 groundwater samples collected during a period of 4 yr from boreholes and 10 wells grouped in two well clusters were analyzed for dissolved Cl concentration. The soil characterization and groundwater monitoring results agreed, showing a very slow change in subsurface Cl contamination with time. These data also calibrated a vertical one-dimensional advective-dispersive transport model across the deposits. Advective transport dominated only in the top 3.37 m of the floodplain deposit, with dispersion being the main transport mechanism below this depth. Due to the silty nature of the aquitard, dispersion rather than diffusion was the main transport mechanism into the floodplain-aquitard system. Soil and groundwater quality data confirmed a Cl concentration at the floodplain surface near the highway runoff drainage outlets of 2450 mg L. The model estimated a vertical dispersivity at the site of 8 mm and a vertical hydrodynamic dispersion coefficient of 3.71 × 10 m s. These data confirmed the aquitards capacity to contain deicing agents, protecting the underlying aquifer from contamination.

Case Study of Steady Oxygen Concentration Gradients in a Groundwater Plume from a Highway Infiltration Basin

We measure and model the steady transport of specific conductivity and dissolved oxygen through a groundwater plume from a highway infiltration basin in southeastern Massachusetts. Specific conductivity is treated as a conservative surrogate for runoff contamination, and the data calibrate a 0.27-m vertical dispersivity α of the aquifer and the bottom streamline elevation of the plume, which falls to an 8-m depth below the water table. The dissolved oxygen degrades as a first order reactant in the plume to levels below 1 mg/L, with a decay constant λ of 0.12  day−1 . The latter may be attributed in part to the historical use of an alternative de-icing agent calcium magnesium acetate on the highway, since acetate is a readily biodegradable substrate for microorganisms. The calibrated kinetics suggest that plume microbes and geochemistry degrade oxygen over two orders of magnitude faster than their ambient groundwater counterparts, which impose a linear decrease of dissolved oxygen concentration below the p...

Bacterial Diversity in Soil Exposed to Highway Runoff and De-icing Agents

Bacterial communities were profiled through two drilled soil cores below an infiltration basin that receives highway runoff and de-icing agents. Analyses of groundwater dissolved oxygen, physical and chemical properties of the soil complemented molecular phylogenetic determinations to distinguish ambient and contaminated plume zones. The bacterial community was previously characterized (when the site received high levels of acetate as de-icing agent) by being dominated by members of Geobacteraceae family. In this study, bacterial 16S rDNA gene clones showed highly diverse microbial communities, both into the plume and in the ambient aquifer, in which Geobacter spp. represents only a small fraction of them. The clones were affiliated with 32 and 23 classes identified from the sediment cores along the contaminant plume. The plume in the infiltration basin was anaerobic and iron-reducing, while the sediments in the underlying ambient aquifer were dominated by aerobes due to the presence of aerated ambient groundwater. These data indicate shifts in microbial communities in correlation with depth, substrate and oxygen availability in a de-icing agent impacted subsurface.

Monod Kinetics for Aerobic Biodegradation of Calcium Magnesium Acetate: Soil Microcosms from a Highway Shoulder

Monod kinetics were used to recalibrate published sets of soil microcosm data that document the aerobic biodegradation of calcium magnesium acetate in a highway shoulder. The natural attenuation is important because acetate can exert an appreciable oxygen demand on ground-water if it persists from the ground surface to the water table. Biomass dynamics were modeled, as well as acetate degradation, in the assumed abundance of oxygen and nutrients. The maximum specific growth rate (μM) and half saturation constant (KS) quantify the ability of an individual aerobe to degrade acetate and accordingly calibrate microcosm data from all locations in the shoulder. Thirty-two microcosm sets from SR-25 in southeastern Massachusetts yielded a calibrated μM of 0.00133 h[minus]1 and a KS of 0.087 kg acetate/m3 soil moisture. Calibrated (not measured) initial biomass concentrations varied from 10 to 1,000 μg biomass/g dry soil, with the largest values near the surface of the loamy sand cover layer and the smallest values above the capillary fringe consisting of uniform sand. The initial biomass concentrations and Monod kinetics yielded reaction rates consistent with field scale estimates of acetate degradation.

Groundwater Plume from an Infiltration Basin

We model and measure the groundwater plume from an infiltration basin in the Plymouth-Carver Aquifer near State Route 25 in southeastern Massachusetts. The advective transport model superimposes axisymmetric basin hydraulics and two dimensional ambient flow. The basin component incorporates a surface source of finite radius into a Hankel transform model for unconfined aquifers, while the ambient component varies linearly in the horizontal and vertical directions. Contaminant streamlines describe the resulting groundwater plume. Deicing agent constituent data confirm the plume boundaries, and calibrate plausible source concentrations and spread rates.

Soil Microcosm Based Estimates of CMA Oxygen Demand and Bacterial Density in a Highway Infiltration Basin

We measure bacterial density and acetate concentrations in 5 sets of aerobic soil microcosms from an infiltration basin in southeastern Massachusetts. The basin drains runoff from a highway deiced with calcium magnesium acetate (CMA), with the potential to sustain a biomass that exerts an oxygen demand on subsurface moisture. Acetate is measured by ion chromatograph, while biomass is estimated by most probable number analysis of serially sacrified soil microcosms. Monod kinetics calibrate the acetate and biomass data: the maximum specific reaction rate µM is estimated as 0.0337 hr-1, the half saturation constant KS is 0.044 kg acetate/m3 soil moisture, and the yield Y is 0.133 kg biomass/kg substrate. The viable acetate degrading population collapses abruptly once the substrate is consumed, and an endogenous decay rate b of 0.166 hr-1 fits the data, with decay time set equal to zero at the time of observed maximum biomass concentration. The calibration supports a simple simulation which suggests that substrate and viable biomass persist about 1 m into the soil moisture if the biomass is mobile and the soil moisture oxygenated. The simulated, transient oxygen demand of 460 µg oxygen/m2-s is an order of magnitude larger than the steady oxygen flux observed to enter the highway shoulder in the field.