Terminal Electron-accepting Conditions Research Articles

Influence of terminal electron-accepting conditions on the soil microbial community and degradation of organic contaminants of emerging concern

Better understanding of the fate and persistence of trace organic contaminants of emerging concern (CEC) in agricultural soils is critical for assessing the risks associated with using treated wastewater effluent to irrigate crops and land application of wastewater biosolids. This study reports on the influence of prevailing terminal electron-accepting processes (TEAPs, i.e., aerobic, nitrate-reducing, iron(III)-reducing, and sulfate-reducing conditions) and exposure to a mixture of nine trace CEC (90 ng/g each) on both the microbial community structure and CEC degradation in agricultural soil. DNA analysis revealed significant differences in microbial community composition following establishment of different TEAPs, but no significant change upon exposure to the mixture of CEC. The largest community shift was observed after establishing nitrate-reducing conditions and the smallest shift for sulfate-reducing conditions. Two of the CEC (atrazine and sulfamethoxazole) showed significant degradation in both bioactive and abiotic (i.e., sterilized) conditions, with half-lives ranging from 1 to 64 days for different TEAPs, while six of the CEC (amitriptyline, atenolol, trimethoprim, and three organophosphate flame retardants) only degraded in bioactive samples, with half-lives ranging from 27 to 90 days; carbamazepine did not degrade appreciably within 90 days in any of the incubations. Amplicon sequence variants (ASVs) from Firmicutes Hydrogenispora, Gemmatimonadetes Gemmatimonadaceae, and Verrucomicrobia OPB34 soil group were identified as potentially responsible for the biodegradation of organophosphate flame retardants, and ASVs from other taxa groups were suspected to be involved in biodegrading the other target CEC. These results demonstrate that CEC fate and persistence in agricultural soils is influenced by the prevailing TEAPs and their influence on the microbial community, suggesting the need to incorporate these factors into contaminant fate models to improve risk assessment predictions.

15N-nitrate and 34S-sulfate isotopic fractionation reflects electron acceptor ‘recycling’ during hydrocarbon biodegradation

The analysis of stable carbon isotopes for the assessment of contaminant fate in the aquifer is impeded in the case of petroleum hydrocarbons (TPH) by their chain length. Alternatively, the coupled nitrogen–sulfur–carbon cycles involved into TPH biodegradation under sulfate- and nitrate reducing conditions can be investigated using nitrogen (δ15N) and sulfur (δ34S) isotopic shifts in terminal electron acceptors (TEA) involved in anaerobic TPH oxidation. Biodegradation of a paraffin-rich crude oil was studied in anaerobic aquifer microcosms with nitrate (NIT), sulfate (SUL), nitrate plus sulfate (MIX) and nitrate under sulfate reduction suppression by molybdate (MOL) as TEA. After 8 months, TPH biodegradation was not different (around 33%) in experiments receiving only nitrate (NIT, MOL) versus under mixed TEA-conditions (MIX), despite higher biodiversity under mixed conditions (H′NIT and H′MOL≈5.9, H′MIX=8.0). Molybdate addition effected higher nitrate depletion, possibly by increasing the production of nitrate reductase. Additional sulfate depletion under mixed conditions suggested bioconversion of polar intermediates. Microcosms only receiving sulfate (SUL) showed no significant TEA and TPH decrease. A Rayleigh kinetic isotope enrichment model for isotopic 15N/14N and 34S/32S shifts in residual TEA gave apparent enrichment factors ɛN,NIT and ɛN,MOL values of −16.7 to −18.0‰ for nitrate as sole TEA and ɛN,MIX of −6.0‰ and ɛS,MIX of −4.1‰ under mixed electron accepting conditions. The low isotopic fractionation under mixed terminal electron accepting conditions was attributed to lithotrophic, sulfide-dependent denitrification by Thiobacillus species, while it was hypothesized that Desulfovibrio replenished the reduced sulfur pool via oxidation of polar hydrocarbon metabolites. Concurrently, organotrophic denitrification was performed by Pseudomonas species, with isotopic fractionation expressed by ɛN,MIX representing the superposition of both denitrification processes. This is, to our knowledge, the first characterization of sulfur and nitrogen isotopic shifts associated to concurrent organotrophic and lithotrophic denitrification in a hydrocarbon-contaminated environment, and offers the prospect of improved understanding of biogeochemical cycles including in situ hydrocarbon biotransformation.

Microbial enzymes that oxidize hydrocarbons

EDITORIAL article Front. Microbiol., 13 November 2013Sec. Microbiological Chemistry and Geomicrobiology https://doi.org/10.3389/fmicb.2013.00338

Microbially Mediated Abiotic Transformation of the Antimicrobial Agent Sulfamethoxazole under Iron-Reducing Soil Conditions

Large quantities of antimicrobial agents used in livestock production are released to soils by land application of manure, but only limited information is available on mechanisms that contribute to antimicrobial fate in soils under variable biogeochemical conditions. Dissipation of the sulfonamide antimicrobial sulfamethoxazole was examined in soil microcosms incubated under different terminal electron-accepting conditions (aerobic, nitrate-reducing, Fe(III)-reducing, and sulfate-reducing). Somewhat unexpectedly, sulfamethoxazole dissipation was fastest under Fe(III)-reducing conditions, with concentrations decreasing by >95% within 1 day. The rapid transformation was attributed to abiotic reactions between sulfamethoxazole and Fe(II) generated by microbial reduction of Fe(III) soil minerals. Separate experiments demonstrated that sulfamethoxazole was abiotically transformed in Fe(II)-amended aqueous suspensions of goethite (α-FeOOH((s))), and observed rate constants varied with the extent of Fe(II) sorption to goethite. Sulfamethoxazole transformation is initiated by a 1-electron reductive cleavage of the N-O bond in the isoxazole ring substituent, and observed products are consistent with Fe(II)-mediated reduction and isomerization processes. These findings reveal potentially important, but previously unrecognized, pathways that may contribute to the fate of sulfamethoxazole and related chemicals in reducing soil environments.

TBA biodegradation in surface-water sediments under aerobic and anaerobic conditions.

The potential for [U-14C] TBA biodegradation was examined in laboratory microcosms under a range of terminal electron accepting conditions. TBA mineralization to CO2 was substantial in surface-water sediments under oxic, denitrifying, or Mn(IV)-reducing conditions and statistically significant but low under SO4-reducing conditions. Thus, anaerobic TBA biodegradation may be a significant natural attenuation mechanism for TBA in the environment, and stimulation of in situ TBA bioremediation by addition of suitable terminal electron acceptors may be feasible. No degradation of [U-14C] TBA was observed under methanogenic or Fe(III)-reducing conditions.

Effect of redox conditions on MTBE biodegradation in surface water sediments.

Microbial degradation of methyl tert-butyl ether (MTBE) was observed in surface water-sediment microcosms under anaerobic conditions. The efficiency and products of anaerobic MTBE biodegradation were dependent on the predominant terminal electron-accepting conditions. In the presence of substantial methanogenic activity, MTBE biodegradation was nominal and involved reduction of MTBE to the toxic product, tert-butyl alcohol (TBA). In the absence of significant methanogenic activity, accumulation of [14C]TBA generally decreased, and mineralization of [U-14C]MTBE to 14CO2 generally increased as the oxidative potential of the predominant terminal electron acceptor increased in the order of SO4, Fe(III), Mn(IV) < NO3 < O2. Microbial mineralization of MTBE to CO2 under Mn(IV)-or SO4-reducing conditions has not been reported previously. The results of this study indicate that microorganisms inhabiting the sediments of streams and lakes can degrade MTBE effectively under a range of anaerobic terminal electron-accepting conditions. Thus, anaerobic bed sediment microbial processes may provide a significant environmental sink for MTBE in surface water systems throughout the United States.

Is interspecies hydrogen transfer needed for toluene degradation under sulfate-reducing conditions?

Sediments from a hydrocarbon-contaminated aquifer, where periodic shifts between sulfate reduction and methanogenesis occurred, were examined to determine whether the degradation of toluene under sulfate-reducing conditions depended on interspecies hydrogen transfer. Toluene degradation under sulfate-reducing conditions was inhibited by the addition of 5 mM sodium molybdate, but the activity was not restored upon the addition of an actively growing, hydrogen-using methanogen. Toluene degradation was not inhibited in microcosms where hydrogen levels were maintained at a level theoretically sufficient to inhibit toluene degradation if the process proceeded via interspecies hydrogen transfer. Finally, the addition of carbon monoxide, a potent inhibitor of hydrogenase activity, inhibited hydrogen but not toluene consumption in sulfate-reducing microcosms. These results suggest that toluene is degraded directly by sulfate-reducing bacteria without the involvement of interspecies hydrogen transfer. The sequence of experiments used to reach this conclusion could be applied to determine the role of interspecies hydrogen transfer in the degradation of a variety of compounds in different environments or under different terminal electron-accepting conditions.

Is interspecies hydrogen transfer needed for toluene degradation under sulfate-reducing conditions?

Sediments from a hydrocarbon-contaminated aquifer, where periodic shifts between sulfate reduction and methanogenesis occurred, were examined to determine whether the degradation of toluene under sulfate-reducing conditions depended on interspecies hydrogen transfer. Toluene degradation under sulfate-reducing conditions was inhibited by the addition of 5 mM sodium molybdate, but the activity was not restored upon the addition of an actively growing, hydrogen-using methanogen. Toluene degradation was not inhibited in microcosms where hydrogen levels were maintained at a level theoretically sufficient to inhibit toluene degradation if the process proceeded via interspecies hydrogen transfer. Finally, the addition of carbon monoxide, a potent inhibitor of hydrogenase activity, inhibited hydrogen but not toluene consumption in sulfate-reducing microcosms. These results suggest that toluene is degraded directly by sulfate-reducing bacteria without the involvement of interspecies hydrogen transfer. The sequence of experiments used to reach this conclusion could be applied to determine the role of interspecies hydrogen transfer in the degradation of a variety of compounds in different environments or under different terminal electron-accepting conditions.

Detection of Phenol and Benzoate as Intermediates of Anaerobic Benzene Biodegradation under Different Terminal Electron-Accepting Conditions

A sulfate-reducing bacterial enrichment that anaerobically metabolized benzene was obtained from a petroleum-contaminated aquifer. During biodegradation, we observed the transient accumulation of p...

Fe(III) reduction activity and cytochrome content of Shewanella putrefaciens grown on ten compounds as sole terminal electron acceptor

Shewanella putrefaciens was grown on a series of ten alternate compounds as sole terminal electron acceptor. Each cell type was analyzed for Fe(III) reduction activity, absorbance maxima in reduced-minus-oxidized difference spectra and heme-containing protein content. High-rate Fe(III) reduction activity, pronounced difference maxima at 521 and 551 nm and a predominant 29.3 kDa heme-containing protein expressed by cells grown on Fe(III), Mn(IV), U(VI), SO 2− 3 and S 2O 2− 3, but not by cells grown on O 2, NO 3, NO 2 , TMAO or fumarate. These results suggest that microbial Fe(III) reduction activity is enhanced by anaerobic growth on metals and sulfur compounds, yet is limited under all other terminal electron-accepting conditions.

Does lead affect microbial metabolism in aquifer sediments under different terminal electron accepting conditions?

High concentrations of aliphatic organic acids (∼1 mM acetate) in groundwater from a petroleum hydrocarbon‐contaminated aquifer are associated with elevated concentrations (∼30 μg/L) of lead. This pattern suggests that toxic effects of lead have contributed to the observed accumulation of organic acids. This hypothesis was tested by examining the effects of lead on carbon mineralization and organic acid accumulation in petroleum‐contaminated aquifer material incubated under different terminal electron‐accepting conditions. Lead was found to inhibit fermentation (organic acid accumulation) as well as microbial carbon mineralization (CO2 or CH4 production) under aerobic, nitrate‐reducing, sulfate‐reducing, and methanogenic conditions. Thus Pb does not appear to be the primary cause of organic acid accumulation at the field site. Rather, substantial accumulation of aliphatic organic acids occurred only in methanogenic microcosms, and only trace amounts of acetic acid were detected in sulfate‐reducing microcosms. This pattern parallels field observations in which high organic acid concentrations were detected in methanogenic zones, but only low concentrations of acetic acid were detected in sulfate‐reducing zones.