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A Novel Electrophototrophic Bacterium Rhodopseudomonas palustris Strain RP2, Exhibits Hydrocarbonoclastic Potential in Anaerobic Environments
Krishnaveni Venkidusamy, Mallavarapu Megharaj
KI-Zusammenfassung
72 % KonfidenzThis paper isolates and characterizes Rhodopseudomonas palustris strain RP2, a novel electrophototrophic bacterium from hydrocarbon-fed microbial electrochemical remediation systems (MERS). The strain exhibits direct electrode respiration, dissimilatory Fe(III) reduction, anaerobic nitrate reduction, nitrogen fixation, and the ability to degrade diesel-range hydrocarbons (C9–C36) under anoxic phototrophic conditions. In acetate-fed MFCs, RP2 generated 305 ± 10 mA/m² current density and 131.65 ± 10 mW/m² power density with 46.7 ± 1.3% coulombic efficiency. When using 800 mg/L diesel hydrocarbons in MERS, the strain achieved 47.4 ± 2.7% hydrocarbon removal and 21 ± 3 mA/m² current density, though only under phototrophic conditions.
Zentrale Erkenntnisse
Schwellenwert Electrochemical activity is strictly anaerobic: anaerobically grown cells showed redox peaks at −394 to −399 mV (reduction) and −200 to +100 mV (oxidation), but aerobically grown cells lacked all electrochemical activity.
Was heißt das. Maintain strict anaerobic conditions in the anode chamber; aerobic exposure will eliminate electrode respiration capability regardless of other conditions.
Schwellenwert Hydrocarbon degradation requires light: photosynthetic anoxic conditions achieved 30.6 ± 0.68% DRH removal (90 days, sulfate), but chemosynthetic anaerobic and aerobic samples showed no degradation or biomass increase.
Was heißt das. Light is non-negotiable for hydrocarbon oxidation in this strain; dark MERS or subsurface applications will not work with RP2 alone.
Zielkonflikt Acetate-fed MFCs outperform hydrocarbon-fed MERS by 14.5-fold in current density: acetate generated 305 ± 10 mA/m² vs. 21 ± 3 mA/m² on 800 mg/L diesel hydrocarbons, despite similar anaerobic conditions.
Was heißt das. Acetate is a far superior electron donor for power generation; use it for energy recovery, but accept much lower current if hydrocarbon remediation is the primary goal.
Schwellenwert Fe(III) reduction is light-dependent: photoheterotrophic conditions achieved 69.5 ± 0.41% Fe(III) oxide reduction, but chemosynthetic grown cells showed no reduction.
Was heißt das. If using RP2 for metal remediation (e.g., iron oxide), ensure illumination; dark conditions will not mobilize Fe(III).
Zielkonflikt Coulombic efficiency increases with current density: CE rose from ~20% at low current to 46.7% at 259.90 mA/m², indicating better substrate utilization at higher electrochemical flux.
Was heißt das. Operate at higher current densities (closer to 300 mA/m²) to maximize coulombic efficiency and substrate conversion; low-current operation wastes substrate.
Widersprüche1
Nur Konflikte, die die Publikation selbst zeigt, jeweils am Text geprüft.
Innerhalb der Publikation Hydrocarbon degradation rate differs dramatically between MERS and batch photosynthetic incubations despite both being anaerobic and illuminated.
Eine SeiteMERS achieved 47.4 ± 2.7% DRH removal in 30 days at 800 mg/L
“An average of 47.4 ± 2.7% decrease in DRH was observed in closed circuit MERS inoculated with the strain RP2 by the end of experiment (30days).”
Andere SeitePhotosynthetic anoxic batch incubations achieved 30.6 ± 0.68% DRH removal in 90 days at 4000 mg/L (5× higher substrate concentration)
“A higher percentage of DRH removal (30.6 ± 0.68%, 90th day) was noticed in the triplicates of sulfate containing anoxic, photoheterotrophic samples.”
Einordnung der Autoren. Authors do not explicitly reconcile this. The difference likely reflects: (1) MERS provides continuous electrochemical driving force (21 mA/m² current), whereas batch incubations rely on chemical gradients; (2) electrode biofilm may concentrate cells and enhance mass transfer; (3) MERS operates at lower substrate concentration (800 vs. 4000 mg/L), reducing inhibition. The paper states 'The current density and degradation profiles suggest that MERS could utilize the DRH as a sole source of energy generation' but does not quantify the electrochemical enhancement factor.
Ansatzpunkte6
Konkrete Lücken in dieser Publikation, jeweils mit einem Experiment, das sie schließen würde.
Hoch Nitrate reduction kinetics and N2O/NO2− accumulation during anaerobic hydrocarbon degradation are not measured. The strain possesses 'anaerobic nitrate reduction' capability but no experiments report NO3− consumption rate, final nitrogen species, or N2O production.
Abstract lists 'anaerobic nitrate reduction' as a salient property; Methods section mentions '10 mM nitrate' as an electron acceptor in growth experiments (Culture Conditions section), but Results section (Hydrocarbonosclastic Potential) shows only sulfate, iron, and 'photosynthetic only' conditions in Figure 6A. No nitrogen balance or speciation data provided.
Versuch. Operate MERS with 10 mM NO3− as sole electron acceptor at 800 mg/L DRH; measure NO3−-N consumption (mg/L), NO2−-N accumulation, N2O headspace concentration (ppm or µmol), and final nitrogen species by ion chromatography over 30 days. Compare to sulfate and iron conditions.
Hoch Cathode potential and catholyte pH are not controlled or reported. All MFC experiments use air cathodes with platinum catalyst but no cathode potential (vs. SHE or Ag/AgCl) or pH profile is given.
Methods state 'Single chamber air cathodes contained 0.5 mg cm⁻² of platinum catalyst' and 'dual chamber cubic MFCs were constructed using two lexan glasses separated by a cation exchange membrane' but Results section provides only voltage (open circuit 700 mV, closed circuit 446 mV) and current. No cathode half-reaction potential, oxygen reduction kinetics, or pH drift in cathode chamber reported.
Versuch. Measure cathode potential (vs. Ag/AgCl) and pH in dual-chamber MERS with DRH at 800 mg/L over 30 days. Vary cathode potential (−0.2 to +0.6 V) and measure DRH removal rate and current density to identify optimal cathode operating window.
Hoch Anode potential during hydrocarbon oxidation is not reported. Cyclic voltammetry shows redox peaks (−394 to −399 mV reduction, −200 to +100 mV oxidation) but no steady-state anode potential during MERS operation is given.
CV studies (Figure 5) show quasi-reversible redox behavior with midpoint potential ~−278 mV, but MERS Results section (Figure 6B–C) reports only current density and voltage across the external load (1000 Ω). No anode potential vs. reference electrode during DRH oxidation provided.
Versuch. Insert reference electrode (Ag/AgCl) in anode chamber of MERS fed with 800 mg/L DRH. Record anode potential (vs. Ag/AgCl) and current density simultaneously over 30 days. Correlate anode potential to DRH removal rate and identify potential window for maximum hydrocarbon oxidation.
Mittel Hydraulic retention time (HRT) and mass transfer limitations are not explored. All experiments are batch or fed-batch with no continuous-flow operation or HRT variation.
MERS experiments report '30 days' total duration and '90 h lag time' before steady current, but no HRT, volumetric flow rate, or residence time distribution provided. Batch mode may mask mass transfer constraints on hydrocarbon bioavailability.
Versuch. Operate continuous-flow MERS with DRH at 800 mg/L and vary HRT (1, 3, 7, 14 days). Measure DRH removal (%), current density (mA/m²), and effluent nitrogen species (NO3−, NO2−, NH4+) to identify optimal residence time.
Mittel C/N ratio effects on hydrocarbon degradation and nitrogen cycling are not tested. Experiments use fixed 10 mM nitrate but DRH concentration varies (800–4000 mg/L), implying C/N ratios from ~80:1 to ~400:1 (by mass), yet no analysis of nitrogen limitation or excess.
Methods state cultures were 'supplemented with Wolfe's trace elements and vitamins' and '10 mM nitrate' was used, but no C/N ratio optimization or nitrogen balance (input vs. output) reported. Figure 6A shows nitrate as one electron acceptor option but no kinetic comparison or N-species measurement.
Versuch. Fix DRH at 800 mg/L and vary NO3− from 1 to 20 mM (C/N ratio ~800:1 to ~40:1 by mass). Measure DRH removal rate, current density, and final NO3−, NO2−, NH4+, and N2O to identify optimal C/N for coupled hydrocarbon oxidation and denitrification.
Mittel Dissolved oxygen intrusion and its effect on electrode respiration are not quantified. The paper emphasizes anaerobic conditions but does not report dissolved oxygen (DO) measurements or the impact of micro-aerophilic conditions on current generation and hydrocarbon degradation.
Methods state 'anodic chamber was flushed with nitrogen gas' and 'sealed with aluminum crimps' but no DO sensor data, oxygen ingress rate, or tolerance to low-level O2 provided. CV studies compare 'aerobically grown cells' (no activity) to 'anaerobically grown cells' (active) but do not test the transition or threshold DO.
Versuch. Operate MERS with DRH at 800 mg/L and gradually increase headspace O2 (0%, 0.1%, 0.5%, 1%, 5%) while measuring current density, anode potential, and DRH removal. Identify the maximum tolerable DO for electrode respiration.
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Zusammenfassung
An electrophototrophic, hydrocarbonoclastic bacterium Rhodopseudomonas palustris stain RP2 was isolated from the anodic biofilms of hydrocarbon fed microbial electrochemical remediation systems (MERS). Salient properties of the strain RP2 were direct electrode respiration, dissimilatory metal oxide reduction, spore formation, anaerobic nitrate reduction, free living diazotrophy and the ability to degrade n-alkane components of petroleum hydrocarbons (PH) in anoxic, photic environments. In acetate fed microbial electrochemical cells, a maximum current density of 305 ± 10 mA/m² (1000Ω) was generated (power density 131.65 ± 10 mW/m²) by strain RP2 with a coulombic efficiency of 46.7 ± 1.3%. Cyclic voltammetry studies showed that anaerobically grown cells of strain RP2 is electrochemically active and likely to transfer electrons extracellularly to solid electron acceptors through membrane bound compounds, however, aerobically grown cells lacked the electrochemical activity. The ability of strain RP2 to produce current (maximum current density 21 ± 3 mA/m²; power density 720 ± 7 μW/m², 1000 Ω) using PH as a sole energy source was also examined using an initial concentration of 800 mg l⁻¹ of diesel range hydrocarbons (C9-C36) with a concomitant removal of 47.4 ± 2.7% hydrocarbons in MERS. Here, we also report the first study that shows an initial evidence for the existence of a hydrocarbonoclastic behavior in the strain RP2 when grown in different electron accepting and illuminated conditions (anaerobic and MERS degradation). Such observations reveal the importance of photoorganotrophic growth in the utilization of hydrocarbons from contaminated environments. Identification of such novel petrochemical hydrocarbon degrading electricigens, not only expands the knowledge on the range of bacteria known for the hydrocarbon bioremediation but also shows a biotechnological potential that goes well beyond its applications to MERS.
Schlagwörter
Kennungen
- PubMed
- 27462307
- Zeitschrift
- Frontiers in Microbiology
- Jahr
- 2016
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