Deconstructing the redox cascade: what role do microbial exudates (flavins) play?

Abstract

Environmental contextRedox potential is a controlling variable in aquatic chemistry. Through time series data, we show that microbial exudates released by bacteria may control trends in redox potential observed in natural waters. In particular, electron transfer between these exudates and the electrode could explain the values measured in the presence of abundant oxidants such as oxygen and nitrate. AbstractRedox electrodes are commonly used to measure redox potentials (EH) of natural waters. The recorded EH values are usually interpreted in terms of the dominant inorganic redox couples. To further advance the interpretation of measured EH distributions along temporal and spatial redox gradients, we performed a series of reactor experiments in which oxidising and reducing conditions were alternated by switching between sparging with air and N2. Starting from a simple electrolyte solution and ending with a complex biogeochemical system, common groundwater solutes, metabolic substrates (NO3− and C3H5O3−), bacteria (Shewanella oneidensis MR-1) and goethite (α-FeOOH(s)) were tested by increasing the system complexity with each subsequent experiment. This systematic approach yielded a redox cascade ranging from +500 to −350 mV (pH ~7.4). The highest and lowest EH values registered by the platinum (Pt) electrode agreed with Nernstian redox potentials predicted for the O2/H2O2 and FeOOH/Fe2+(aq) couples respectively. Electrode poisoning by the organic pH buffer (MOPS) and addition of bacteria to the aerated solutions resulted in marked decreases in measured EH values. The latter effect is attributed to the release of flavins by Shewanella oneidensis MR-1 to the medium. As expected, equilibrium with the non-electroactive NO3−/NO2−/NH4+ redox couples could not account for the EH values recorded during dissimilatory nitrate reduction to ammonium (DNRA). However, the observed EH range for DNRA coincided with that bracketed by EH values measured in separate abiotic solutions containing either the oxidised (+324 ± 29 mV) or reduced (−229 ± 40 mV) forms of flavins. The results therefore suggest that the Pt electrode detected the presence of the electroactive flavins, even at submicromolar concentrations. In particular, flavins help explain the fairly low EH values measured in the presence of strong oxidants, such as O2 and NO3−.

Structural Incorporation of Sorbed Molybdate during Iron(II)-Induced Transformation of Ferrihydrite and Goethite under Advective Flow Conditions

“…Aqueous Fe speciation was simulated using the Nernst equation and input pe and total Fe values. However, previous studies have reported issues with Eh measurements in the presence of organic buffers and the Ferrozine method detection limit (i.e., 0.3 μM) is above anticipated effluent Fe­(III) concentrations during Phase II . Consequently, we modified Eh for consistency with reported values for Fe­(II) oxidation coupled with Fe­(III) (oxyhydr)­oxide reduction. , Finally, we performed a sensitivity analysis of Eh values (range 0 to −300 mV) and found that associated variations in modeled SIs did not strongly affect data interpretations.…”

Genomic evidence for a chemical link between redox conditions and microbial community composition

“…Because the electroactivity of specific redox couples is affected by kinetic barriers that are not well understood, ORP measurements generally have a more difficult interpretation. Nevertheless, they can yield information about electrochemical reactions that are relevant to microbial growth, especially if the measurements are made continuously in time (49).…”

Oxidation of Fe(II) by Flavins under Anoxic Conditions

“…The total concentrations of RBF and FMN were determined at 450 nm on a UV–vis spectrometer kept in the anaerobic glove box following established protocols . In the complexation experiments of Fe 2+ and Fe 3+ with RBF and FMN, the concentrations of free (i.e., un-complexed) RBF and FMN were measured by fluorescence at 520 nm after excitation at 450 nm using a Flexstation Multimode Microplate Reader. (Note: complexed RBF and FMN are non-fluorescent).…”