Deconstructing the redox cascade: what role do microbial exudates (flavins) play?
Ekaterina Markelova A B E , Christopher T. Parsons A , Raoul-Marie Couture A C , Christina M. Smeaton A , Benoit Madé D , Laurent Charlet A B and Philippe Van Cappellen A
+ Author Affiliations
- Author Affiliations
A Ecohydrology Research Group, Department of Earth and Environmental Sciences and Water Institute, University of Waterloo, 200 University Avenue W, Waterloo, ON N2L 3G1, Canada.
B University Grenoble Alpes, University Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, 38000 Grenoble, France.
C Norwegian Institute for Water Research-NIVA, Gaustadalléen 21, 0349 Oslo, Norway.
D Andra, National Radioactive Waste Management Agency, Research and Development Division, Transfer Migration Group, 1/7 Rue Jean Monnet, 92298 Chatenay Malabry Cedex, France.
E Corresponding author. Email: emarkelo@uwaterloo.ca
Environmental Chemistry 14(8) 515-524 https://doi.org/10.1071/EN17158
Submitted: 12 September 2017 Accepted: 8 November 2017 Published: 22 March 2018
Environmental context. Redox 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.
Abstract. Redox 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−.
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