Electron‐transfer coupling in microbial fuel cells: 1. comparison of redox‐mediator reduction rates and respiratory rates of bacteria

Abstract

AbstractRedox mediators promote electron transfer in microbial fuel cells. The reduction of a range of redox mediators by bacteria was studied in some detail in order to identify effective mediator—organism combinations. Rates of reduction of mediator dyes by bacteria were measured spectrophotometrically at 30°C under anaerobic conditions for standardised concentrations of organism, substrate and dye. The kinetics of dye reduction showed two general patterns: a simple, exponential curve or a complex curve with an initial linear rate followed by a faster exponential rate of reduction. Dye‐reduction rates were greater than rates of oxygen consumption (QO2) for several combinations of organism and redox dye. Thionine, brilliant cresyl blue, methylene blue and benzyl viologen were tested in combination withAlcaligenes eutrophus, Azotobacter chroococcum, Bacillus subtilis, Escherichia coli, Proteus vulgaris, Pseudomonas aeruginosaandPseudomonas putida, using glucose and succinate as substrates. Rates of reduction of alizarin brilliant blue, 2,6‐dichlorophenolindophenol, gallocyanine, new methylene blue,N,N‐dimethyl‐disulphonated thionine, phenazine ethosulphate, resorufin, safranine‐O, phenothiazinone and toluidine blue‐O were also measured withPr. vulgarisonly. ForE. coli, both QO2and the rate of thionine reduction increased with increasing temperature in the range 25 to 37°C, but forPr. vulgaristhionine reduction rates did not correlate with temperature in this way. Dye‐reduction rates and QO2forAz. chroococcumwere dependent on the components of the washing solution and/or the temperature at which cell suspensions were prepared. The results are discussed in relation to the use of these dyes as electron‐transfer mediators in microbial fuel cells.