Synergistic iron reduction and citrate dissimilation by Shewanella alga and Aeromonas veronii

Two bacterial isolates from Great Bay Estuary, New Hampshire, in co-culture carried out anaerobic dissimilation of citric acid with Fe(III) as the terminal electron acceptor. Neither isolate oxidized citrate with Fe(III) anaerobically in axenic culture. The Fe(III) reducer, Shewanella alga strain BrY, did not grow anaerobically with citrate as an energy source. The citrate utilizer, Aeromonas veronii, did not reduce iron axenically with a variety of electron donors including citrate. The onset of iron reduction by the co-culture occurred after initiation of citrate dissimilation and just prior to initiation of growth by either organism (as measured by viable plate counts). Anaerobic culture growth rates and final cell densities of each bacterial strain were greater in co-culture than in axenic cultures. By 48 h of growth, the co-culture had consumed 27 mM citrate as compared with 12 mM dissimilated by the axenic culture of A. veronii. By 48 h the coculture produced half as much formate (6 mM) and twice as much acetate (40 mM) as did A. veronii grown axenically (12 mM and 20 mM, respectively). Formate produced from citrate by A. veronii appeared to have supported growth and Fe(III) reduction by S. alga. Although not obligatory, nutrient coupling between these two organisms illustrates that fermentative (A. veronii-type) organisms can convert organic compounds such as citrate to those used as substrates by dissimilatory Fe(III) reducers, including S. alga. This synergism broadens the range of substrates available for iron reduction, stimulates the extent and rate of organic electron donor degradation (and that of iron reduction) and enhances the growth of each participant.