Evidence for microbial-mediated iron oxidation at a neutrophilic groundwater spring

Abstract

The relative abundance of two morphologically distinct iron-oxidizing bacteria (FeOB), in particular, helically stalked Gallionella ferruginea and sheathed Leptothrix ochracea, decreased downstream from a neutral pH groundwater spring that feeds Ogilvie Creek near Deep River, Ontario, Canada. Redox potential and dissolved oxygen increased with distance from the spring, whereas concentrations of ferrous and total iron decreased, dropping from 2.8 and 4.5 mg/l at the source to <0.5 mg/l 3.0 m downstream. Rates of ferrous iron oxidation measured in microcosms included (i) a chemical control with filtered spring water to assess the rate of inorganic chemical oxidation, (ii) a negative control containing 50 ml of sodium azide (0.1 M final concentration)-treated FeOB from the spring to evaluate the impact of bacteriogenic iron oxides on the chemical oxidation rate, and (iii) an experimental microcosm with 50 ml of live FeOB from the spring to ascertain the influence of FeOB on oxidation rates. The rate constant in the presence of FeOB (0.147 min −1) was 2.6 times greater than the sodium azide-treated FeOB (0.056 min −1) and 6.1 times greater than the chemical control (0.024 min −1). The change in chemical speciation of total iron in the microcosm systems over time indicated further that BIOS behave as potent substrates for ferric iron precipitation with up to 75% of the total iron precipitated after 25 min in the sodium azide treated and live FeOB microcosms, whereas only 30% of the total iron precipitated in the chemical control.