Effect of three Electron Shuttles on Bioreduction of Ferric Iron in two Acidic and Calcareous soils

Iron cycle is one of the most important biogeochemical processes which affect the availability of iron in soils. Ferric iron oxides are the most abundant forms of iron in soils and sediments. Ferric iron is highly insoluble at circumneutral pH. Present investigations have shown that the structural ferric iron bound in clay minerals is reduced by some microorganisms. Anaerobic bacteria reduce ferric iron which bound to soil clay minerals under anaerobic conditions. They have the ability to use ferric iron as a terminal electron acceptor. Many studies presented that dissimilatory iron reducing bacteria (DIRB) mediate the transfer of electrons from small organic molecules like acetate and glucose to various humic materials (electron shuttles) which then pass electrons abiotically to ferric iron oxyhydroxide and phyllosilicate minerals. Electron shuttles like AQDS, a tricyclic quinone, increase the rate of iron reduction by iron reducing bacteria on sites of iron oxides and oxyhydroxides. By increasing the rate of bioreduction of ferric iron, the solubility and availability of iron enhanced meaningfully. Royer et al. (2002) showed that bioreduction of hematite (common iron mineral in soils) increased more than three times in the presence of AQDS and Shewanella putrefaciens comparedto control treatments. Previous works have mostly used synthetic minerals as electron acceptor in bioreduction process. Furthermore, the effect of quinones as electron acceptor for microorganisms were studied with poorly crystalline ferric iron oxides . The main objective of this study was to study the effect of AQS, humic acid and fulvic acid (as electron shuttle) and Shewanella sp. and Pseudomonas aeruginosa, on bioreduction of native ferric iron in two acidic and calcareous soils.
An experiment was conducted in a completely randomized design with factorial arrangement and three replications in vitro condition. The soil samples collected from locations in Mashhad and Guilan cities, Iran, in 2015. The soil samples were air dried in a glasshouse and later subjected to general analysis. Some part of the soil samples were kept at 4 oC as fresh soil samples for bioreduction assay. In that part of experiment, all soil samples were treated with glucose (10 mM) as electron donor. Native ferric iron considered as electron acceptor. Then soil samples were treated with AQS, humic acid and fulvic acid (as electron shuttles) and inoculated with bacterial cells (Shewanella sp. and P. aeruginosa) and they were incubated for 30 days in an incubator at 30 and 37 oC according to the optimum temperature for bacteria in an anaerobic condition. At the end of incubation time, ferrous and acid extractable iron were determined with Ferrozine assay by spectrophotometer in 562 nm (8, 25).
Results showed that the AQS had a noticeable effect on ferrous iron concentrations in both acidic and calcareous soils. In these cases ferrous iron concentrations were 8 and 15.7 times higher compared to initial concentration in acidic and calcareous soils, respectively. The Shewanella sp. intensified ferrous iron concentration 7.2 and 16.3 fold in acidic and calcareous soils, respectively but P. aeruginosa increased it 5.6 and 12.1 fold compared to initial concentration of ferrous iron. In acidic soil, in the presence of Shewanella sp. and AQS, ferrous and acid extractable iron concentrations were 1.45 and 4.50 mg g-1, respectively. Results showed that 11.7 fold enhancements occur in the presence of Shewanella sp. and AQS compared to initial (0.385 mg g-1) concentration of iron in acidic soil. When P. aeruginosa was inoculated in acidic soil in the presence of AQS, soluble ferrous iron concentration was 1.27 mg g-1. The acid extractable iron in this treatment was 2.85 mg g-1. The concentration of soluble ferrous iron in calcareous soil was 0.81 mg g-1, when AQS was added to Shewanella sp. treatments. That value was 0.54 when P. aeruginosa was added. The acid extractable iron was 3.90 mg g-1 in the presence of AQS and Shewanella sp. By adding P. aeruginosa, acid extractable iron was 2.84 mg g-1 compared to control treatments.
Dissimilatory ferric iron reduction is a potentially important process in controlling contaminant fate. It has the potential for being particularly useful in the remediation of metals and radionuclides. Means for stimulating ferric iron reduction will be useful in enhancing bioremediation process. Results illustrated that the Shewanella sp. and P. aeruginosa were enhanced the bioreduction of ferric iron in the presence of AQS, humic acid and fulvic acid in soils. When soil samples were inoculated with Shewanella sp., and AQS was added to the soil samples (in acidic and calcareous soil samples) the concentration of ferrous iron increased intensively.