The iron cycling mediated by a single strain Shewanella oneidensis MR-1 and its implication for nitrogen removal

Microbially mediated nitrate-reducing Fe(II) oxidation (NRFO) process has been recognized as an effective route for removing nitrogen in wastewater. However, the need for continuous dose of iron blocks NRFO technology to be scaled up. Microbial iron cycling can improve iron utilization efficiency, but whether it can be mediated by a single strain remains elusive. Here, Shewanella oneidensis strain MR-1 was employed to mediate Fe redox process with the subsequent addition of Fe(III), nitrate, and lactate under anaerobic conditions. Fe(II) kinetic curve showed that strain MR-1 could efficiently reduce Fe(III) with a reduction rate of 2.55 mmol·L−1·d−1, producing a great amount of Fe(II). After nitrate addition, strain MR-1 mediated the NRFO process. Fe(III) reduction and NRFO constituted Fe cycle, indicating that strain MR-1 successfully carried out Fe redox cycling. It is noted that two iron redox cycles were achieved, but the second cycle was weaker. That is, about 86.7 % of total iron was reduced in the first cycle, but only 4.6 % was reduced in the second cycle. As nitrate reduction is the key process for iron cycling, the unsustainability should be related to nitrate reduction inhibition. The possible mechanisms were investigated, revealing that nitrate reduction was inhibited by cell encrustation and nutrient shortage. These findings expand the understanding of microbial iron cycle and provide a new microbial iron cycling model. This will help to improve the biological wastewater treatment technology based on NRFO process.