Organic Acid-Dependent Iron Mineral Formation by a Newly Isolated Iron-Reducing Bacterium, Shewanella sp. HN-41

The influence of organic acids used as substrates for bacterial respiration coupled to dissimilatory Fe(III) reduction on the mineralogy of formed iron minerals was examined. A newly isolated dissimilatory iron-reducing bacterium, Shewanella strain HN-41 exhibited different mineral formation patterns resulting from the reduction of poorly crystalline Fe(III)-oxyhydroxide, akaganeite (β-FeOOH; ∼ 70 mM) depending upon the presence of the different organic acids: lactate, pyruvate, and formate (10 mM) under anaerobic conditions. X-ray diffraction analysis identified the minerals as magnetite, siderite, and a mixture of magnetite and siderite, which were produced by strain HN-41 using lactate, pyruvate, and formate as a sole electron donor, respectively. With the descending order of the amount of Fe(II) in aqueous phase, the pyruvate-, formate-, and lactate-incubations released Fe(II) into aqueous phase from the poorly crystalline akaganeite by the bacterial Fe(III) reduction. The amount of Fe(II) released into aqueous phase was inversely related to the degree of crystallinity. Magnetite produced during lactate-incubation with the least amount of Fe(II) in aqueous phase, and final pH of 8.6 and Eh of –408 mV showed the highest crystallinity, while siderite formed by pyruvate-incubation with the largest release of Fe(II) in aqueous phase, and final pH of 8.1 and Eh of –394 mV showed the lowest crystallinity. When strain HN-41 was incubated with the organic acid substrates at 10 mM and Fe(III)-citrate at 20 mM, total inorganic carbon (TIC) contents, which is the collective term of free carbon dioxide [CO ₂ ], carbonate ion [CO₃ ^{2 −} ], bicarbonate ion [HCO ₃ ⁻], and carbonic acid [H ₂ CO ₃ ], also showed different trends with different organic acids as electron donors. TICs produced from both pyruvate-and formate-incubations were relatively fast and decreased with time after early phase increases, while TIC from the lactate-incubation increased gradually. Therefore, low molecular weight organic acids lactate, pyruvate, and formate, may differentially affect mineral formation through chemistry change of the environmental conditions created by the newly isolated dissimilatory iron-reducing bacterium Shewanella strain HN-41.

Keywords:

• dissimilatory iron-reducing bacteria
• Shewanella
• magnetite
• siderite
• organic acids

Acknowledgments

Authors Lee, Kim and Hur are all affiliated with the Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology.

We thank Dr. Robert A. Kanaly (Department of Technology and Ecology, Graduate School of Global Environmental Studies, Kyoto University, Japan), and Professor Jaeweon Cho (GIST) for their kind advices and critical reading of the manuscript. We thank Mr. Sungyun Lee and Ms. Su-Jin Kim (GIST) for technical assistance of TOC analyzer and TEM, respectively. This work was supported in part by National Core Research Center program of Ministry of Science and Technology (Grant #: R15-2003-012-02002-0), and the 21C Frontier Microbial Genomics and Applications Center Program, Ministry of Science and Technology, Republic of Korea.