Organic acid-dependent iron mineral formation by a newly isolated iron-reducing bacterium, Shewanella sp HN-41

Abstract

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 (beta-FeOOH; similar to 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(I) in aqueous phase, and final pH of 8.1 and Eh of -394mVshowed the lowest crystallinity. When strain HN- 41 was incubated with the organic acid substrates at 10 mM and Fe(III)-citrate at 20mM, total inorganic carbon (TIC) contents, which is the collective term of free carbon dioxide [CO2], carbonate ion [CO32-], bicarbonate ion [HCO3-], and carbonic acid [H2CO3], 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.