Potential function of added minerals as nucleation sites and effect of humic substances on mineral formation by the nitrate-reducing Fe(II)-oxidizer Acidovorax sp. BoFeN1.

The mobility of toxic metals and the transformation of organic pollutants in the environment are influenced and in many cases even controlled by iron minerals. Therefore knowing the factors influencing iron mineral formation and transformation by Fe(II)-oxidizing and Fe(III)-reducing bacteria is crucial for understanding the fate of contaminants and for the development of remediation technologies. In this study we followed mineral formation by the nitrate-reducing Fe(II)-oxidizing strain Acidovorax sp. BoFeN1 in the presence of the crystalline Fe(III) (oxyhydr)oxides goethite, magnetite and hematite added as potential nucleation sites. Mössbauer spectroscopy analysis of minerals precipitated by BoFeN1 in (57)Fe(II)-spiked microbial growth medium showed that goethite was formed in the absence of mineral additions as well as in the presence of goethite or hematite. The presence of magnetite minerals during Fe(II) oxidation induced the formation of magnetite in addition to goethite, while the addition of humic substances along with magnetite also led to goethite but no magnetite. This study showed that mineral formation not only depends on the aqueous geochemical conditions but can also be affected by the presence of mineral nucleation sites that initiate precipitation of the same underlying mineral phases.

Green rust formation during Fe(II) oxidation by the nitrate-reducing Acidovorax sp. strain BoFeN1.

Green rust (GR) as highly reactive iron mineral potentially plays a key role for the fate of (in)organic contaminants, such as chromium or arsenic, and nitroaromatic compounds functioning both as sorbent and reductant. GR forms as corrosion product of steel but is also naturally present in hydromorphic soils and sediments forming as metastable intermediate during microbial Fe(III) reduction. Although already suggested to form during microbial Fe(II) oxidation, clear evidence for GR formation during microbial Fe(II) oxidation was lacking. In the present study, powder XRD, synchrotron-based XAS, Mössbauer spectroscopy, and TEM demonstrated unambiguously the formation of GR as an intermediate product during Fe(II) oxidation by the nitrate-reducing Fe(II)-oxidizer Acidovorax sp. strain BoFeN1. The spatial distribution and Fe redox-state of the precipitates associated with the cells were visualized by STXM. It showed the presence of extracellular Fe(III), which can be explained by Fe(III) export from the cells or extracellular Fe(II) oxidation by an oxidant diffusing from the cells. Moreover, GR can be oxidized by nitrate/nitrite and is known as a catalyst for oxidation of dissolved Fe(II) by nitrite/nitrate and may thus contribute to the production of extracellular Fe(III). As a result, strain BoFeN1 may contribute to Fe(II) oxidation and nitrate reduction both by an direct enzymatic pathway and an indirect GR-mediated process.