Biological and chemical processes of microbially mediated nitrate-reducing Fe(II) oxidation by Pseudogulbenkiania sp strain 2002

In the microbially mediated nitrate-reducing Fe(II) oxidation system, it is recognized that chemical oxidation of Fe(II) by nitrite, which is a bioreduction intermediate of nitrate, can occur under anoxic conditions (chemo-denitrification), but it is still difficult to quantitatively evaluate the contributions of biological Fe(II) oxidation and chemodenitrification. Here, nitrate reduction coupled with Fe(II) oxidation by a suggested lithoautotro phic nitrate-reducing Fe(II)-oxidizing bacterium, Pseudogulbenkiania sp. strain 2002, was investigated in PIPES buffered medium without any organic cosubstrate through reaction kinetics, nitrogen isotope fractionation, and secondary mineral characterization. Substantial Fe(II) oxidation was observed in the presence of cells and nitrate, and nitrite (0.59 mM) was able to quickly oxidize Fe(II). Stored carbon in strain 2002 harvested during pre-incubation can serve as carbon source for nitrate reduction. Furthermore, the N isotopic composition (delta N-15) of N2O in Cell + NO3- + Fe(II) was much more negative than those in Cell + NO3-/ NO2-, Cell + NO2- + Fe (II), and NO2- + Fe(II), implying that Fe(II) affects N fractionation associated with the reduction of nitrate to nitrite. Goethite was formed in Fe(II)+ NO2-, while lepidocrocite was the main mineral phase in Cell + Fe(II) + NO3-. The morphology and cell-mineral interactions determined by electron microscopy showed that secondary minerals were formed outside of cells in Cell + NO2- + Fe(II), while cell encrustation was observed in the periplasmic space of cells in Cell + NO3- + Fe(II). The secondary minerals present in the different treatments further illustrated the co-occurrence of biological, chemical, and coupling processes in the microbially mediated nitrate-reducing Fe(II) oxidation system. This study highlights the involvements of the biological Fe(II) oxidation and chemical Fe(II) oxidation by nitrite in microbially mediated nitrate-reducing Fe(II) oxidation.