Stable isotopes and iron oxide mineral products as markers of chemodenitrification.

When oxygen is limiting in soils and sediments, microorganisms utilize nitrate (NO3-) in respiration--through the process of denitrification--leading to the production of dinitrogen (N2) gas and trace amounts of nitrous (N2O) and nitric (NO) oxides. A chemical pathway involving reaction of ferrous iron (Fe2+) with nitrite (NO2-), an intermediate in the denitrification pathway, can also result in production of N2O. We examine the chemical reduction of NO2- by Fe(II)--chemodenitrification--in anoxic batch incubations at neutral pH. Aqueous Fe2+ and NO2- reacted rapidly, producing N2O and generating Fe(III) (hydr)oxide mineral products. Lepidocrotite and goethite, identified by synchrotron X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy, were produced from initially aqueous reactants, with two-line ferrihydrite increasing in abundance later in the reaction sequence. Based on the similarity of apparent rate constants with different mineral catalysts, we propose that the chemodenitrification rate is insensitive to the type of Fe(III) (hydr)oxide. With stable isotope measurements, we reveal a narrow range of isotopic fractionation during NO2- reduction to N2O. The location of N isotopes in the linear N2O molecule, known as site preference, was also constrained to a signature range. The coexistence of Fe(III) (hydr)oxide, characteristic 15N and 18O fractionation, and N2O site preference may be used in combination to qualitatively distinguish between abiotic and biogenically emitted N2O--a finding important for determining N2O sources in natural systems.

Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces.

Molecular hydrogen (H(2)) is derived from the hydrothermal alteration of olivine-rich planetary crust. Abiotic and biotic processes consume H(2) to produce methane (CH(4)); however, the extent of either process is unknown. Here, we assess the temporal dependence and limit of abiotic CH(4) related to the presence and formation of mineral catalysts during olivine hydrolysis (i.e., serpentinization) at 200 °C and 0.03 gigapascal. Results indicate that the rate of CH(4) production increases to a maximum value related to magnetite catalyzation. By identifying the dynamics of CH(4) production, we kinetically model how the H(2) to CH(4) ratio may be used to assess the origin of CH(4) in deep subsurface serpentinization systems on Earth and Mars. Based on our model and available field data, low H(2)/CH(4) ratios (less than approximately 40) indicate that life is likely present and active.

Additive and competitive effects of bacteria and Mn oxides on arsenite oxidation kinetics.

Arsenic (As) is a redox-active metalloid whose toxicity and mobility in soil depend on oxidation state. Arsenite [As(III)] can be oxidized to arsenate [As(V)] by both minerals and microbes in soil however, the interaction between these abiotic and biotic processes is not well understood. In this study, the time dependency of As(III) oxidation by two heterotrophic soil bacteria (Agrobacterium tumefaciens and Pseudomonas fluorescens) and a poorly crystalline manganese (Mn) oxide mineral (δ-MnO(2)) was determined using batch experiments. The apparent rate of As(V) appearance in solution was greater for the combined batch experiments in which bacteria and δ-MnO(2) were oxidizing As(III) at the same time than for either component alone. The additive effect of the mixed cell- δ-MnO(2) system was consistent for short (<1 h) and long (24 h) term coincubation indicating that mineral surface inhibition by cells has little effect the As(III) oxidation rate. Surface interactions between cells and the mineral surface were indicated by sorption and pH-induced desorption results. Total sorption of As on the mineral was lower with bacteria present (16.1 ± 0.8% As sorbed) and higher with δ-MnO(2) alone (23.4 ± 1%) and As was more easily desorbed from the cell-δ-MnO(2) system than from δ-MnO(2) alone. Therefore, the presence of bacteria inhibited As sorption and decreased the stability of sorbed As on δ-MnO(2) even though As(III) was oxidized fastest in a mixed cell-δ-MnO(2) system. The additive effect of biotic (As-oxidizing bacteria) and abiotic (δ-MnO(2) mineral) oxidation processes in a system containing both oxidants suggests that mineral-only results may underestimate the oxidative capacity of natural systems with biotic and abiotic As(III) oxidation pathways.

Arsenite modifies structure of soil microbial communities and arsenite oxidization potential.

The influence of arsenite [As(III)] on natural microbial communities and the capacity of exposed communities to oxidize As(III) has not been well explored. In this study, we conducted soil column experiments with a natural microbial community exposed to different carbon conditions and a continuous flow of As(III). We measured the oxidation rates of As(III) to As(V), and the composition of the bacterial community was monitored by 454 pyrosequencing of 16S rRNA genes. The diversity of As(III)-oxidizing bacteria was examined with the aox gene, which encodes the enzyme involved in As(III) oxidation. Arsenite oxidation was high in the live soil regardless of the carbon source and below detection in sterilized soil. In columns amended with 200 µmol kg(-1) of As (III), As(V) concentrations reached 158 µmol kg(-1) in the column effluent, while As(III) decreased to unmeasurable levels. Although the number of bacterial taxa decreased by as much as twofold in treatments amended with As(III), some As(III)-oxidizing bacterial groups increased up to 20-fold. Collectively, the data show the large effect of As(III) on bacterial diversity, and the capacity of natural communities from a soil with low initial As contamination to oxidize large inputs of As(III).