Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts.
Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, Cambridge, Massachusetts.
Geobiology. 2018 Jul;16(4):399-411. doi: 10.1111/gbi.12288. Epub 2018 Apr 25.
The ability of micro-organisms to oxidize manganese (Mn) from Mn(II) to Mn(III/IV) oxides transcends boundaries of biological clade or domain. Many bacteria and fungi oxidize Mn(II) to Mn(III/IV) oxides directly through enzymatic activity or indirectly through the production of reactive oxygen species. Here, we determine the oxygen isotope fractionation factors associated with Mn(II) oxidation via various biotic (bacteria and fungi) and abiotic Mn(II) reaction pathways. As oxygen in Mn(III/IV) oxides may be derived from precursor water and molecular oxygen, we use a twofold approach to determine the isotope fractionation with respect to each oxygen source. Using both O-labeled water and closed-system Rayleigh distillation approaches, we constrain the kinetic isotope fractionation factors associated with O atom incorporation during Mn(II) oxidation to -17.3‰ to -25.9‰ for O and -1.9‰ to +1.8‰ for water. Results demonstrate that stable oxygen isotopes of Mn(III/IV) oxides have potential to distinguish between two main classes of biotic Mn(II) oxidation: direct enzymatic oxidation in which O is the oxidant and indirect enzymatic oxidation in which superoxide is the oxidant. The fraction of Mn(III/IV) oxide-associated oxygen derived from water varies significantly (38%-62%) among these bio-oxides with only weak relationship to Mn oxidation state, suggesting Mn(III) disproportionation may account for differences in the fraction of mineral-bound oxygen from water and O . Additionally, direct incorporation of molecular O suggests that Mn(III/IV) oxides contain a yet untapped proxy of δ18OO2 of environmental O , a parameter reflecting the integrated influence of global respiration, photorespiration, and several other biogeochemical reactions of global significance.
微生物将锰 (Mn) 从 Mn(II) 氧化为 Mn(III/IV) 氧化物的能力超越了生物谱系或域的界限。许多细菌和真菌通过酶活性或通过产生活性氧物质间接将 Mn(II) 氧化为 Mn(III/IV) 氧化物。在这里，我们确定了通过各种生物（细菌和真菌）和非生物 Mn(II) 反应途径氧化 Mn(II) 时相关的氧同位素分馏因子。由于 Mn(III/IV) 氧化物中的氧可能来自前体水和分子氧，因此我们采用双重方法来确定与每个氧源的同位素分馏。使用 O 标记的水和封闭系统瑞利蒸馏方法，我们约束了与 Mn(II)氧化过程中 O 原子掺入相关的动力学同位素分馏因子，对于 O 为 -17.3‰ 至 -25.9‰，对于水为 -1.9‰ 至 +1.8‰。结果表明，Mn(III/IV)氧化物的稳定氧同位素具有区分两种主要生物 Mn(II)氧化类型的潜力：直接酶促氧化，其中 O 是氧化剂，间接酶促氧化，其中超氧化物是氧化剂。这些生物氧化物中与 Mn(III/IV)氧化物相关的氧的一部分来自水，变化很大（38%-62%），与 Mn 氧化态的关系较弱，这表明 Mn(III)歧化可能解释了来自水和 O 的矿物结合氧的分数差异。此外，分子 O 的直接掺入表明 Mn(III/IV)氧化物包含环境 O 的 δ18OO2 的未开发代理，该参数反映了全球呼吸、光呼吸和其他几个具有全球重要意义的生物地球化学反应的综合影响。
