Abstract Selenium (Se) isotope fractionation has been widely used for constraining redox conditions and microbial processes in both modern and ancient environments, but our knowledge of the controls on fractionation during microbial reduction of Se-oxyanions is based on a limited number of studies. Here we complement and expand the currently available pure culture data for Se isotope fractionation by investigating for the first time six phylogenetically diverse, mesophilic, and non-respiring bacterial strains that reduce Se-oxyanions to elemental Se [Se(0)]. Experiments were performed with either selenate [Se(VI)] or selenite [Se(IV)] at lower, more environmentally-relevant Se (9–47 μM) and carbon (500 μM) concentrations than previously investigated. Enterobacter cloacae SLD1a-1, Desulfitobacterium chlororespirans Co23 and Desulfitobacterium sp. Viet-1 were incubated with Se(VI) and Se(IV). Geobacter sulfurreducens PCA, Anaeromyxobacter dehalogenans FRC-W and Shewanella sp. (NR) were examined for their ability reducing Se(IV) to Se(0). Our data confirm that microbial reduction of both Se-oxyanions is accompanied by large kinetic isotopic fractionation (reported as 82/76e = 1000×(82/76α-1) in ‰). Under our experimental conditions, microbial reduction of Se(VI) shows consistently greater isotope fractionation (e = −9.2‰ to −11.8‰) than reduction of Se(IV) (e = −6.2 to −7.8‰) confirming the difference in metabolic pathways for the reduction of the two Se-oxyanions. For Se(VI), an inverse relationship between normalized cell specific reduction rate (cSRR) and Se isotope fractionation suggests that the kinetic isotope effect for Se(VI) reduction is governed by an enzymatically-specific pathway related to the bacterial strain-specific physiology. In contrast, the lack of correlation between normalized cSRR and isotope fractionation for Se(IV) reduction indicates a non-enzyme specific pathway which is dominantly extracellular. Our study highlights the importance to understand microbially-mediated Se isotope fractionation depending on Se species, and cell-specific reduction rates before Se isotope ratios can become a fully applicable tool to interpret Se isotopic changes in modern and ancient environments.

中文翻译：

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系统发育多样性细菌对硒氧阴离子的微生物还原过程中质量依赖的硒同位素分馏

摘要 硒 (Se) 同位素分馏已广泛用于限制现代和古代环境中的氧化还原条件和微生物过程，但我们对 Se-氧阴离子微生物还原过程中分馏控制的了解是基于有限数量的研究。在这里，我们通过首次调查将 Se 氧阴离子还原为元素 Se [Se(0)] 的六种系统发育多样、嗜温且无呼吸的细菌菌株，补充和扩展了目前可用的 Se 同位素分馏纯培养数据。实验是用硒酸盐 [Se(VI)] 或亚硒酸盐 [Se(IV)] 进行的，与之前的研究相比，硒 (9–47 μM) 和碳 (500 μM) 浓度更低、更环保。阴沟肠杆菌 SLD1a-1、氯呼吸脱硫杆菌 Co23 和脱硫杆菌属。Viet-1 与 Se(VI) 和 Se(IV) 一起孵育。硫还原地杆菌 PCA、脱卤厌氧菌 FRC-W 和希瓦氏菌属。检查 (NR) 将 Se(IV) 还原为 Se(0) 的能力。我们的数据证实，微生物对硒氧阴离子的还原伴随着大的动力学同位素分馏（报告为 82/76e = 1000×(82/76α-1) in ‰）。在我们的实验条件下，Se(VI) 的微生物还原显示出始终比 Se(IV) 的还原（e = -6.2 到 -7.8‰）更大的同位素分馏（e = -9.2‰ 到 -11.8‰），证实了代谢的差异两种硒氧阴离子的还原途径。对于硒（VI），归一化细胞特异性还原率 (cSRR) 和 Se 同位素分馏之间的反比关系表明，Se(VI) 还原的动力学同位素效应受与细菌菌株特异性生理学相关的酶特异性途径控制。相比之下，标准化 cSRR 和 Se(IV) 还原的同位素分馏之间缺乏相关性表明主要是细胞外的非酶特异性途径。我们的研究强调了在硒同位素比率成为解释现代和古代环境中硒同位素变化的完全适用的工具之前，了解微生物介导的硒同位素分馏取决于硒种类和细胞特异性减少率的重要性。归一化 cSRR 和 Se(IV) 还原的同位素分馏之间缺乏相关性表明主要是细胞外的非酶特异性途径。我们的研究强调了在硒同位素比率成为解释现代和古代环境中硒同位素变化的完全适用的工具之前，了解微生物介导的硒同位素分馏取决于硒种类和细胞特异性减少率的重要性。归一化 cSRR 和 Se(IV) 还原的同位素分馏之间缺乏相关性表明主要是细胞外的非酶特异性途径。我们的研究强调了在 Se 同位素比率成为解释现代和古代环境中 Se 同位素变化的完全适用工具之前，了解微生物介导的 Se 同位素分馏取决于 Se 物种和细胞特异性减少率的重要性。