Department of Geology and Environmental Earth Science, Miami University, Oxford, OH 45056, USA.
Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99354, USA.
Sci Total Environ. 2020 Nov 1;741:140213. doi: 10.1016/j.scitotenv.2020.140213. Epub 2020 Jun 19.
Previous studies have shown that humic substances can serve as electron shuttle to catalyze bioreduction of structural Fe(III) in clay minerals, but it is unclear if clay-sorbed humic substances can serve the same function. It is unknown if the electron shuttling function is dependent on electron donor type and if humic substances undergo change as a result. In this study, humic acid (HA) and fulvic acid (FA) were sorbed onto nontronite (NAu-2) surface. Structural Fe(III) in HA- and FA-coated NAu-2 samples was bioreduced by Shewanella putrefaciens CN32 using H and lactate as electron donors. The results showed a contrasting effect of humic substances on bioreduction of structural Fe(III), depending on the electron donor type. With H as electron donor, humic substances had little effect on bioreduction of Fe(III) (the reduction extent: 26.2%, 27.4%, 29.3% for HA-coated, FA-coated, and uncoated NAu-2, respectively). In contrast, these substances significantly enhanced bioreduction of Fe(III) with lactate as electron donor (the reduction extent: 20.2%, 20.7%, 11.5% for HA-coated, FA-coated, and uncoated NAu-2, respectively). This contrasting behavior is likely caused by the difference in reaction free energy and electron transport process between H and lactate. When H served as electron donor, more energy was released than when lactate served as electron donor. In addition, because of different cellular locations of lactate dehydrogenase (inner membrane) and H hydrogenase (the periplasm), electrons generated by H hydrogenase may pass through the electron transport chain more rapidly than those generated from lactate dehydrogenase. Through their functions as electron shuttle and/or carbon source, clay-sorbed HA/FA underwent partial transformation to amino acids and other compounds. The availability of external carbon source played an important role in the amount and type of secondary product generation. These results have important implications for coupled iron and carbon biogeochemical cycles in clay- and humic substance-rich environments.
先前的研究表明，腐殖质可以作为电子穿梭体来催化粘土矿物中结构 Fe(III) 的生物还原，但不清楚粘土吸附的腐殖质是否具有相同的功能。目前尚不清楚电子穿梭功能是否依赖于电子供体类型，以及腐殖质是否因此发生变化。在这项研究中，腐殖酸 (HA) 和富里酸 (FA) 被吸附到非绿脱石 (NAu-2) 表面。Shewanella putrefaciens CN32 使用 H 和乳酸作为电子供体还原 HA 和 FA 涂覆的 NAu-2 样品中的结构 Fe(III)。结果表明，腐殖质对结构 Fe(III) 生物还原的影响因电子供体类型而异。当 H 作为电子供体时，腐殖质对 Fe(III) 的生物还原几乎没有影响（还原程度：HA 涂覆、FA 涂覆和未涂覆的 NAu-2 分别为 26.2%、27.4%和 29.3%）。相比之下，当使用乳酸作为电子供体时，这些物质显著增强了 Fe(III) 的生物还原（还原程度：HA 涂覆、FA 涂覆和未涂覆的 NAu-2 分别为 20.2%、20.7%和 11.5%）。这种对比行为可能是由于 H 和乳酸之间的反应自由能和电子传递过程的差异造成的。当 H 作为电子供体时，释放的能量比乳酸作为电子供体时多。此外，由于乳酸脱氢酶（内膜）和 H 氢化酶（周质）的细胞位置不同，H 氢化酶产生的电子可能比来自乳酸脱氢酶的电子更快地通过电子传递链。通过作为电子穿梭体和/或碳源的功能，粘土吸附的 HA/FA 部分转化为氨基酸和其他化合物。外部碳源的可用性对次生产物生成的数量和类型起着重要作用。这些结果对粘土和腐殖质丰富环境中耦合的铁和碳生物地球化学循环具有重要意义。
