Geomicrobiology Group, Center for Applied Geoscience (ZAG), University of Tübingen, Schnarrenbergstrasse 94-96, Tübingen, D-72076, Germany.
Experimental Biogeochemistry, BayCEER, University Bayreuth, Dr.-Hans-Frisch-Str. 1-3, Bayreuth, 95448, Germany.
Environ Sci Process Impacts. 2020 Sep 23;22(9):1877-1887. doi: 10.1039/d0em00244e.
Cadmium (Cd) adversely affects human health by entering the food chain via anthropogenic activity. In order to mitigate risk, a better understanding of the biogeochemical mechanisms limiting Cd mobility in the environment is needed. While Cd is not redox-active, Cd speciation varies (i.e., aqueous, complexed, adsorbed), and influences mobility. Here, the cycling of Cd in relation to initial speciation during the growth of Geobacter sulfurreducens was studied. Either fumarate or ferrihydrite (Fh) was provided as an electron acceptor and Cd was present as: (1) an aqueous cation, (2) an aqueous complex with cysteine, which is often present in metal stressed soil environments, or (3) adsorbed to Fh. During microbial Fe(iii) reduction, the removal of Cd was substantial (∼80% removal), despite extensive Fe(ii) production (ratio Fe(ii)total : Fetotal = 0.8). When fumarate was the electron acceptor, there was higher removal from solution when Cd was complexed with cysteine (97-100% removal) compared to aqueous Cd (34-50%) removal. Confocal laser scanning microscopy (CLSM) demonstrated the formation of exopolymeric substances (EPS) in all conditions and that Cd was correlated with EPS in the absence of Fe minerals (r = 0.51-0.56). Most notable is that aqueous Cd was more strongly correlated with Geobacter cells (r = 0.72) compared to Cd-cysteine complexes (r = 0.51). This work demonstrates that Cd interactions with cell surfaces and EPS, and Cd solubility during metabolic activity are dependent upon initial speciation. These processes may be especially important in soil environments where sulfur is limited and Fe and organic carbon are abundant.
镉 (Cd) 通过人为活动进入食物链，从而对人类健康产生不利影响。为了降低风险，需要更好地了解限制环境中 Cd 迁移的生物地球化学机制。虽然 Cd 没有氧化还原活性，但 Cd 的形态（即水相、络合、吸附）会发生变化，从而影响其迁移性。在此，研究了在 Geobacter sulfurreducens 生长过程中与初始形态相关的 Cd 循环。以延胡索酸盐或水铁矿 (Fh) 作为电子受体，Cd 存在于以下三种形态：（1）水合阳离子，（2）与半胱氨酸形成的水合络合物，半胱氨酸通常存在于受金属胁迫的土壤环境中，（3）吸附在 Fh 上。在微生物还原 Fe(iii) 的过程中，尽管 Fe(ii) 的产量很大（Fe(ii)total : Fetotal = 0.8），但 Cd 的去除量相当可观（∼80%）。当延胡索酸盐作为电子受体时，与半胱氨酸络合的 Cd 的去除率（97-100%）明显高于水合 Cd（34-50%）。共聚焦激光扫描显微镜 (CLSM) 表明在所有条件下都形成了胞外聚合物物质 (EPS)，并且在没有 Fe 矿物的情况下，Cd 与 EPS 相关（r = 0.51-0.56）。值得注意的是，与 Cd-半胱氨酸络合物（r = 0.51）相比，水合 Cd 与 Geobacter 细胞的相关性更强（r = 0.72）。这项工作表明，Cd 与细胞表面和 EPS 的相互作用以及代谢活性过程中 Cd 的溶解度取决于初始形态。这些过程在硫有限而铁和有机碳丰富的土壤环境中可能尤为重要。
