Lopez-Adams Rebeca, Newsome Laura, Moore Katie L, Lyon Ian C, Lloyd Jonathan R
Department of Earth and Environmental Sciences, University of Manchester, Manchester, United Kingdom.
Camborne School of Mines, Environment and Sustainability Institute, University of Exeter, Exeter, United Kingdom.
Front Microbiol. 2021 Feb 22;12:640734. doi: 10.3389/fmicb.2021.640734. eCollection 2021.
Microbial metabolism plays a key role in controlling the fate of toxic groundwater contaminants, such as arsenic. Dissimilatory metal reduction catalyzed by subsurface bacteria can facilitate the mobilization of arsenic the reductive dissolution of As(V)-bearing Fe(III) mineral assemblages. The mobility of liberated As(V) can then be amplified reduction to the more soluble As(III) by As(V)-respiring bacteria. This investigation focused on the reductive dissolution of As(V) sorbed onto Fe(III)-(oxyhydr)oxide by model Fe(III)- and As(V)-reducing bacteria, to elucidate the mechanisms underpinning these processes at the single-cell scale. Axenic cultures of sp. ANA-3 wild-type (WT) cells [able to respire both Fe(III) and As(V)] were grown using C-labeled lactate on an arsenical Fe(III)-(oxyhydr)oxide thin film, and after colonization, the distribution of Fe and As in the solid phase was assessed using nanoscale secondary ion mass spectrometry (NanoSIMS), complemented with aqueous geochemistry analyses. Parallel experiments were conducted using an mutant, able to respire Fe(III) but not As(V). NanoSIMS imaging showed that most metabolically active cells were not in direct contact with the Fe(III) mineral. Flavins were released by both strains, suggesting that these cell-secreted electron shuttles mediated extracellular Fe(III)-(oxyhydr)oxide reduction, but did not facilitate extracellular As(V) reduction, demonstrated by the presence of flavins yet lack of As(III) in the supernatants of the A deletion mutant strain. 3D reconstructions of NanoSIMS depth-profiled single cells revealed that As and Fe were associated with the cell surface in the WT cells, whereas for the A mutant, only Fe was associated with the biomass. These data were consistent with sp. ANA-3 respiring As(V) in a multistep process; first, the reductive dissolution of the Fe(III) mineral released As(V), and once in solution, As(V) was respired by the cells to As(III). As well as highlighting Fe(III) reduction as the primary release mechanism for arsenic, our data also identified unexpected cellular As(III) retention mechanisms that require further investigation.
微生物代谢在控制有毒地下水污染物（如砷）的归宿方面起着关键作用。地下细菌催化的异化金属还原作用可促进砷的迁移，即含砷（V）的铁（III）矿物组合的还原溶解。释放出的砷（V）的迁移性随后可通过砷（V）呼吸细菌将其还原为更易溶的砷（III）而增强。本研究聚焦于通过典型的铁（III）还原菌和砷（V）还原菌对吸附在铁（III）-（氢）氧化物上的砷（V）的还原溶解，以阐明在单细胞尺度上支撑这些过程的机制。使用C标记的乳酸盐在含砷的铁（III）-（氢）氧化物薄膜上培养嗜砷还原地杆菌（Geobacter arsenatis）sp. ANA-3野生型（WT）细胞的无菌培养物[能够同时呼吸铁（III）和砷（V）]，在细胞定殖后，使用纳米二次离子质谱（NanoSIMS）评估固相中铁和砷的分布，并辅以水相地球化学分析。使用一种突变体进行平行实验，该突变体能够呼吸铁（III）但不能呼吸砷（V）。NanoSIMS成像显示，大多数代谢活跃的细胞并未与铁（III）矿物直接接触。两种菌株均释放黄素，这表明这些细胞分泌的电子穿梭体介导了细胞外铁（III）-（氢）氧化物的还原，但并未促进细胞外砷（V）的还原，这一点通过A缺失突变体菌株上清液中存在黄素但不存在砷（III）得以证明。对NanoSIMS深度剖析的单细胞进行的三维重建显示，在WT细胞中，砷和铁与细胞表面相关联，而对于A突变体，只有铁与生物质相关联。这些数据与嗜砷还原地杆菌sp. ANA-3以多步骤过程呼吸砷（V）一致；首先，铁（III）矿物的还原溶解释放出砷（V），一旦进入溶液，砷（V）就被细胞呼吸为砷（III）。除了强调铁（III）还原是砷的主要释放机制外，我们的数据还确定了需要进一步研究的意外的细胞内砷（III）保留机制。
