A Microbial Arsenic Cycle in Sediments of an Acidic Mine Impoundment: Herman Pit, Clear Lake, California

The involvement of prokaryotes in the redox reactions of arsenic occurring between its +5 [arsenate; As(V)] and +3 [arsenite; As(III)] oxidation states has been well established. Most research to date has focused upon circum-neutral pH environments (e.g., freshwater or estuarine sediments) or arsenic-rich “extreme” environments like hot springs and soda lakes. In contrast, relatively little work has been conducted in acidic environments. With this in mind we conducted experiments with sediments taken from the Herman Pit, an acid mine drainage impoundment of a former mercury (cinnabar) mine. Due to the large adsorptive capacity of the abundant Fe(III)-rich minerals, we were unable to initially detect in solution either As(V) or As(III) added to the aqueous phase of live sediment slurries or autoclaved controls, although the former consumed added electron donors (i.e., lactate, acetate, hydrogen), while the latter did not. This prompted us to conduct further experiments with diluted slurries using the live materials from the first incubation as inoculum. In these experiments we observed reduction of As(V) to As(III) under anoxic conditions and reduction rates were enhanced by addition of electron donors. We also observed oxidation of As(III) to As(V) in oxic slurries as well as in anoxic slurries amended with nitrate. We noted an acid-tolerant trend for sediment slurries in the cases of As(III) oxidation (aerobic and anaerobic) as well as for anaerobic As(V) reduction. These observations indicate the presence of a viable microbial arsenic redox cycle in the sediments of this extreme environment, a result reinforced by the successful amplification of arsenic functional genes (<i>aioA</i>, and <i>arrA</i>) from these materials.

原核生物(prokaryotes)参与砷在+5价[砷酸盐(arsenate; As(V))]与+3价[亚砷酸盐(arsenite; As(III))]氧化态之间的氧化还原反应，这一结论已得到充分证实。迄今为止，绝大多数相关研究聚焦于pH近中性的环境（例如淡水或河口沉积物），或是富砷的“极端”环境，如热泉与苏打湖。与之相对，针对酸性环境的相关研究则相对匮乏。有鉴于此，我们从赫尔曼矿坑（Herman Pit）采集沉积物开展实验，该矿坑为一处前辰砂(cinnabar)汞矿的酸性矿坑排水蓄水塘。由于该环境中富含大量富三价铁(Fe(III))矿物，其吸附能力极强，我们最初无法在活性沉积物悬液与高压灭菌对照的水相中检测到添加的As(V)与As(III)；不过前者会消耗添加的电子供体（即乳酸盐、乙酸盐与氢气），而后者则无此现象。为此我们开展后续实验：以首次培养获得的活性物料作为接种物，采用稀释后的悬液开展试验。在这些实验中，我们观测到厌氧条件下As(V)被还原为As(III)，且添加电子供体可提升还原速率。此外，我们还在好氧悬液以及添加硝酸盐的厌氧悬液中，观测到As(III)被氧化为As(V)的现象。针对As(III)氧化（好氧与厌氧条件）以及厌氧条件下As(V)还原的实验，我们发现沉积物悬液呈现出耐酸性的特征。上述观测结果表明，该极端环境沉积物中存在活跃的微生物驱动的砷氧化还原循环；从这些沉积物样本中成功扩增得到砷功能基因(aioA与arrA)，进一步佐证了这一结论。