Nicholas David R, Ramamoorthy Srividhya, Palace Vince, Spring Stefan, Moore Johnnie N, Rosenzweig R Frank
Division of Biological Sciences, Program in Microbial Ecology, University of Montana, Missoula, MT 59812-4824, USA.
Biodegradation. 2003 Apr;14(2):123-37. doi: 10.1023/a:1024031700533.
Arsenic is a wide-spread contaminant of soils and sediments, and many watersheds worldwide regularly experience severe arsenic loading. While the toxicity of arsenic to plants and animals is well recognized, the geochemical and biological transformations that alter its bioavailability in the environment are multifaceted and remain poorly understood. This communication provides a brief overview of our current understanding of the biogeochemistry of arsenic in circumneutral freshwater sediments, placing special emphasis on microbial transformations. Arsenic can reside in a number of oxidation states and complex ions. The common inorganic aqueous species at circumneutral pH are the negatively charged arsenates (H2As(V)O4(-) and Has(V)O4(2-)) and zero-charged arsenite (H3As(III)O3(0)). Arsenic undergoes diagenesis in response to both physical and biogeochemical processes. It accumulates in oxic sediments by adsorption on and/or co-precipitation with hydrous iron and manganese oxides. Burial of such sediments in anoxic/suboxic environments favors their reduction, releasing Fe(II), Mn(II) and associated adsorbed/coprecipitated As. Upward advection can translocate these cations and As into the overlying oxic zone where they may reprecipitate. Alternatively, As may be repartitioned to the sulfidic phase, forming precipitates such as arsenopyrite and orpiment. Soluble and adsorbed As species undergo biotic transformations. As(V) can serve as the terminal electron acceptor in the biological oxidation of organic matter, and the limited number of microbes capable of this transformations are diverse in their phylogeny and physiology. Fe(III)-respiring bacteria can mobilize both As(V) and As(III) bound to ferric oxides by the reductive dissolution of iron-arsenate minerals. SO4(2-)-reducing bacteria can promote deposition of As(III) as sulfide minerals via their production of sulfide. A limited number of As(III)-oxidizing bacteria have been identified, some of which couple this reaction to growth. Lastly, prokaryotic and eukaryotic microbes can alter arsenic toxicity either by coupling cellular export to its reduction or by converting inorganic As to organo-arsenical compounds. The degree to which each of these metabolic transformations influences As mobilization or sequestration in different sedimentary matrices remains to be established.
砷是土壤和沉积物中广泛存在的污染物，全球许多流域经常经历严重的砷负荷。虽然砷对动植物的毒性已得到充分认识，但改变其在环境中生物有效性的地球化学和生物转化是多方面的，目前仍知之甚少。本通讯简要概述了我们目前对中性淡水沉积物中砷生物地球化学的理解，特别强调了微生物转化。砷可以以多种氧化态和络合离子的形式存在。在中性pH值下常见的无机水相物种是带负电荷的砷酸盐（H2As(V)O4(-)和Has(V)O4(2-)）和不带电荷的亚砷酸盐（H3As(III)O3(0)）。砷会因物理和生物地球化学过程而发生成岩作用。它通过吸附在含水铁和锰氧化物上和/或与之共沉淀而在有氧沉积物中积累。将此类沉积物埋藏在缺氧/亚缺氧环境中有利于其还原，释放出Fe(II)、Mn(II)以及相关的吸附/共沉淀的砷。向上平流可以将这些阳离子和砷转移到上覆的有氧区域，在那里它们可能会重新沉淀。或者，砷可能会重新分配到硫化物相中，形成诸如毒砂和雌黄之类的沉淀物。可溶性和吸附态的砷物种会发生生物转化。As(V)可以作为有机物生物氧化过程中的终端电子受体，能够进行这种转化的微生物数量有限，它们在系统发育和生理方面各不相同。铁还原呼吸细菌可以通过砷酸铁矿物的还原溶解来释放与三价铁氧化物结合的As(V)和As(III)。硫酸根还原细菌可以通过产生硫化物促进As(III)以硫化物矿物的形式沉淀。已经鉴定出数量有限的As(III)氧化细菌，其中一些将这种反应与生长耦合。最后，原核和真核微生物可以通过将细胞输出与其还原耦合或通过将无机砷转化为有机砷化合物来改变砷的毒性。这些代谢转化中的每一种在不同沉积基质中影响砷的迁移或固定的程度仍有待确定。
