Tamaki S, Frankenberger W T
Department of Soil and Environmental Sciences, University of California, Riverside 92521.
Rev Environ Contam Toxicol. 1992;124:79-110. doi: 10.1007/978-1-4612-2864-6_4.
Microorganisms are involved in the redistribution and global cycling of arsenic. Arsenic can accumulate and can be subject to various biotransformations including reduction, oxidation, and methylation. Bacterial methylation of inorganic arsenic is coupled to the methane biosynthetic pathway in methanogenic bacteria under anaerobic conditions and may be a mechanism for arsenic detoxification. The pathway proceeds by reduction of arsenate to arsenite followed by methylation to dimethylarsine. Fungi are also able to transform inorganic and organic arsenic compounds into volatile methylarsines. The pathway proceeds aerobically by arsenate reduction to arsenite followed by several methylation steps producing trimethylarsine. Volatile arsine gases are very toxic to mammals because they destroy red blood cells (LD50 in rats; 3.0 mg kg-1). Further studies are needed on dimethylarsine and trimethylarsine toxicity tests through inhalation of target animals. Marine algae transform arsenate into non-volatile methylated arsenic compounds (methanearsonic and dimethylarsinic acids) in seawater. This is considered to be a beneficial step not only to the primary producers, but also to the higher trophic levels, since non-volatile methylated arsenic is much less toxic to marine invertebrates. Freshwater algae like marine algae synthesize lipid-soluble arsenic compounds and do not produce volatile methylarsines. Aquatic plants also synthesize similar lipid-soluble arsenic compounds. In terrestrial plants, arsenate is preferentially taken up 3 to 4 times the rate of arsenite. In the presence of phosphate, arsenate uptake is inhibited while in the presence of arsenate, phosphate uptake is only slightly inhibited. There is a competitive interaction between arsenate and phosphate for the same uptake system in terrestrial plants. The mode of toxicity of arsenate is to partially block protein synthesis and interfere with protein phosphorylation but the presence of phosphate prevents this mode of action. There appears to be a higher affinity for phosphate than arsenate with a discriminate ratio of 4:1. It is estimated that as much as 210 x 10(5) kg of arsenic is lost to the atmosphere in the vapor state annually from the land surface. The continental vapor flux is about 8 times that of the continental dust flux indicating that the biogenic contribution may play a significant role in cycling of arsenic. It has not been established whether volatile arsenic can be released by plants. Further studies are needed to determine mass balances in the rate of transfer (fluxes) of arsenic in the environment.
微生物参与了砷的重新分布和全球循环。砷能够积累，并可经历包括还原、氧化和甲基化在内的各种生物转化过程。在厌氧条件下，产甲烷菌中无机砷的细菌甲基化与甲烷生物合成途径相关联，这可能是一种砷解毒机制。该途径通过将砷酸盐还原为亚砷酸盐，随后甲基化生成二甲基胂来进行。真菌也能够将无机和有机砷化合物转化为挥发性甲基胂。该途径在有氧条件下通过将砷酸盐还原为亚砷酸盐，随后经过几个甲基化步骤生成三甲基胂来进行。挥发性胂气体对哺乳动物毒性极大，因为它们会破坏红细胞（大鼠的半数致死剂量为3.0毫克/千克）。需要通过对目标动物进行吸入二甲基胂和三甲基胂的毒性测试开展进一步研究。海洋藻类在海水中将砷酸盐转化为非挥发性甲基化砷化合物（甲基胂酸和二甲基胂酸）。这不仅被认为对初级生产者有益，对更高营养级也有益，因为非挥发性甲基化砷对海洋无脊椎动物的毒性要小得多。与海洋藻类一样，淡水藻类合成脂溶性砷化合物，且不产生挥发性甲基胂。水生植物也合成类似的脂溶性砷化合物。在陆生植物中，砷酸盐的吸收优先于亚砷酸盐，速率是其3至4倍。在有磷酸盐存在时，砷酸盐的吸收受到抑制，而在有砷酸盐存在时，磷酸盐的吸收仅略有抑制。在陆生植物中，砷酸盐和磷酸盐在同一吸收系统上存在竞争性相互作用。砷酸盐的毒性作用方式是部分阻断蛋白质合成并干扰蛋白质磷酸化，但磷酸盐的存在可阻止这种作用方式。对磷酸盐的亲和力似乎高于砷酸盐，区分率为4:1。据估计，每年从陆地表面以气态形式进入大气的砷多达210×10⁵千克。大陆气相通量约为大陆粉尘通量的8倍，这表明生物源贡献在砷的循环中可能发挥重要作用。植物是否能释放挥发性砷尚未确定。需要开展进一步研究以确定环境中砷转移（通量）速率的质量平衡。
