Levy Jacqueline L, Stauber Jennifer L, Adams Merrin S, Maher William A, Kirby Jason K, Jolley Dianne F
Centre for Environmental Contaminants Research, Commonwealth Scientific and Industrial Research Organization, Energy Technology, Private Mail Bag 7, Bangor, New South Wales 2234, Australia.
Environ Toxicol Chem. 2005 Oct;24(10):2630-9. doi: 10.1897/04-580r.1.
The toxicity of As(V) and As(III) to two axenic tropical freshwater microalgae, Chlorella sp. and Monoraphidium arcuatum, was determined using 72-h growth rate-inhibition bioassays. Both organisms were tolerant to As(III) (72-h concentration to cause 50% inhibition of growth rate [IC50], of 25 and 15 mg As[III]/L, respectively). Chlorella sp. also was tolerant to As(V) with no effect on growth rate over 72 h at concentrations up to 0.8 mg/L (72-h IC50 of 25 mg As[V]/L). Monoraphidium arcuatum was more sensitive to As(V) (72-h IC50 of 0.25 mg As[V]/L). An increase in phosphate in the growth medium (0.15-1.5 mg PO4(3-)/L) decreased toxicity, i.e., the 72-h IC50 value for M. arcuatum increased from 0.25 mg As(V)/L to 4.5 mg As(V)/L, while extracellular As and intracellular As decreased, indicating competition between arsenate and phosphate for cellular uptake. Both microalgae reduced As(V) to As(III) in the cell, with further biological transformation to methylated species (monomethyl arsonic acid and dimethyl arsinic acid) and phosphate arsenoriboside. Less than 0.01% of added As(V) was incorporated into algal cells, suggesting that bioaccumulation and subsequent methylation was not the primary mode of detoxification. When exposed to As(V), both species reduced As(V) to As(III); however, only M. arcuatum excreted As(III) into solution. Intracellular arsenic reduction may be coupled to thiol oxidation in both species. Arsenic toxicity most likely was due to arsenite accumulation in the cell, when the ability to excrete and/or methylate arsenite was overwhelmed at high arsenic concentrations. Arsenite may bind to intracellular thiols, such as glutathione, potentially disrupting the ratio of reduced to oxidized glutathione and, consequently, inhibiting cell division.
采用72小时生长速率抑制生物测定法，测定了五价砷（As(V)）和三价砷（As(III)）对两种无菌热带淡水微藻——小球藻（Chlorella sp.）和弓形单歧藻（Monoraphidium arcuatum）的毒性。两种生物对As(III)均具有耐受性（导致生长速率50%抑制的72小时浓度[IC50]分别为25和15 mg As(III)/L）。小球藻对As(V)也具有耐受性，在浓度高达0.8 mg/L时，72小时内生长速率无影响（72小时IC50为25 mg As(V)/L）。弓形单歧藻对As(V)更敏感（72小时IC50为0.25 mg As(V)/L）。生长培养基中磷酸盐增加（0.15 - 1.5 mg PO4(3-)/L）会降低毒性，即弓形单歧藻的72小时IC50值从0.25 mg As(V)/L增加到4.5 mg As(V)/L，同时细胞外砷和细胞内砷减少，表明砷酸盐和磷酸盐在细胞摄取方面存在竞争。两种微藻均在细胞内将As(V)还原为As(III)，并进一步生物转化为甲基化产物（一甲基砷酸和二甲基砷酸）以及磷酸砷核糖苷。添加的As(V)中不到0.01%被整合到藻类细胞中，这表明生物累积及随后的甲基化并非解毒的主要方式。当暴露于As(V)时，两种藻类均将As(V)还原为As(III)；然而，只有弓形单歧藻将As(III)排泄到溶液中。两种藻类细胞内的砷还原可能与硫醇氧化相关。当高砷浓度下砷的排泄和/或甲基化能力不堪重负时，砷毒性很可能是由于亚砷酸盐在细胞内积累所致。亚砷酸盐可能与细胞内的硫醇（如谷胱甘肽）结合，潜在地破坏还原型谷胱甘肽与氧化型谷胱甘肽的比例，从而抑制细胞分裂。
