Arsenite bacterial oxidation

AW Turner, JW Legge. Bacterial oxidation of arsenite. II. Description of bacteria isolated from cattle-dipping fluids. Aust J Biol Sci 7 452-478, 1954. 

In 1953, Quastel and Scholefield (10) observed bacterial oxidation of arsenite to arsenate in soil perfusion experiments, using Audus s modification (11) of the airlift column designed by Lees and Quastel (12). They noted that sodium arsenite in aqueous solution at 2.5 X 10 M concentration when perfused through soil from Cardiff (Wales) became oxidized to arsenate. In the initial perfusion, a lag was observed before arsenite oxidation occurred. No lag was observed on reperfusion of the same soil column. Addition of sulfanilamide increased the initial lag in arsenite oxidation but not the oxidation on reperfusion. The oxidation was inhibited when they added 0.1% sodium azide to the arsenite solution. The initial lag and the effect of the azide indicated to the investigators that the oxidation was biological, but they made no attempt to isolate the arsenite-oxidizing organisms from the soil. They did show that ammonia was not oxidized in these columns. 

The foregoing shows that arsenite in aerobic environments can undergo bacterial oxidation to arsenate. Since, as shown in the chapter on arsenate reduction, some anaerobic bacteria have the ability to reduce As(V) to different lower oxidation states, bacterial arsenite oxidation must represent part of a microbial arsenic cycle. Microbial activity can also mobilize arsenic in some minerals as arsenite and/ or arsenate. These microbial activities have to be considered in any assessment of environmental arsenic pollution. 

JW Legge, AW Turner. Bacterial oxidation of arsenite. ni. Cell-free arsenite dehydrogenase. Aust J Sci 7 496-503, 1954. 

L Carlson, EB Lindstrom, KB Hallberg, OH Tuovinen. Solid-phase products of bacterial oxidation of arsenical pyrite. Appl Environ Microbiol 58 1046-1049, 1992. WD Cassity, B Pesic. Interactions of Thiobacillus ferrooxidans with arsenite, arsenate and arsenopyrite. In R Amils, A Ballester, eds. Biohydrometallurgy and the Environment Toward the Mining of the 21st Century, Part A. Amsterdam Elsevier, 1999, pp 521-532. 

Figure 1 Bacterial reduction of arsenate and oxidation of arsenite. (A). Cytoplasmic arsenate reductase (ArsC) as encoded by bacterial ars operons along with chromosomaUy encoded Pit and Pst phosphate transport systems with arsenate as an alternative substrate. After reduction from arsenate to arsenite, arsenite is removed from the cell by the ArsB membrane protein. (B). Anaerobic periplasmic arsenate reductase. (C). Aerobic periplas-mic arsenite oxidase, hnked via azurin to the respiratory chain.

In addition to plasmid arsenic resistance that is well understood and for which clusters of genes have been isolated and sequenced, there are bacterial arsenic metabolism systems that involve oxidation of arsenite to arsenic. Arsenite oxidation by aerobic pseudomonads was first found with bacteria isolated from cattle dipping solutions where arsenicals were used as agents against ticks around the time of World War I. They were subsequently isolated by Turner and Legge... 

The arsenic and iron in solution did not reflect the full extent to which the arsenopyrite had been oxidized. Acidiflcation of the culture medium in each flask with 1 ml of concentrated HCl at the end of the experiment increased the arsenic concentration in solution 1.6-fold and the iron concentration 4.4-fold in uninoculated flasks and 1.6- and 7.2-fold, respectively, in inoculated flasks. The increase in dissolved As and Fe on acidification suggests that a portion of the mobilized iron and arsenic was precipitated as iron arsenate and arsenite in inoculated as well as uninoculated flasks. The weight ratios of Fe/As were always higher over 21 days in uninoculated flasks than in inoculated flasks, and in both types of flasks dropped in the first few days of incubation and then increased again. Precipitation of ferric arsenate (scorodite) as well as potassium jarosite [KFcs (804)2(011)6] in bacterial arsenical pyrite oxidation was reported by Carlson et al.(35). 

Simple chemical oxidation of arsenite by ferric iron at acid pH has been questioned by Barrett et al. (37). They found experimentally that Fe could not oxidize As02 chemically at pH 1.3 at either 70 or 45°C in the presence of a mixed culture capable of growing on Fe and pyrite (FeS2). However, when they added pyrite to the reaction mixture, the bacteria did promote oxidation of arsenite at 45°C. They explained the effect of the pyrite as a heterogeneous catalyst, the role of the bacteria being the regeneration of a clean catalytic surface on the pyrite and the reoxidation of Fe + generated in the oxidation of arsenite by Fe. Mandl and Vyskovsky (38) developed a kinetic model for the catalytic role of pyrite in this form of bacterial arsenite oxidation by Fe. They performed the experiments on which they based their model with T. ferrooxidans strain CCM 4253. 

Both oxidation and methylation are microbial transformations involved in the redistribution and global cycling of arsenic. Oxidation involves the conversion of toxic arsenite to less toxic arsenate. Bacterial methylation of inorganic arsenic under anaerobic conditions may be a mechanism of arsenic detoxification. Fungi also transform inorganic and organic arsenic compounds into volatile methylar-sines. However, unlike methylated selenium which is nontoxic, the volatile arsine... 

Three bacterial systems have been described that differ from the E. colt and animal enzyme systems in that they oxidize pyruvate without participation of lipoic acid, as shown by studies in the presence of specific inhibitors. One of the classical specific inhibitors of a-keto acid oxidation has been inorganic arsenite. This inhibitor acts by binding lipoic acid through combination with both sulfhydryl groups. The nonlipoic acid systems are completely insensitive to arsenite. 

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