Arsenic bacteria

A fascinating story that was also published in 2010 in the leading journal Science is the story of the discovery of arsenic bacteria. Figure 11.6 shows the essence of the claim, namely that some bacteria, which grew in a pond that contains a high concentration of arsenic, could use arsenic instead of phosphorus in their DNA. The... 

Wolfe-Simon s experiments showed that the microbes could live in high levels of arsenic and very low levels of phosphate. This was an exciting result, but still, just the beginning. Survival in arsenate does not mean the cell is made of arsenate. Atoms are too small to see with a microscope, so Wolfe-Simon and company looked at the cells themselves, and observed bacteria that looked inflated and shot through with huge holes filled with arsenate. Bacteria usually do this when faced with a toxin—they shove the toxin inside a bubble like they re cleaning house by shoving all the extra stuff in a closet. Just don t open the closet. 

Arsenic is another element with different bioavailabiUty in its different redox states. Arsenic is not known to be an essential nutrient for eukaryotes, but arsenate (As(V)) and arsenite (As(III)) are toxic, with the latter being rather more so, at least to mammals. Nevertheless, some microorganisms grow at the expense of reducing arsenate to arsenite (81), while others are able to reduce these species to more reduced forms. In this case it is known that the element can be immobilized as an insoluble polymetallic sulfide by sulfate reducing bacteria, presumably adventitiously due to the production of hydrogen sulfide (82). Indeed many contaminant metal and metalloid ions can be immobilized as metal sulfides by sulfate reducing bacteria. 

The most recent comprehensive assessment of the quality of rivers in the USA is that of Smith et al. 12), This followed an earlier assessment by Wolman et al. in 1971 (iS). The former 1987 assessment was based on 24 water quality measures from 161-383 stations around the country covering the period 1974-1981. Trends observed included major increases in nitrate, phosphorous, sodium, suspended sediment, fecal bacteria, dissolved oxygen deficit, arsenic and selenium. Major decreases were observed with nitrate, suspended sediment, fecal bacteria, dissolved oxygen deficit and lead. 

Arsenite is also an intermediate in the fungal biomethylation of arsenic (Bentley and Chasteen 2002) and oxidation to the less toxic arsenate can be accomplished by heterotrophic bacteria including Alcaligenes faecalis. Exceptionally, arsenite can serve as electron donor for chemolithotrophic growth of an organism designated NT-26 (Santini et al. 2000), and both selenate and arsenate can be involved in dissimilation reactions as alternative electron acceptors. 

Clearly pollution hazards exist when inorganic arsenic compounds are introduced into an environment where anaerobic bacteria are growing. Arsenic impurities in the phosphate used in detergents and for agricultural practices may pose serious problems because of the continuing synthesis of deadly poisonous methylarsenic compounds. 

Alkaline phosphatases [AP, orthophosphoric-monoester phosphorylase (alkaline optimum) EC 3.1.3.1] represent a large family of almost ubiquitous isoenzymes found in organisms from bacteria to animals. In mammals, there are two forms of AP, one form present in a variety of tissues and another form found only in the intestines. They share common attributes in that the phosphatase activity is optimal at pH 8-10, is activated by the presence of divalent cations, and is inhibited by cysteine, cyanides, arsenate, various metal chelators, and phosphate ions. Most conjugates created with AP utilize the form isolated from calf intestine. 

Several inorganic arsenic compounds are weak inducers of chromosomal aberrations, sister chromatid exchange, and in vitro transformation of mammalian and piscine cells. However, there is no conclusive evidence that arsenic causes point mutations in any cellular system (Pershagen and Valuer 1979 Belton et al. 1985 Lee et al. 1985 Deknudt et al. 1986 Manna and Mukheijee 1989). Studies with bacteria suggest that arsenite is a comutagen, or may inhibit DNA repair (Belton et al. 1985). 

Arsenicals were ineffective in controlling certain bacterial and viral infections. Mice experimentally infected with bacteria (Klebsiella pneumonias) or viruses (pseudorabies, encephalitis, encephalmyocarditis) showed a significant increase in mortality when treated with large doses of arsenicals compared to nonarsenic-treated groups (NAS 1977 Aranyi et al. 1985). 

Freeman, M.C. 1985. The reduction of arsenate to arsenite by an Anabaena bacteria assemblage isolated from the Waikato River. N.Z. Jour. Mar. Freshwater Res. 19 277-282. 

The process can be used to immobilize heavy metals such as Cd, Zn, Cu, Pb, Ni and Co. Cr(VI) can be reduced by some metal-reducing bacteria to the less toxic and less soluble form Cr(III). Arsenate [As(V)] can be reduced to the more mobile arsenite [As(III)] which precipitates as AS2S3, and is insoluble at low pH. Several laboratory-scale tests (batch and column) are currently available to study the feasibility of this process. However, only a few field tests have been performed to date. Two such tests have been conducted in Belgium, one at a non-ferrous industrial site, where the groundwater was contaminated with Cd, Zn, Ni and Co, and the other which was treated by injection of molasses in order to reduce chromium (VI) to chromium (III). A third demonstration in The Netherlands has been performed at a metal surface treatment site contaminated by Zn. The outcomes of a batch test of a groundwater heavily contaminated by Zn, Cd, Co and Ni are presented in Table 5. The initial sulphate concentration was 506mg/l. With the addition of acetate, a nearly... 

Macy JM, Santini JM, Pauling BV, et al. 2000. Two new arsenate/sulfate-reducing bacteria mechanism of arsenate reduction. Arch Microbiol 173 49-57. 

At the beginning of the twentieth century, arsenic compounds were used to kill spirochete bacteria, which cause the sexually transmitted disease syphilis. After this amphetamine compound was used to treat syphilis in Europe, the disease rate was reduced by more than half. The antibiotic penicillin has replaced arsenic for most medical purposes... 

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