Microbial arsenite oxidation

N Wakao, H Koyatsu, Y Komai, H Shimokawara, Y Sakurai, H Shiota. Microbial oxidation of arsenite and occurrence of arsenite-oxidizing bacteria in acid mine water from a sulfur-pyrite mine. Geomicrobiol 6 11-24, 1988. 

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. 

A strong association between As and Fe-oxides in river and lake sediments has been reported (eg., Aggett and Roberts, 1986 Belzile and Tessier, 1990 Brannon and Patrick, 1987). This association is attributed to the adsorption of arsenate onto the Fe-oxide coatings on sediment particles. Arsenite ion, although also adsorbed, is not as rapidly or completely adsorbed onto Fe-oxide as arsenate (Belzile and Tessier, 1990 Pierce and Moore, 1982 Swedlund and Webster, 1999). This observation has important implications for the re-release of adsorbed As under anoxic conditions. Microbial reduction of arsenate can occur even when As is bound to Fe-oxide (Langner and Inskeep, 2000), and reduction or methylation of sedimentary arsenate under anoxic conditions is the principal mechanism for re-release of As into the water column (Aggett and... 

Two oxidation states of arsenic, As(V) and As(III), predominate in surface and near-surface environments. In solution, arsenic exists primarily as oxyanions arsenate [As(V) as H3ASO4] has pKa values of 2.2, 6.9, and 11.5, while arsenite [As(ni) as H3ASO3] has values of 9.2, 12.1, and 13.4 (Smith et al., 1998 Goldberg and Johnston, 2001). Thus, at circumneutral pH, H2ASO4, HAsOa , and H3ASO3 species dominate. Plant and microbial activity may methylate As(V) or As(III), forming, for example, dimethylarsenic acid (DMAA) and monomethy-larsonous acid (MMAA) (Cullen and Reimer, 1989). However, methylated species... 

Table 5 Approximate Time Scales of Microbiologically Mediated Oxidation of Arsenite to Arsenate, Using Examples of Pure Culture Isolates or Microbial Populations Present in Hot-Spring Ecosystems...

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... 

---