Fungi arsenic methylation

Some metals, arsenic and mercury for example, may be volatilized by methylation due to activity of anaerobic microorganisms. Arsenic can be methylated by methanogenic Archaea and fungi to volatile toxic dimethylarsine and trimethylarsine or can be converted to less toxic nonvolatile methanearsonic and dimethylarsinic acids by algae [42]. 

Bacterial methylation of arsenate by a methanogen was first reported by McBride and Wolfe (135) in 1971, and reports of nonmethanogenic bacterial methylation followed. These transformations are now known to be effected by a range of bacteria, and the mechanisms are likely to be similar to those proposed for fungi (32). 

The methylation of arsenic is entirely or almost entirely biotic (Frankenberger and Arshad, 2002), 367. Specifically, certain fungi (including yeasts) and bacteria are capable of methylating arsenic ((Bentley and Chasteen, 2002), 257-260 (Cullen and Reimer, 1989), 717-724 Chapter 4). Only limited evidence exists for the chemical (abiotic) methylation of arsenic. As mentioned earlier, some volatile arsines have been produced in the laboratory from photochemical reactions involving As(III), carboxylic acids, and ultraviolet radiation (Guo et al., 2005 McSheehy et al., 2005). 

Perhaps as much as 26 200 t of arsenic may annually volatilize into the atmosphere from soils (Matschullat, 2000), 300-301. Much of this volatilization is due to microbial activity (Frankenberger and Arshad, 2002), 363-364. Under reducing conditions in soils, fungi and other microorganisms may produce gaseous arsine and methylated arsines, such as methylarsine, dimethylarsine and trimethylarsine ((Mandal and Suzuki, 2002), 205 (Lrankenberger and Arshad, 2002), 363 (Oremland and Stolz, 2003), 939 Chapter 4). 

The third group of As species in natural freshwaters are the methylated arsenic species. Arsenic can be methylated by bacteria, algae and fungi to form gaseous mono-, di- and tri-methyl arsine (CH3) As) (Baker et al., 1983 Cullen and Reimer, 1989 Maeda et al, 1987), which dissolve in water forming the methylarsenic oxyacids, monomethylarsonic acid (MMAA) and... 

Many chemicals can also exist as various species or states of ionization. For example, nitrogen can exist as nitrate, nitrite, or ammonia, arsenic can exist as arsenate or arsenite, and lead can exist as lead nitrate or lead chloride. The species or ionization state may depend upon abiotic variables such as soil or water pH, amount of dissolved oxygen in the water, and presence of other chemicals. Alternatively, bacteria and fungi may change the species or ionization state of a chemical. For example, bacteria can convert arsenite to arsenate, and add methyl groups to ionic mercury to produce methylmercury. 

Numerous bacteria, fungi, yeasts, and algae are able to transform arsenic compounds. Among the transforming processes mediated by microorganisms are oxidation (9,10), reduction (11), demethylation (12,13), and methylation reactions (12-16). Biological transformations of arsenic in soil are illustrated in Figure 1 (17). 

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... 

The more troublesome problem is the biotransformation of arsenic into arsine gas. Four common soil fungi, two anaerobic bacteria and algae in surface water can generate a methylated version of arsine. Generally, see Arsenic Speciation in the Enviromnent by W. R. Cullen and K. J. Reimer, Chemistry Review (1989) Arsenic in the Soil Environment by E. Smith, R. Naidu, and A. M. Alston, Advances in Agronomy (1998) and Arsenic Chemistry in Soils An Overview of Thermodynamic Predictions and Field Observations by Muhammed Sadiq, Water Air and Soil Pollution (1997). 

The more troublesome problem is the biotransformation of arsenic into arsine gas. Four common soil fungi, two anaerobic bacteria, and algae in surface water can generate a methylated version of arsine. It is unknown whether these same biological processes can convert antimony to stibine. 

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