Arsenic methylation in marine environments

Methylarsenic usually represents less than 10% of the total arsenic in seawater (Le, 2002), 97. The principal methylarsenic species in seawater are MMA(V) and DMA(V) (Francesconi and Edmonds, 1994), 222. Several other organoarsenicals may also be present. However, analytical methods are not always available to identify every organoarsenical in seawater and other materials. In some circumstances, unidentified organoarsenicals may represent a significant portion of the total arsenic in coastal marine waters, perhaps 25 % (Francesconi and Edmonds, 1994), 224. 

At their sampling sites in the Pacific Ocean, Santosa et al. (1997) found that MMA(V) and DMA(V) concentrations were highest at the surface with 0.012-0.016 pg L-1 and 0.048-0.185 pg L-1, respectively. The concentrations sharply declined to depths of 200 m. From depths of 200 to at least 5000 m, MMA(V) and DMA(V) concentrations stabilized at about 0.003 pg L-1 (Santosa et al., 1997). Santosa et al. (1996), 703 argue that the presence of methylarsenic in deep ocean waters is probably not due to diffusion from ocean floor sediments. Instead, the deep water methyl forms may result from the diffusion of methylarsenic-bearing surface waters, the circulation of surface waters to greater depths, and the tendency of methylarsenic not to appreciably sorb onto iron (oxy)(hydr)oxides particles. 

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