Is arsenic biotransformation a detoxification mechanism for microorganisms?

Arsenic (As) is extremely toxic to living organisms at high concentration. In aquatic systems, As exists in different chemical forms. The two major inorganic As (iAs) species are As(V), which is thermodynamically stable in oxic waters, and As(III), which is predominant in anoxic conditions. Photosynthetic microorganisms (e.g., phytoplankton and cyanobacteria) take up As(V), biotransform it to As(III), then biomethylate it to methylarsenic (MetAs) forms. Although As(III) is more toxic than As(V), As(III) is much more easily excreted from the cells than As(V). Therefore, majority of researchers consider the reduction of As(V) to As(III) as a detoxification process. The biomethylation process results in the conversion of toxic iAs to the less toxic pentavalent MetAs forms (monomethylarsonate; MMA(V), dimethylarsonate; DMA(V), and trimethylarsenic oxide; TMAO(V)) and trimethylarsine (TMAO(III)). However, biomethylation by microorganisms also produces monomethylarsenite (MMA(III)) and dimethylarsenite (DMA(III)), which are more toxic than iAs, as a result of biomethylation by the microorganisms, demonstrates the need to reconsider to what extent As biomethylation contributes to a detoxification process. In this review, we focused on the discussion of whether the biotransformation of As species in microorganisms is really a detoxification process with recent data.