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 AsV, which is thermodynamically stable in oxic waters, and AsIII, which is predominant in anoxic conditions. Photosynthetic microorganisms (e.g., phytoplankton and cyanobacteria) take up AsV, biotransform it to AsIII, then biomethylate it to methylarsenic (MetAs) forms. Although AsIII is more toxic than AsV, AsIII is much more easily excreted from the cells than AsV. Therefore, majority of researchers consider the reduction of AsV to AsIII as a detoxification process. The biomethylation process results in the conversion of toxic iAs to the less toxic pentavalent MetAs forms (monomethylarsonate; MMAV, dimethylarsonate; DMAV, and trimethylarsenic oxide; TMAOV) and trimethylarsine (TMAOIII). However, biomethylation by microorganisms also produces monomethylarsenite (MMAIII) and dimethylarsenite (DMAIII), 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.