Wolfe-Simon et al. (2010) discovered a bacterium that can grow by using arsenic instead of phosphorus. This article surprised us because we have believed that phosphorus is an element essential for life and nothing can substitute for phosphorus in biomolecules. The authors showed that this bacterium can replace phosphorus to arsenic in not only its proteins but also its DNA. Their NanoSIMS analyses, combined with the evidence for intracellular arsenic by ICP-MS and radiolabeled ⁷³AsO₄^3- experiments demonstrated that intracellular AsO₄^3- was incorporated into key biomolecules, specifically DNA.

This bacterium, which is named strain GFAJ^-1 in this article, was isolated from Mono Lake, located in eastern California, USA. The water of this lake is hypersaline and alkaline with high dissolved arsenic concentrations. The 16S rRNA sequence comparison (Supplementary Fig. S1 of this article) shows that this bacterium belongs to the genus Halomonas. It is so reasonable that Halomonas was isolated from Mono Lake. Interestingly the authors showed the cells driven by AsO₄^3- in the growth medium have morphological differences from the cells driven by PO₄^3- (Figs. 1C, 1D, and 1E of this article). I am so interested in whether vacuole-like structures (Fig. 1E) in the cells driven by AsO₄^3- are associated with the arsenic uptake or not.

Arsenic is toxic to not only human but also most of bacteria. However, some bacteria can discharge arsenic from the cell and are resistant to arsenic (Cai et al. 1998; Cervantes et al. 1994; Diorio et al. 1995; Willsky and Malamy 1980). In those systems, arsenic is not accumulated in the bacterial cells.

In the Science article (Wolfe-Simon et al. 2010), the authors cited a paper (reference number 12, Takeuchi et al. 2007) about bacteria accumulating intracellular arsenic. Takeuchi et al. (2007) indicated that Halomonas marina has high arsenic resistance, which was cultured with medium amended with arsenate at 310 mg As l^-1. In addition, they showed that Marinomonas communis (510 mg As l^-1) and Vibrio alginolyticus (730 mg As l^-1) have higher arsenic resistance than Halomonas marina. Arsenic accumulation amounted to 2.3 mg As g^-1 dry weight of the Marinomonas cells under incubation on medium containing 5 mg As l^-1 (Takeuchi et al. 2007). Thus, I am so interested in relation between the high arsenic resistance with accumulation and the ability to use arsenic instead of phosphorus. I think that other bacteria than Halomonas sp. GFAJ^-1 can substitute arsenic for phosphorus.

Part of microbiologists knows that some bacteria accumulate arsenic in the cells. However, nobody has known where the accumulated arsenic exists in the bacterial cell at the molecular level. Wolfe-Simon et al. (2010) elucidated the place of the arsenic in Halomonas sp. GFAJ^-1. It was beyond my imagination that Halomonas sp. GFAJ^-1 can substitute arsenic for phosphorus in its DNA. I have believed that the DNA structure is one of the most fundamental structures of organisms on earth.

According to this article (Wolfe-Simon et al. 2010), I recognized that microorganisms (bacteria) have high adaptability for their environments beyond our expectations. I am so interested in the mechanism of substituting arsenic for phosphorus. It is suggested that the mechanism of substituting arsenic for phosphorus in Halomonas sp. GFAJ^-1 had been formed during the course of evolution. I expect that the evolutionary process will be elucidated on the basis of genome sciences.

Generally archaea and bacteria have genome plasticity. Gene acquisition and loss have occurred frequently. It will be useful for elucidation of the mechanism of substituting arsenic for phosphorus to compare the genomes of bacteria accumulating intracellular arsenic.