Arsenic. Can this toxic metalloid sustain life?

It was recently reported that a bacterium, Halomonas species GFAJ-1, isolated from arsenic-rich Mono Lake and further selected for growth under conditions of high arsenate and low phosphate, is able to grow using arsenic instead of phosphorus. This claim, and subsequent studies to evaluate GFAJ-1, has brought new attention to the question of whether arsenic can play an essential or sustaining role for living organisms. If true, this would be in stark contrast to the well known toxicity of this element and its ability to cause a number of diseases, including cancer of the skin, lung, bladder, liver, and kidney. However, while deadly at high doses, arsenic oxide is also an approved and effective chemotherapeutic drug for the treatment of acute promyelocytic leukemia (APL). This review examines the evidence that arsenic may be a beneficial nutrient at trace levels below the background to which living organisms are normally exposed. It also examines whether arsenic can be used to sustain organisms growing under high arsenic conditions, specifically the results from recent studies of arsenic biochemistry motivated by the report of GFAJ-1. Both of these topics are considered in the context of the toxicity of this element and its ability to cause cancer and other diseases, yet its Janus-faced ability to effectively treat APL.