Microbe GFAJ-1 supposedly substitutes arsenic for phosphorus in its molecular components. As of now researchers have only looked at its DNA, but they still think the backbone is made with phosphorus otherwise it would fall apart in water, and the bacteria are getting the phosphorus from contaminants in what should be a media without it. Basically they found life growing in a pool with a lot of arsenic, which is deadly to humans, so it must be cool.
I feel sort of obliged to post this due to the interest in astrobiology the media seems to have suddenly attained (or blame it on NASA’s hype), but it is not at all unexpected. Arsenic and phosphorus share similar chemical properties. I won’t substantiate my claim and instead tell you to look at a periodic table. In fact, arsenic is deadly to humans because of its similarity to phosphorus; it replaces it and primarily screws up cellular metabolism.
Based on this research, it is unclear if bacteria can actually grow in a medium deficient of phosphorus. There is no evidence of “arsenic based life”! I have a feeling we will learn that the bacteria simply make extensive use of arsenic, but still cannot live without phosphorus– similar to organisms that incorporate both silicon and carbon. The general theme is that life works with what it can efficiently acquire (or else it is no longer life) and chemistry doesn’t give a crap. The longterm astrobiological goal is to figure out the minimum requirements for life, which is hard as hell because we have a life-containing-object-traveling-through-space sample size of one, of which we have much to explore.