Or, Star Trek to NASA: you've found my Horta!
What is life? We used to think we had a good answer for that. Recently we thought there were a handful of chemicals that were necessary for life, among them, Phosphorus. Phosphorus is found throughout any living body – bones, cell walls, energy transport, proteins, even DNA. In fact it is so necessary for life and so common inside a cell that it is possible to compute how much phosphorus must be contained in a given cell in order for it to function.
What our scientists have found is a microbe along the shores of Mono Lake that is able to substitute arsenic for phosphorous. The measurement of phosphorus in the microbes after they've "grown up" is by far too little for them to have done so. There clearly still is phosphorous involved, but there is also clearly arsenic in large amounts. This happened because the microbes were transferred into an environment that was otherwise normal except the phosphorous in the environment was replaced with arsenic.
This isn't quite Spock's friend, the Horta, which "Bones" noted had replaced all of it's carbon with silicon, but the same process is at work. In Star-Trek the sensors couldn't "see" the Horta as anything other than moving rocks. Replacing the basic chemicals of life re-defines what we have to consider as "life", and changes what we have to look for. It also means we have to look at a wider range of environments as being possibly "habitable". What else would using arsenic to replace phosphorous enable in a microbe? It has a different energy of reaction…
As Dr Wolfe-Simon has said, "this opens the doors" to a whole new set of paradigms. We thought we knew what to look for. Now, we need to look for a whole lot more.