A bacterium found in the arsenic-filled waters of a Californian lake is poised to overturn scientists' understanding of the biochemistry of living organisms. The microbe seems to be able to replace phosphorus with arsenic in some of its basic cellular processes — suggesting the possibility of a biochemistry very different from the one we know, which could be used by organisms in past or present extreme environments on Earth, or even on other planets. Scientists have long thought that all living things need phosphorus to function, along with other elements such as hydrogen, oxygen, carbon, nitrogen and sulphur.