Arsenic is a deadly poison and yet it can be adapted to by bacterial species given enough time living at lower concentrations which they can tolerate. Increasing concentrations of toxic metals have been tolerated by the bacteria in Mono Lake, California, which has no outlet to the sea. Evaporation takes away only volatile substances, which leads to increased constration of non-volatile heavier elements like arsenic.
Mark Twain wrote a story about Mono Lake back in 1872 where he almost gets marooned there on a small island when his rowboat gets blown away from the island shore. The lake has no one living near it and he was rightly worried about dying of thirst. He said the water was so caustic that it instantly burned your finger if you touched it, ever so quickly, so swimming even the half mile back to the lakeshore was impossible. Since Twain’s visit, the source of fresh water flowing into Mono Lake has been largely shunted off to Los Angeles and used there for drinking water and so with even less fresh water coming in, the lake has become even more concentrated with natural chemicals. Thus, there has been a slow buildup of arsenic along with other heavy metals which forms a perfect natural experiment for adaptation by living things. Most things when subjected to these concentrations of arsenic just die and leave no progeny, but some bacteria have managed to survive and adapt to this extremely hostile environment.
Bacteria have a very short life cycle and thus their species can adapt to concentrations of chemicals like arsenic much more quickly than much larger living things, such as humans. Without some form of advanced artificial selection process controlled by humans, any animal large enough to be seen with your naked eye would take longer to adapt than you would live. So, you would never have the opportunity to see it happen. But, in the artificial experiments performed by Dr. Wolfe-Simon and her colleagues from Arizona State University, on a bacterium known as strain GFAJ-1 of the Halomonadaceae family of Gammaproteobacteria, they could accelerate the selection process quickly enough to see results in a few years. These minute bacterial life forms already had relatives who lived in various extreme (by human standards) environments such as Mono Lake and at the bottom of the sea in such places as Black Smokers at the mid-oceanic ridge and White Smokers a few kilometers away from the ridges.
The experiments were done with known species of extremophiles and those are much closer to being adapted to the Earth as it existed several billion years ago than to the present surface conditions. They can survive in these extreme conditions because they still have adaptations available from that distant past life experience. In the extreme environment of Mono Lake they don’t have to compete with more advanced life forms. It is because they don’t have to compete that they can maintain their lives at a not very efficient level when in the lake. When they have the opportunity to live in a more benign environment they revert as quickly as possible to not using arsenic in their life processes. When deprived of phosphorus but supplied with arsenic, a similar element, they are able to incorporate it into their living metabolism. It’s like my eating spinach as as an adult but hating it when a child although even now I will quickly revert to chocolate cake given the chance.
Extremophiles choose phosphorus instead of arsenic when given the chance.