Halomonadaceae

The Halomonadaceae are a family of halophilic proteobacteria. The family was originally created in 1988 to contain the genera "Halomonas" and "Deleya". Many of its members were only recently discovered, and research is discovering new facts about them.
GFAG-1.
GFAJ-1 is a strain of rod-shaped extremophile bacteria in the Halomonadaceae. It was claimed that, when it did not have phosphorus, it took the usually poisonous element arsenic as a substitute. 
Geomicrobiologist Felisa Wolfe-Simon collected lake-bottom sediments in the shallow waters of Mono Lake in California. Wolfe-Simon cultured the organisms from this hypersaline (very salty) and highly alkaline environment. They grew in an arsenic-containing medium.
Unfortunately, the conclusions taken from this research have been disproved. A group of researchers analyzed the DNA of GFAJ-1, and could not detect any arsenic. The phosphorus in its DNA cannot be, and is not, replaced with arsenic.
Background.
Life is mostly composed of the elements carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. Although these six elements make up nucleic acids, proteins, and lipids and thus the bulk of living matter, it is possible in theory that some other elements in the periodic table might serve the same functions.
It is believed that all life needs phosphorus as a basic piece of the ‘backbone’ that holds DNA together. Likewise ATP (adenosine triphosphate), a key molecule in the cells' energy cycle.