It was announced in 2010 in the journal Science that a species of bacteria was found which was able to metabolize (meaning to convert a substance into energy for uses like growth) arsenic. This bacteria exists in extreme conditions (hence it being called an "extremophile"), and normally metabolizes phosphorous. The scientific team which discovered this bacterium noted that the use of arsenic as a "food source" occurred when the bacteria's environment was deprived of phosphorous. Some scientists have doubted that the bacterium in question was indeed metabolizing arsenic, and scientific research into the veracity of this claim is still being performed.
This has great implications on the search for extraterrestrial life. The reason this is important is that when searching for extraterrestrial life, astrobiologists typically begin with assumptions about what is and is not possible, and then work outward to determine how many likely extraterrestrial civilizations there are in existence. For example, one common assumption used by astrobiologists is that liquid water must be present for life to evolve. This assumption is based on the observation that most chemical reactions for life take place using water as a solvent – these reactions form much of the field of organic chemistry. If someone were to find a way to have organisms here on Earth which didn't need water for life giving molecular reactions, it would force astrobiologists to accept the possibility that life could evolve on planets on which there was no liquid water. So the constraints on what scientists believe are likely to occur in the evolution of extraterrestrials are dependent upon what we've seen occur here on Earth in the formation of organisms. These assumptions based on observation form the basis of the astrobiologist's "tool chest".
One assumption which has been made about the evolution of extraterrestrial life involves what is and is not needed to be present when life forms. Phosphorous was one of the major components which had been assumed as necessary for the formation of life. Meanwhile, arsenic is toxic to most life forms. Some researchers have hypothesized that arsenic can actually be beneficial in some organisms, since it could be used in place of phosphorous. However, there are some differences between arsenic and phosphorous, such as the fact that arsenic molecules are larger than phosphorous molecules and that the needed arsenic reactions are less stable in water than phosphorous reactions.
However, if the discoverers of this bacteria are correct in their conclusion that this form of life can use arsenic as a food source when phosphorous is not present, it greatly increases the number of possible places in which extraterrestrial life may have formed, since now a place which doesn't have phosphorous at all can still be considered a candidate for life forms to evolve.