All forms of previously known life require six elements: nitrogen, oxygen, hydrogen, carbon, sulfur and phosphorus.
The salt-loving bacterium in the new study, a member Halomonadaceae family of proteobacteria, was able to survive and thrive on arsenic in laboratory conditions in the complete absence of phosphorus.
The discovery, part of an astrobiology study sponsored by the US space agency NASA, is considered a breakthrough in life science on Earth and in the search for extraterrestrial life.
'We've cracked open the door to what's possible elsewhere in the universe, and that's profound,' Felisa Wolfe-Simon, the NASA research fellow who led the study team, said Thursday in a NASA press conference.
Arsenic and phosphorus are both soft metals - in adjoining spots on the periodic table of the elements - and behave similarly in many chemical reactions.
The bacterium was found in Mono Lake, on the desertified eastern slopes of California's Sierra Nevada mountains. Just east of Yosemite National Park, the lake is briny and toxic but 'teeming' with microorganisms such as algae and bacteria, Wolfe-Simon said.
'It is terrestrial life,' said NASA senior scientist for astrobiology Mary Voytek, 'but not life as we know it.'
Beyond increasing influencing the search for life forms outside Earth, the findings could have practical applications in waste-water treatment, cleaning up toxic waste and replacing scarce phosphorous in bio-energy production.
Researchers grew the bacteria in a laboratory. In Petri dishes, arsenic was used to gradually replace phosphate salt, and eventually the bacterium grew using arsenic entirely, replacing phosphorus with arsenic in its molecules.
James Elser, an evolutionary ecologist who specializes in phosphorus in the environment, said it was 'quite shocking' for the bacterium to use arsenic instead.
The findings will require changes in science textbooks.
'Certainly, some paragraphs and sentences are going to have to be rewritten,' said Elser, a professor at Arizona State University.
