Until now, biologists have said life requires six elements -- carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.
Arsenic is right below phosphorus on the periodic table, and the two are very similar.
However arsenic is different enough that it cannot substitute for phosphorus, and typically slows down an organism enough that it kills it, hence why it is considered poison.
"What is new here is arsenic is being used as a building block for the organism," AFP quoted co-author of the study Ariel Anbar as saying.
According to the research published in the journal Science, the finding can help scientists understand the the way life arose on Earth - and how many times it may have done so.
It also gives weight to the long-standing idea that life on other planets may have a radically different chemical makeup
"We have had this idea that life requires these six elements with no exceptions and here it turns out, well maybe there is an exception," Anbar said.
Mono Lake, where the bacteria were found, is known for its high levels of salt and arsenic.
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