In December, NASA held a press conference to announce that a bacteria discovered in a salty California lake appeared to use arsenic in its DNA. But the findings ran into criticism right from the start, and now the journal Science has published the critiques of the work, as well as a response from the team that did the original work.

It is unusual for a journal to publish direct critiques this way; usually it might publish letters to the editor or even full-on research papers by other scientists supporting or refuting the work.

Researchers Felisa Wolfe-Simon of NASA's Astrobiology Institute published their discovery in Science on Dec. 2. In her paper, she said she found a bacteria in California's Mono Lake, called GFAJ-1, that could and use arsenic in place of phosphorous to build DNA and other molecules.

All life on earth is made up of six basic chemicals: carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorous. Phosphorous makes up the backbone of DNA and is vital to getting energy out of sugars and transferring energy within cells.

If Wolfe-Simon is correct then her discovery would show that life can use other chemicals beyond those six. Most scientists take it as a given that for life to exist anywhere - including on other planets-it has to contain those chemical elements.

The discovery was criticized almost immediately. Even at the press conference announcing the discovery, Steven Benner, of the Foundation for Applied Molecular Evolution in Gainesville, Fla., said he was skeptical because arsenic does not bond as strongly to other atoms at room temperature.

The critiques published by Science cover several points. Some focus on experimental design or methods. Stefan Oehler, visiting researcher at the Alexander Fleming Biomedical Sciences Research Center in Greece, wrote that while it would be very interesting if such a life form were found, Wolfe-Simon and her colleagues didn't do the experiments that would say for sure if they did by checking if the arsenic was actually incorporated into the relevant molecules such as DNA.

Another critique, for James Cotner, of the Department of Ecology at the University of Minnesota, says Wolfe-Simon's bacteria aren't that different from ones he found in other lake systems. Wolfe-Simon says in her original paper that typical bacteria need about 1-3 percent phosphorous by weight. But Cotner says he has found bacteria surviving with much less than that.

Rosemary Redfield, a professor at the University of British Columbia, has been one of the harshest critics. She says the experiment Wolfe-Simon and her team did suffers from problems with contamination. Another issue not taking certain steps to purify the DNA they extracted from the bacteria.

Wolfe-Simon, in her response, says the data from Cotner's work actually supports the argument that GFAJ-1 had unusually low levels of phosphorous. She notes that when her team cultured cells in a phosphorous-depleted but arsenic rich environment, the highest values for total phosphorous content of the cells was much less than what he found in bacterial populations. She also stood by her method of purifying the DNA.