Two new reports show that a bacterium, known as GFAJ-1, requires small amounts of phosphate to grow—and that it cannot substitute arsenic for phosphorus to survive, as a 2010 report in Science had suggested.

The GFAJ-1 bacterium, which was discovered in the arsenic-rich sediments of California’s Mono Lake, became the center of a controversy last year after Felisa Wolfe-Simon of the NASA Astrobiology Institute and the U.S. Geological Survey and colleagues reported that the microorganism could incorporate arsenic into its DNA when phosphorus wasn’t available.

Toxicologists have long held that the dose makes the poison: A substance can cause harm only in amounts high enough to overwhelm the body’s defenses. But a major conceptual shift is underway, a leading expert said recently at AAAS, with much more attention being paid to low-dose chemical exposures and the impact they can have even many years later.