In the waters around Hawaii, tumors are growing on the face and flippers and internal organs of endangered green turtles. This deadly disease, fibropapillomatosis, is a leading cause of death for the turtles, and now scientists have figured out why turtles in certain areas seem to suffer disproportionately from it. Nitrogen runoff from cities and farms is triggering outbreaks of the disease, Duke University reports.

In 2010, researchers found that turtles living in parts of the ocean with higher concentrations of nitrogen also suffered a higher instance of the disease. To better establish that link, the researchers studied how algae—which turtles eat—stores nitrogen. As Duke describes, algae converts excess nitrogen into arginine, an amino acid. Turtles that suffered from fibropapillomatosis, the team reports in the journal PeerJ, had higher levels of arginine than those that were healthy. Algae in the water where they lived also contained elevated levels of the compound.

Arginine, in turn, supports the growth of the virus that causes fibropapollomatosis. As Kyle Van Houtan, lead author of the study, told Duke, "If this disease is a car, arginine its fuel." Proline and glycine, molecules commonly found in human cancer tissue, also turned up in elevated levels in the turtles.

Van Houtan and his colleagues call for more research in order to definitively link the nitrogen influx with the disease, but at this point they say that it's pretty clear that there is a connection and that, until that runoff is better controlled, turtles will most likely continue to suffer.