Photosynthesis, the process by which plants utilize the sun's energy to create their own, leaves behind a unique calling card in the form of a chemical signature that is spelled out with stable oxygen isotopes.

Photosynthesis by microscopic plants forms the base of the oceanic food chain, but it is difficult to measure how productive these plants are in natural settings. This research will make it easier to do so.

Most oxygen atoms contain eight protons and eight neutrons and are represented by the symbol O-16. More than 99.9 percent of Earth's oxygen is O-16, but two heavier oxygen isotopes exist in trace amounts: O-17, with one extra neutron, and O-18, with two.

Scientists know that plants and animals sometimes process heavy isotopes like O-17 and O-18 at a different pace than O-16. For instance, when sea temperatures decrease, corals and mollusks produce calcium carbonate -- the raw material of ocean reefs and clam shells -- that contains greater amounts of heavy oxygen isotopes. As a result, scientists have used isotopic ratios from carbonate fossils to estimate global temperatures in the distant past.

In the new study, the researchers examined "clumped" oxygen isotopes, oxygen molecules that contain two heavy isotopes. Such molecules, which have masses of 35 or 36, are exceptionally rare; less than a handful exist in every trillion oxygen molecules. Today's mass spectrometers, however, are sophisticated enough to tally them and allow scientists the opportunity to compare their relative abundance in various circumstances.

The new research shows that biological assembly of molecules produces molecules that have pairings of isotopes that violate expectations from both thermodynamics and sheer chance.

"We've found a new type of biosignature," said lead author Laurence Yeung, an assistant professor of Earth science at Rice University. "We show that plants and plankton impart this type of biosignature on the oxygen they produce during photosynthesis."

 Published in Science.