Coastline Vital to Ocean Life

The discovery that sunlight produces significant amounts of nitrogen from dissolved organic matter and makes it available to microscopic life forms will alter current views of how coastal environments support life in the seas.

The research was published May 30 in the British journal Nature.

"Biologists and chemists have often worked in isolation on these kinds of questions," said Dr. Mary Ann Moran of the department of marine science at the University of Georgia. "The two fields have come together in this paper."

The research provides yet more evidence that the land-sea margin is crucial in supporting the chain of life in oceans. Moran and her colleagues now believe there is some 20 percent more terrestrial nitrogen available to microorganisms that had been known earlier.

"The nitrogen had really been hiding in plain sight," said Dr. Robert Hodson, head of the UGA School of Marine Programs and a co-author of the paper. "We knew there was a huge reserve of organic matter in the oceans, but we didn't think that nitrogen from the land was a significant part of the action. Geochemists had been interested but not biologists. The finding is important because we know the biotic activity on this planet controls the cycles of so many important elements."

Dissolved organic matter in marine and freshwater ecosystems constitutes one of the largest active organic matter reserves on Earth. Chemically identifiable, low-molecular-weight compounds have been studied for more than three decades, but little has been known about molecules with higher molecular weights that make up most dissolved organic matter (DOM).

The University of Georgia team studied a part of DOM called "humic substances" -- groups of compounds that are composed mostly of high-molecular-weight organic acids. Using bacterial bioassays, the team found that exposure to sunlight causes the release from DOM of nitrogen-rich compounds that are biologically available and help degrade humic substances.

Photochemical studies subsequently showed that ammonium was among the nitrogen compounds released when the substances were exposed to light, largely in the ultraviolet range. This ammonium production has important implications for nitrogen production in coastal oceans.

"Scientists had generally believed that, although humic substances can make up the largest fraction of DOM, all the available nitrogen had been extracted from it," said Moran. "That's why it had received so little attention as a potential source of nitrogen for planktonic organisms and the microbial food webs that are crucial to the health of the seas."

Thus, the research opens a new area of study for oceanographers. The study could also alter ideas regarding the effects of changing global climate and precipitation patterns.

"We believe this demonstrates yet another connection between the terrestrial environment and coastal systems," said Hodson.