Dissecting DOM

Aquatic organisms release organic molecules that dissolve and persist in the water, providing detailed chemical fingerprints of an ecosystem's inhabitants over thousands of years. Now researchers have found a way to unlock the secrets of dissolved organic matter (DOM), an important step in elucidating the environmental origins and impacts of the complex chemical mixture (Environ. Sci. Technol., DOI: 10.1021/es103425s).

DOM in ocean water is remarkably stable, with an average age of 6,000 years. However, if global warming or other environmental changes speed up DOM's decomposition by even 1%, the resulting carbon dioxide flux into the atmosphere will be greater than that produced by human fossil fuel usage, says Andre Simpson, a chemist at the University of Toronto, Scarborough. Because DOM represents a major carbon reservoir as well as a historic record of aquatic life, scientists are eager to characterize its makeup.

But this analysis is no simple feat. "DOM is believed to be the world's most complex organic mixture," Simpson says. A single milliliter of water can contain thousands or even millions of distinct organic molecules. Such a mixture confounds classical analytical techniques like mass spectrometry and NMR: Molecular signatures overlap, making it impossible for researchers to identify individual structures.

Previously, researchers tried to separate components of DOM using high-performance liquid chromatography (HPLC). However, DOM's chemical diversity and the tendency of its components to aggregate into large "supermolecules" foiled these attempts. So Simpson and his colleagues turned to a variant of HPLC, known as hydrophilic interaction chromatography (HILIC). Because DOM has many polar constituents, they separate more readily by HILIC than by HPLC methods based on hydrophobic interactions or size exclusion.

Using HILIC, the researchers separated DOM in a water sample from Georgia's Suwannee River into 80 fractions. Two-dimensional NMR spectra revealed the success of their approach. In just one of the fractions, the team identified more unique chemical structures —27—than they did in the entire unfractionated DOM sample.

Many DOM molecules appear to be derivatives of terpenoids, Simpson says. These compounds "are made by living species as pigments, steroids, and scents." Because scientists have established that many species produce unique terpenoids, DOM could serve as a chemical record of "long-extinct species, changes in ocean populations over time, and the impact of climate change on ocean life," he says.

Brian Kelleher, a chemist at Dublin City University in Ireland, says that Simpson's technique has the potential to "expose a treasure chest of compounds, some known and some novel." He adds that identifying DOM components may provide clues as to why many components of the mixture are unusually stable, whereas others decompose and release carbon dioxide into the atmosphere.