Scientists analyze first direct images of dissolved organic carbon from the ocean

Using an atomic force microscopy technique developed by IBM, the team was able to visualize individual atoms and bonds, yielding clues about their persistence in the marine environment. Findings were published today in the American Geophysical Union journal Geophysical Research Letters.

"To understand processes happening on the scale of ocean basins, it is sometimes necessary to view objects that are orders of magnitude smaller," said study co-author Ellen Druffel, professor and Fred Kavli Chair of Earth System Science at UCI. "By seeing with our own eyes the double bonds and rings of dissolved organic carbon molecules, we are better equipped to explain how they remain in the ocean for tens of thousands of years."

The molecules that were imaged were collected by UC Santa Cruz researchers from waters in the northern central Pacific Ocean.

The marine dissolved organic carbon pool, comparable in size to the atmospheric CO₂ reservoir, is about 200 times larger than the amount of carbon contained in all the plants and animals on the planet. Because of its complexity, diverse origins and varied reactions to environmental conditions, only about 10 percent of DOC has been characterized.

"We are still trying to figure out how the vast majority of this substance is going to be impacted by the ongoing addition of new CO₂ from fossil fuel burning and from increasing temperatures due to global climate change," said co-author Brett Walker, a UCI assistant researcher in Earth system science.

Most DOC near the ocean surface is derived from the remains of recently living phytoplankton. However, radiocarbon dating of DOC in the deep ocean shows it to be much older than expected -- by as much as 4,500 years -- indicating that a portion of this DOC survives multiple ocean mixing cycles.

Researchers have suggested that the chemical structure of DOC is responsible for its endurance in the environment, and through the atomic force microscopy technique, scientists are now able to see real-space images of the bound atoms in these compounds. The team has found that molecules from the deep ocean frequently exhibit aromaticity, meaning they're flat rings of atoms that are very stable and do not break apart easily.

"These atom-scale visualizations help demonstrate that the old age of small DOC molecules in the deep ocean has to do with their chemical structure, which bacteria do not seem to utilize," Walker said. "This is a crucial finding that will help researchers better understand the cycling of carbon in the oceans and the overall health of our planet's marine environments."

The work was funded by the University of Zurich, IBM Research-Zurich, the National Science Foundation and the American Chemical Society.