The new study determined that much of the ocean’s dissolved organic matter is made up of novel polysaccharides — long chains of sugar molecules created by photosynthetic bacteria in the upper ocean. Bacteria begin to slowly break these polysaccharides, tearing out pairs of carbon and phosphorus atoms from their molecular structure. In the process, the microbes create methane, ethylene, and propylene gasses as byproducts. Most of the methane escapes back into the atmosphere.Graphic courtesy of Eric Taylor, Woods Hole Oceanographic Institution

The new study determined that much of the ocean’s dissolved organic matter is made up of novel polysaccharides — long chains of sugar molecules created by photosynthetic bacteria in the upper ocean. Bacteria begin to slowly break these polysaccharides, tearing out pairs of carbon and phosphorus atoms from their molecular structure. In the process, the microbes create methane, ethylene, and propylene gasses as byproducts. Most of the methane escapes back into the atmosphere.Graphic courtesy of Eric Taylor, Woods Hole Oceanographic Institution

For decades, marine chemists have faced an elusive paradox. The surface waters of the world’s oceans are supersaturated with the greenhouse gas methane, yet most species of microbes that can generate the gas can’t survive in oxygen-rich surface waters.

So where exactly does all the methane come from?

This longstanding riddle, known as the “marine methane paradox,” may have finally been cracked thanks to a new study by researchers from the Woods Hole Oceanographic Institution and the University of Hawaiʻi at Mānoa.

According to the study, published last week in the journal Nature Geoscience, the answer may lie in the complex ways that bacteria break down dissolved organic matter, a cocktail of substances excreted into seawater by living organisms.

The researchers found that much of the ocean’s dissolved organic matter is made up of novel polysaccharides — long chains of sugar molecules created by photosynthetic bacteria in the upper ocean. Bacteria begin to slowly break these polysaccharides, tearing out pairs of carbon and phosphorus atoms (called C-P bonds) from their molecular structure. In the process, the microbes create methane, ethylene, and propylene gasses as byproducts. Most of the methane escapes back into the atmosphere

“All the pieces of this puzzle were there, but they were in different parts, with different people, in different labs, at different times,” said WHOI geochemist and lead author Dan Repeta. “This paper unifies a lot of those observations.”

Methane is produced when bacteria breakdown long-chain sugar molecules. Graphic courtesy of UH SOEST