Scientists say they've finally established where the ocean's methane is coming from.
Up to four percent of the methane on Earth comes from the ocean's oxygen-rich waters, but scientists have been unable to work out the source.
Now, though, a University of Illinois-led team says it's found the culprit: a bit of 'weird chemistry' practised by the most abundant microbes on the planet.
The discovery was made by accident: the researchers were actually searching for new antibiotics, examining phosphonates - already in use in agriculture and medicine.
Many microbes produce phosphonates to thwart their competitors. Phosphonates mimic molecules the microbes use, but tend to be more resistant to enzymatic breakdown. The secret of their success is the durability of their carbon-phosphorus bond.
"We're looking at all kinds of antibiotics that have this carbon-phosphorus bond," says University of Illinois professor William Metcalf.
"So we found genes in a microbe that we thought would make an antibiotic. They didn't. They made something different altogether."
The microbe was Nitrosopumilus maritimus, one of the most abundant organisms on the planet, which lives in the oxygen-rich regions of the open ocean.
When scanning microbial genomes for promising leads, postdoctoral researcher Benjamin Griffin noticed that N. maritimus had a gene for an enzyme that resembled other enzymes involved in phosphonate biosynthesis.
He saw that the microbe also contained genes to make a molecule, called HEP, which is an intermediate in phosphonate biosynthesis.
The team cloned the gene for the mysterious enzyme, expressed it in a bacterium, and ramped up production of the enzyme. When the researchers added HEP, the chemical reaction that ensued produced a long sought-after compound, methylphosphonic acid, that could explain the origin of methane in the aerobic ocean.
Subsequent experiments indicated that the methylphosphonate was bound to another molecule, likely a sugar attached to the microbe's surface. When N. maritimus dies, other marine microbes break the carbon-phosphorus bond of the methylphosphonate to consume the phosphorus, generating methane.
The new findings will help those modeling the geochemistry of the ocean to understand climate change, says Metcalf.
"We know that about 20 percent of the greenhouse effect comes from methane and four percent of that comes from this previously unexplained source," he says.
"You have to know where the methane comes from and where it goes to understand what will happen when the system changes."