August 31, 2012
Mechanism Discovered in Deep Ocean Production Of Methane
Brett Smith for redOrbit.com - Your Universe Online
While searching for new and more effective antibiotics, a team of biologists and chemists has stumbled upon the mechanism responsible for the deep ocean production of methane, a greenhouse gas that drives climate change.
Researchers from the University of Illinois and the University of Washington began their studies examining microbes´ production of phosphonates that these tiny creatures use to disrupt bacteria and other microscopic organisms by mimicking nutrients. After being taken in by these targeted microorganisms, phosphonates inhibit cellular processes because the resilience of their carbon-phosphorus bond makes them difficult to break down.
"We're looking at all kinds of antibiotics that have this carbon-phosphorus bond," said University of Illinois microbiology 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 in question was Nitrosopumilus maritimus, one of the smallest and most abundant organisms on the planet that lives in oxygen-rich regions of the open ocean. When performing a genomic analysis of N. maritimus, the scientists noticed a gene for an enzyme involved in phosphonate production machinery. They also spotted genes that lead to the production of HEP, an intermediate in phosphonate biosynthesis.
Thinking they were on the trail of an attractive phosphonate antibiotic, chemist Robert Cicchillo from the University of Illinois cloned the gene for the key enzyme and expressed it in E. coli, which he then cultured to produce the enzyme. When the researchers added HEP to the enzyme, the chemical reaction produced methylphosphonic acid, a long sought-after compound that could explain why vast quantities of methane were being created in the ocean.
In 2008, David Karl at the University of Hawaii and some colleagues published a hypothesis to explain how methane, known to only be produced by anaerobes, was arising in the aerobic ocean. They posited that if certain microbes could cleave the carbon-phosphorus bond in methylphosphonic acid, it would release methane as a byproduct.
"There was just one problem with this theory," van der Donk said. "Methylphosphonic acid has never been detected in marine ecosystems. And based on known chemical pathways, it was difficult to see how this compound could be made without invoking unusual biochemistry."
Van der Donk´s team was able to replicate methylphosphonic acid through a “Herculean effort” of chemistry that involved culturing 100 liters of microbes to produce a few milligrams of cells.
"Organisms that make phosphonates tend to use weird chemistry for all kinds of things," van der Donk said. "But this is very unusual. One of the carbon atoms of the HEP is oxidized by four electrons and the other is turned into a methyl group. I'm not aware of any other cases where that happens."
He added that the results of the study could have wider implications for climate change research.
"We know that about 20 percent of the greenhouse effect comes from methane and 4 percent of that comes from this previously unexplained source," van der Donk said. "You have to know where the methane comes from and where it goes to understand what will happen when the system changes."