August 31, 2012 | 3 Comments
There is a lot of methane, the prime component of natural gas in the crust of the earth and on the surface. Souring where it all comes from is a topic of considerable debate with several theories on how the molecule forms. The theories range from simple to complex, some even suggest methane is seeping up from deep in the earth. Many will prove to be true.
And yet, much of the ocean waters are simply saturated with methane, and it continually seeps into the atmosphere frightening the global warming crowd with some merit, as methane is pretty good at holding heat.
Last April saw Eric Kort of NASA’s Jet Propulsion Laboratory uncovering a surprising and potentially important new source of Arctic methane: the ocean itself. This comes as no great surprise; organic life produces methane at prodigious rates.
Kort’s team observed increased methane levels while flying five HIAPER Pole-to-Pole Observations at low altitudes over the remote Arctic Ocean, north of the Chukchi and Beaufort Seas. The methane level was about one-half percent larger than normal background levels.
The question then becomes, “Where was the methane coming from?” The team detected no carbon monoxide in the atmosphere that would point to possible contributions from human combustion activities. In addition, based on the time of year, location and nature of the emissions, it was extremely unlikely the methane was coming from high-latitude wetlands or geologic reservoirs.
By comparing locations of the enhanced methane levels with airborne measurements of carbon monoxide, water vapor and ozone, they pinpointed a source: the ocean surface, through cracks in Arctic sea ice and areas of partial sea ice cover. The cracks expose open Arctic seawater, allowing the ocean to interact with the air, and methane in the surface waters to escape into the atmosphere. The team detected no enhanced methane levels when flying over areas of solid ice.
Now we’re up to, “How is the methane being produced?” The scientists weren’t yet sure, but Kort hinted biological production from living things in Arctic surface waters may be a likely culprit.
Only four months later University of Illinois microbiology and Institute for Genomic Biology (IGB) professor William Metcalf, study leader with chemistry and IGB professor Wilfred van der Donk report that they have found the culprit: a bit of “weird chemistry” practiced by the most abundant microbes on the planet.
When scanning microbial genomes for promising leads, Benjamin Griffin, a postdoctoral researcher in Metcalf’s lab, noticed that Nitrosopumilus 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.
To stay in step with the story, phosphonates are an unusual class of potential antibiotic agents 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.
To determine whether N. maritimus was actually producing a desirable phosphonate antibiotic, chemistry postdoctoral researcher Robert Cicchillo cloned the gene for the mysterious enzyme, expressed it in E. coli bacterium and ramped up production of the enzyme. When the researchers added HEP to the enzyme, the chemical reaction that ensued produced a long sought-after compound, one that could explain the origin of methane in the aerobic ocean.
The long sought for molecule, methylphosphonic acid, has been pursued for four years due to David Karl at the University of Hawaii, Edward DeLong at MIT and their colleagues publishing an elegant and yet unproven hypothesis to explain how methane was arising in the aerobic ocean. The only microbes known to produce methane until today were anaerobes, unable to tolerate oxygen. And yet the aerobic ocean is essentially saturated with methane.
Karl and DeLong noted that many aerobic marine microbes host an enzyme that can cleave the carbon-phosphorus bond. If that bond were embedded in a molecule with a single carbon atom, methylphosphonic acid, one of the byproducts of this cleavage would be methane. Karl and DeLong even showed that incubation of seawater microbes with methylphosphonic acid led to methane production.
Van der Donk explains the problem that has been lurking for the past four years, “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.”
But van der Donk’s lab conducted further experiments that demonstrated that the N. maritimus was actually synthesizing phosphonic acids. The 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 gobble up the phosphorus, an element that is rare in the oceans but essential to life. This encounter generates the methane.
Van der Donk remarks, “Organisms that make phosphonates tend to use weird chemistry for all kinds of things. 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.”
The connecting research efforts combine into an elegant and practical theory. While experiments and tests need to be performed to harden the theory, the research will very likely stand, and may not be the only oceanic biological activity producing methane.
These research studies offer biofuel and global warming folks a new strong hand up. The ocean and other ideas that man might invent have huge potential to supply natural gas. In coming studies we’ll learn how fast all this takes place and how to industrialize it. It would be unusual that natural gas could become a renewable fuel.
If anything is getting clearer, using natural gas is a good thing for the planet.