New study explains mysterious source of greenhouse gas methane in the ocean

New study explains mysterious source of greenhouse gas methane in the ocean
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. Credit: Illustration by 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 from the Woods Hole Oceanographic Institution (WHOI).

According to WHOI geochemist Dan Repeta, 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.

In a paper released in the November 14, 2016 issue of the journal Nature Geoscience, Repeta and colleagues at the University of Hawaii found that much of the ocean's dissolved organic matter is made up of novel polysaccharides—long chains of sugar molecules created by 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 , 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," says Repeta. "This paper unifies a lot of those observations."

Methane is a potent greenhouse gas, and it is important to understand the various sources of methane in the atmosphere. The research team's findings describe a totally new pathway for the microbial production of methane in the environment, that is very unlike all other known pathways.

Leading up to this study, researchers like Repeta had long suspected that microbes were involved in creating methane in the ocean, but were unable to identify the exact ones responsible.

"Initially, most researchers looked for microbes living in isolated low-oxygen environments, like the guts of fish or shrimp, but they pretty quickly realized that couldn't be a major factor. Too much oxygenated water flows through there," says Repeta. Many researchers also examined flocculent material—snowy-looking bits of animal excrement and other organic material floating in ocean waters. "Some of those also have low-oxygen conditions inside them," he says, "but ultimately they didn't turn out to be a major methane source either."

In 2009, one of Repeta's co-authors, David Karl, found an important clue to the puzzle. In the lab, he added a manmade chemical called methylphosphonate, which is rich in C-P bonds, to samples of seawater. As he did, bacteria within the samples immediately started making methane, proving that they were able to take advantage of the C-P bonds provided by the chemical. Since methylphosphonate had never been detected in the ocean, Repeta and his team reasoned that bacteria in the wild must be finding another natural source of C-P bonds. Exactly what that source was, however, remained elusive.

After analyzing samples of dissolved organic matter from in the northern Pacific, Repeta ran into a possible solution. The polysaccharides within it turned out to have C-P bonds identical to the ones found in methylphosphonate—and if bacteria could break down those molecules, they might be able to access the phosphorus contained within it.

To confirm this idea, Repeta and his team incubated seawater bacteria under different conditions, adding nutrients such as glucose and nitrate to each batch. Nothing seemed to help the bacteria produce methane—until, that is, they added pure polysaccharides isolated from seawater. Once those were in the mix, the bacteria's activity spiked, and the vials began spitting out large amounts of methane.

"That made us think it's a two-part system. You have one species that makes C-P bonds but can't use them, and another species that can use them but not make them," he says.

Repeta and another co-author, Edward DeLong, a microbial oceanographer at the University of Hawaii, then began to explore how bacteria metabolize dissolved . Using a process called metagenomics, DeLong catalogued all the genes he could find in a sample of seawater from the north Pacific. In the process, he found genes responsible for breaking apart C-P bonds, which would allow bacteria to wrench phosphorus away from carbon atoms. Although DeLong was not certain which bacteria could actually do this, one thing was clear: If the gene was active, it would give an organism access to an important but rare nutrient in seawater.

"The middle of the ocean is a nutrient-limited system," says Repeta. "To make DNA, RNA, and proteins, you need nitrogen and phosphorus, but in the open ocean, those nutrients are at such low concentrations that they're almost immeasurable." Instead of using free-floating nutrients in the water, Repeta says, DeLong's study showed that the microbes must somehow be able to crack into nitrogen and phosphorus hidden deep inside organic molecules.

Although Repeta's latest paper confirms that it is indeed possible for bacteria to break apart C-P bonds, he notes that it's still not a particularly easy means of getting nutrients. With phosphorus tied up in organic molecules, it can be exceedingly difficult for bacteria to reach. If microbes can find other sources of the nutrient, he says, they will inevitably use those first.

"Think of it like a buffet," Repeta says. "If you're a microbe, inorganic nutrients are like fruits and meats and all the tasty stuff that you reach for immediately. Organic nutrients are more like leftover liver. You don't really want to eat it, but if you're hungry enough, you will. It takes years for to get around to eating the organic phosphorus in the upper ocean. We don't exactly know why, but there's another really interesting story there if we can figure it out."


Explore further

Methane Formation in the Oceans: New Pathway Discovered

More information: Daniel J. Repeta et al, Marine methane paradox explained by bacterial degradation of dissolved organic matter, Nature Geoscience (2016). DOI: 10.1038/ngeo2837
Journal information: Nature Geoscience

Citation: New study explains mysterious source of greenhouse gas methane in the ocean (2016, November 17) retrieved 20 September 2019 from https://phys.org/news/2016-11-mysterious-source-greenhouse-gas-methane.html
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Nov 17, 2016
For decades, marine chemists have faced an elusive paradox. The surface waters of the world's oceans are supersaturated with the greenhouse gas methane,


........"supersaturated with methane", a Heat Trapping Gas 30 times more potent than CO2 on the part of the surface of the Earth with water covering 3/4 of the planet. Nevertheless, the 1998 Holy Hockey Stick scaremongers want the gullible to think it is humankind living on the small remaining part of the surface who are the GHG problem with their tiny contribution of CO2.

Nov 17, 2016
Nawwww, its' whale farts

Nov 17, 2016
Bacteria Tax?

Nov 18, 2016
Thirty years ago, astrophysicist Thomas Gold proposed, as an alternative to the absurd claim that oil comes from rotted dinosaurs, that anaerobic microorganisms deep in the crust are a major source of hydrocarbon production. If that is the case, the Earth "sweats" hydrocarbons, which escape to the surface eventually. Impervious rock structures like salt domes however trap those molecules to make pools of hydrocarbons. That is why large oil fields are found peripheral to tectonic fault zones and impact craters; these features are habitats. Subsequent to the publication of his theory, such anaerobic microorganisms were found in deep sea vents. ... One implication of this idea is that climate models should incorporate this so-far unquantified source of CO2 (a breakdown product of methane) as an input.

Nov 18, 2016
The idea of Gold's abiogenic gas did not pan out, but the issue is of fugitive emissions is important.

Nov 18, 2016
Anonym, perhaps the claim was absurd because scientists weren't the one making it. Coal and oil are generally recognized as coming from the remains of producers, not consumers. Your phrase "tectonic fault zone" is also peculiar. It implies that these are special faults related to tectonic activity or tectonic plates. These can only be mid-ocean ridges, where you certainly won't find any oil rigs, or subduction zones. I can certainly think of a few areas where oil is found near a subduction zone, but since the over-riding plate scrapes off material from the subducted plate, organic materials can easily be claimed to have come from the scraped-off material. There are areas far away from subduction zones that have oil and gas, so subduction does not appear to be a requisite. My guess is that you simply used the phrase "tectonic fault" with little understanding of its meaning.

Nov 20, 2016


I think Zwicky is god, Benni.
Bow for him. Kneel, you clown.

I also think you are at the least a sociopath.


Read more at: http://phys.org/n...html#jCp

Nevertheless, Benni's misinformation has likely more impact than the facts, because he reacted so timely. I urge all to counteract this source of confusion instantly. One should not underestimate the impact of power of misinformation.
........seems to me that the "misinformation" is your claim that zany Zwicky is a "god" before whom all of mankind should "bow" & "kneel", the short & fat pudgy bowling ball of an asstrophysicist that he was.


Nov 20, 2016
Benni's remarks are unsubstantiated

Nevertheless, Benni's misinformation has likely more impact than the facts, because he reacted so timely. I urge all to counteract this source of confusion instantly. One should not underestimate the impact of power of misinformation.


seconded

Nov 20, 2016
Here are the facts:
https://en.wikipe...e_effect

LOL.
The Retard of the Decade, who is incapable of an independent thought, goes to Wikipedia for his facts. What a surprise.

Nov 20, 2016

........seems to me that the "misinformation" is your claim that zany Zwicky is a "god" before whom all of mankind should "bow" & "kneel", the short & fat pudgy bowling ball of an asstrophysicist that he was.

Not "mankind" Benni.
You. Kneel, worm.
.......I've seen pictures of your zany family icon standing next to other people, and everytime I see one of your posts I picture two bowling balls with stubby little arms stuffing themselves with potato chips.

Nov 20, 2016
I back up my statements with published material, as anyone here should
..........no one cares about clicking on any link you make, the reason being that is the limiting factor for someone such as yourself who has been unable to pass a 1st Semester Physics course. Without WikiPedia you are as lost as your Tired zany family icon ever was.

Nov 20, 2016
@lunaticle
incapable of an independent thought, goes to Wikipedia for his facts. What a surprise.

Your comment directly implies that by independent thought you actually mean fact free thought. This confirms what I have always thought, that you are an advocate of the fact free thought and politics. I despise such people.

LOL.
What a surprise, being the very definition of retard, you are incapable discerning fact from fiction and realizing you despise yourself.

Nov 21, 2016
I enjoyed this article. Thank-you.
As for the sad witless horde jumping to all unwarranted conclusions and sliding into abuse in the comments section, however ...

Nov 21, 2016
This comment has been removed by a moderator.

Nov 21, 2016
This comment has been removed by a moderator.

Nov 21, 2016
By the way a retard is someone with a permanent delay in mental development, often due to lack of oxygen at birth. Thanks for respecting people who had this bad luck. You are so kind. :)

So, your parents tried to suffocate you, or did they drop you repeatedly?

Nov 22, 2016
Out of curiosity, what is your drive to spout your lies here?
And why did the owners of this site not flush you a long time ago?
Are you are professional agitator ?

No curiosity here as to what is your drive to spout your retardness.
Why did your parents not flush you a long time ago?
You are a professional retard, hence the Retard of the Decade award, and still not satisfied you are going for Retard of the Century.

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