World news – Researchers discover an immense hydrocarbon cycle in the world’s ocean


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February 2, 2021

by Harrison Tasoff, University of California-Santa Barbara

Hydrocarbons and petroleum are almost synonymous in environmental science. After all, oil reserves make up almost all of the hydrocarbons we encounter. The few hydrocarbons that originate from biological sources may play a greater ecological role than originally believed by scientists.

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A team of researchers from UC Santa Barbara and the Woods Hole Oceanographic Institution examined this previously neglected area of ​​oceanography for signs of an overlooked global cycle. They also tested how its existence might affect the ocean’s response to oil spills.

« We have shown that there is a massive and rapid hydrocarbon cycle in the ocean, which is different from the ocean’s ability to approach oil spills respond, « said Professor David Valentine, who holds the Norris presidency in the Department of Earth Science at UCSB. The research, led by PhD students Eleanor Arrington and Connor Love, appears in Nature Microbiology.

In 2015, an international team led by scientists from Cambridge University published a study showing that the hydrocarbon pentadecane is produced by marine cyanobacteria was produced in laboratory cultures. The researchers extrapolated that this connection could be important in the ocean. The molecule appears to relieve stress in curved membranes, so it’s found in things like chloroplasts, where tightly packed membranes require extreme curvature, Valentine explained. Certain cyanobacteria still synthesize the compound while other ocean microbes easily use it up for energy.

Valentine wrote a two-page commentary on the article with Woods Hole’s Chris Reddy and decided to pursue the subject with Arrington and Love. They visited the Gulf of Mexico in 2015 and the Western Atlantic in 2017 to collect samples and conduct experiments.

The team collected seawater from a nutrient-poor region of the Atlantic known as the Sargasso Sea, named after the floating Sargassum kelp, who was drawn in from the Gulf of Mexico. This is beautiful clear blue water with a Bermuda flare in the middle, said Valentine.

Obtaining the samples was apparently quite a difficult endeavor. Since pentadecane is a common hydrocarbon found in diesel fuel, the team had to take additional precautions to avoid contamination from the ship itself. They had the captain turn the ship into the wind so the exhaust would not contaminate the samples, and they analyzed the chemical signature of the diesel to make sure it wasn’t the source of any pentadecane found.

Also, no one on deck could smoke, cook, or paint while the researchers collected seawater. « That was a big deal, » said Valentine, « I don’t know if you’ve ever been on a ship for any length of time, but you paint every day. It’s like the Golden Gate Bridge: you start at one and when you end at Arriving at the other end, it’s time to start over. « 

The precautionary measures worked and the team retrieved pristine seawater samples. « When we stood in front of the gas chromatograph in Woods Hole after the 2017 expedition, it was clear that the samples were clean and without any signs of diesel, » said co-lead author Love. « Pentadecane was unmistakable and showed clear oceanographic patterns in the first samples [we] did. »

Because of their large numbers in the world’s oceans, Love continued, « only two species of marine cyanobacteria add to this Ocean adds up to 500 times more hydrocarbons per year than the sum of all other types of petroleum inputs into the ocean, including natural oil seepage, oil spills, fuel dumping, and land spillage. « These microbes together produce 300 to 600 million tons of pentadecane each Year, an amount that dwarfs the 1.3 million tons of hydrocarbons released from all other sources.

These amounts are impressive, but a bit misleading. The authors point out that the pentadecane cycle spans 40% or more of the earth’s surface and more than a trillion quadrillion pentadecane-laden cyanobacterial cells are suspended in the sunlit region of the world’s ocean. However, the life cycle of these cells is typically less than two days. As a result, the researchers estimate that the ocean contains only about 2 million tons of pentadecane at any one point in time.

It’s a rapidly spinning wheel, Valentine explained, so the actual amount that is present at any one point in time isn’t particularly good is great. « Every two days you produce and consume all of the pentadecane in the ocean, » he said.

In the future, the researchers hope to link the genomics of microbes with their physiology and ecology. The team already has genome sequences for dozens of organisms that have multiplied to use up the pentadecane in their samples. « The amount of information that is out there is incredible, » said Valentine, « and I think it shows how much we don’t know about the ecology of many hydrocarbon-consuming organisms. »

After the team’s existence and extent Having confirmed this bio-hydrocarbon cycle, it tried to answer the question of whether its presence could cause the ocean to break down spilled oil. The key question, Arrington explained, is whether these abundant pentadecane-consuming microorganisms serve as a benefit in oil spill cleanup. To investigate this, they added pentane – a petroleum hydrocarbon similar to pentadecane – to seawater taken at various distances from natural oil seepage in the Gulf of Mexico.

They measured the total respiration in each sample to determine how long it took pentanivorous microbes to multiply. The researchers hypothesized that if the pentadecane cycle really prepares microbes to consume other hydrocarbons as well, then all samples should flower at similar rates.

However, this was not the case. Samples near the oil quickly oozed flowers. « We developed an abundant population within a week of adding pentane, » said Valentine. « And that gets slower and slower the further you are, until you can wait months in the North Atlantic and never see a flower. » In fact, after the expedition, Arrington had to stay at the Woods Hole, Massachusetts facility to continue the experiment with the specimens from the Atlantic because these blooms were taking so long to appear.

Interestingly, the team also found evidence that Microbes belonging to another area of ​​life, archaea, may also play a role in the pentadecane cycle. « We learned that a group of mysterious, globally abundant microbes that have not yet been domesticated in the laboratory can run on pentadecane in the surface ocean, » said co-lead author Arrington.

The results raise questions about why the presence of an enormous pentadecane cycle appeared to have no effect on the degradation of the petrochemical pentane. « Oil is different from pentadecane, » Valentine said, « and you need to understand what the differences are and what compounds actually make up oil to understand how the ocean microbes will react to it. »

Ultimately, those are different Genes that microbes commonly use to consume the pentane, of those used for pentadecane. « A microbe that lives in the clear waters off Bermuda is much less likely to encounter the petrochemical pentane than pentadecane, which is produced by cyanobacteria, and is therefore less likely to carry the genes for pentane consumption, » said Arrington.

Many different Microbial species can consume pentadecane, but that doesn’t mean they can consume other hydrocarbons, Valentine continued, especially given the variety of hydrocarbon structures present in petroleum. There are fewer than a dozen common hydrocarbons that marine organisms produce, including pentadecane and methane. Oil is now made up of tens of thousands of different hydrocarbons. In addition, we are now seeing that organisms that can break down complex petroleum products live in greater abundance near natural oil seepage.

Valentine calls this phenomenon « biogeographical priming » – when the microbial population of the ocean turns to a specific energy source in one conditioned by a certain geographic area. « And what we’re seeing in this work is a distinction between pentadecane and petroleum, » he said, « that’s important in understanding how different marine regions will react to oil spills. »

Low-nutrient gyres like Lake Sargasso make impressive things 40% of the earth’s surface. Ignoring the land leaves 30% of the planet to look for other biocarbon cycles. Valentine believes that in regions with higher productivity, the processes will be more complex and possibly better prepare for oil consumption. He also pointed out that nature’s blueprint for the production of biological hydrocarbons holds great promise for the development of the next generation of green energy.

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