Researchers are discovering a massive hydrocarbon cycle in the world’s oceans

Researchers are discovering a massive hydrocarbon cycle in the world's oceans

Researchers extract water samples from the Sargasso Sea. Credit: David Valentine

Hydrocarbons and oil are nearly synonymous in environmental sciences. After all, oil reserves represent almost all of the hydrocarbons we encounter. But the few hydrocarbons with their origins in biological sources may play a greater environmental role than scientists originally thought.

A team of researchers at the University of California, Santa Barbara and the Woods Hole Oceanographic Institute have investigated this previously neglected area of ​​oceanography for signs of a neglected global cycle. They also tested how its presence could influence the ocean’s response to oil spills.

Professor David Valentine, who holds the Norris Presidential Chair in the Earth Department, said: “We have shown that there is a huge and fast hydrocarbon cycle taking place in the ocean, and that it differs from the ocean’s ability to respond to petroleum inputs.” Science at UCSB. The research, led by graduate students Eleanor Arrington and Connor Love, appears in Nature Microbiology.

In 2015, an international team led by scientists at the University of Cambridge published a study showing that the hydrocarbon pentadecane was produced by marine cyanobacteria in laboratory cultures. The researchers concluded that this compound may be important in the ocean. Valentine explained that the molecule appears to relieve pressure in curved membranes, so it is found in things like chloroplasts, where tightly packed membranes require intense bending. Some cyanobacteria still synthesize the compound, while other ocean microbes readily consume it for energy.

Valentine wrote a two-page commentary on the paper, along with Chris Reddy of Woods Hole, and decided to pursue the topic further with Arrington and Love. They visited the Gulf of Mexico in 2015, and then the Western Atlantic in 2017, to collect samples and conduct experiments.

The team took seawater samples from a nutrient-poor area in the Atlantic Ocean known as the Sargasso Sea, named after the Sargassum floating seaweed that swept across the Gulf of Mexico. Valentine said these are beautiful, clear blue waters with Bermuda in the middle.

Obtaining the samples seemed to have been a rather difficult endeavor. Because pentadecan is a common hydrocarbon in diesel, The team had to take extra precautions to avoid contamination from the ship itself. They had the captain turn the ship to the wind so the exhaust wouldn’t spoil the samples and they analyzed the chemical signature of the diesel to make sure it wasn’t the source of any Pentadecans they had found.

Sudden cycle

Large quantities of pentadecan are produced and consumed in the upper layers of the ocean. Credit: David Valentine

Moreover, no one could smoke, cook or paint on the deck while researchers were collecting seawater. Valentine said, “That was a big deal, I don’t know if you’ve rode a ship for a long period of time, but you paint every day. It’s like the Golden Gate Bridge: you start at one o’clock the end and by the time you get to the other end, it’s time to start.” Anew. “

The precautions worked, and the team recovered samples of pure seawater. “It was standing in front of the gas chromatograph in Woods Hole after the 2017 flight, and it was clear that the samples were clean and there was no indication of the presence of diesel,” said co-author Love. “The Pentadecan was unmistakable and was already showing clear oceanographic patterns even in the first couple of samples [we] he ran.”

Given their enormous numbers in the world’s oceans, Loew continued, “There are only two types of marine cyanobacteria that add up to 500 times more hydrocarbons to the ocean annually than the sum of all other types of petroleum inputs into the ocean, including natural oil. Fuel dumping and land runoff. ” Collectively, these microbes produce 300-600 million metric tons of pentadecane annually, an amount that dwarfs the 1.3 million metric tons of hydrocarbons emitted from all other sources.

While these quantities are impressive, they are a bit misleading. The authors indicate that the pentadecan cycle covers 40% or more of the Earth’s surface, and that more than a trillion quadrillion pentadecane-laden blue bacterial cells are suspended in the sunlit region of the world’s ocean. However, the life cycle of these cells is usually less than two days. As a result, researchers estimate that the ocean only contains about 2 million metric tons of pentadecan at any one time.

Valentine explained that it is a rapidly spinning wheel, so the actual amount present at any given time is not particularly large. “Every two days it produces and consumes all the pentadecans in the ocean,” he said.

In the future, researchers hope to link microbial genomes with their physiology and environment. The team already has genome sequences for dozens of organisms that have duplicated to consume pentadecane in their samples. “The amount of information out there is incredible, and I think it reveals how much we don’t know about the many living things that consume hydrocarbons,” Valentin said.

After ascertaining the existence and scale of this vital hydrocarbon cycle, the team sought to address the question of whether its existence could prepare the ocean for the breakup of the spilled oil. The key question, Arrington explained, is whether these pentadecan-consuming microorganisms function as an asset during oil spill clean-up operations. To verify this, they added pentane – a petroleum hydrocarbon similar to pentadecane – to seawater taken from various distances from natural oil spills in the Gulf of Mexico.

Sudden cycle

The amount of pentadecane circulating through the oceans dwarfs the hydrocarbon input from oil. However, the microbes involved in the pentadecan cycle are unlikely to be able to handle the chemical complexity of hydrocarbons from the oil. Credit: David Valentine

They measured the total respiration in each sample to see how long it took the pentane-eating microbes to reproduce. The researchers hypothesized that if the pentadecan cycle was really preparing the microbes to consume other hydrocarbons as well, then all of the samples should evolve at similar rates.

But this was not the case. Soon the samples developed from near the oil spill. “Within about a week of adding the pentane, we saw an abundance of population growth,” Valentine said. “And this gets slower and slower the further you go, even, when you’re in the North Atlantic, you can wait months and never see prosperity.” In fact, Arrington had to stay after the expedition at the facility in Woods Hole, Massachusetts to continue experimenting with samples from the Atlantic because those blooms took so long to appear.

Interestingly, the team also found evidence that microbes belonging to another sphere of life, archaea, may also play a role in the pentadecan cycle. “We learned that a group of mysterious and globally available microbes – which have not yet been domesticated in the laboratory – may be feeding by the pentadecans at the ocean surface,” said co-lead author Arrington.

The results raise the question of why the massive cycle of pentadecane had no effect on the breakdown of petrochemical pentane. “Oil is different from pentadecan,” Valentin said, “and you need to understand the differences, and the compounds that the oil is actually made of, to understand how the ocean’s microbes respond to it.”

In the end, the genes that microbes typically use to consume pentane are different from those used for pentadecans. “The microbe that lives in the clear waters off the shores of Bermuda is much less likely to encounter petrochemical pentane than the pentadecane produced by cyanobacteria, and thus it is less likely to carry the genes for consuming the pentane,” Arrington said.

The loads from different microbial species can consume the pentadecan, but that does not mean that they can also consume other hydrocarbons, Valentine continued, especially given the diversity of hydrocarbon structures found in petroleum. There are fewer than a dozen common hydrocarbons produced by marine organisms, including pentadecans and methane. Meanwhile, petroleum is made up of tens of thousands of different hydrocarbons. Moreover, we are now seeing that organisms capable of breaking down complex petroleum products tend to live more abundantly near natural oil spills.

Valentin calls this phenomenon “biogeographic priming” – when microbial populations in the ocean are adapted to a specific energy source in a particular geographical area. “What we see in this work is the distinction between the Pentadecan and petroleum, and this is important for understanding how different ocean regions respond to oil spills,” he said.

Nutrient-poor eddies such as the Sargasso Sea account for 40% of the Earth’s surface. But, by ignoring Earth, that still leaves 30% of the planet to explore other bio-hydrocarbon cycles. Valentine believes that operations in high-productivity areas will be more complex and possibly provide more preparation for oil consumption. He also noted that the scheme of biological nature hydrocarbon Production bodes well for efforts to develop the next generation of green energy.

A window to another world: Life gushes to the bottom of the sea with petroleum from the depths of the sea

more information:
Connor R. Love et al. Microbial production and consumption of hydrocarbons in the global oceans, Nature Microbiology (2021). DOI: 10.1038 / s41564-020-00859-8

the quote: Researchers Discover a Massive Hydrocarbon Cycle in the World’s Ocean (2021, February 2) Retrieved February 2, 2021 from

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