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

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

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

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

In 2015, an international team led by scientists from the University of Cambridge published a study showing that the hydrocarbon pentadecane was produced by marine cyanobacteria in laboratory cultures. The researchers extrapolated that this connection may be important in the ocean. The molecule appears to relieve stress in curved membranes, so it’s found in things like chloroplasts, in which tightly packed membranes require extreme curvature, Valentine explained. Certain 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 discuss 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 sampled seawater from a nutrient-poor region of the Atlantic Ocean known as the Sargasso Sea, named after the floating sargassum seaweed brought in from the Gulf of Mexico. This is beautiful, clear, blue water with a Bermuda in the center, Valentine said.

Obtaining the samples was apparently quite a tricky endeavor. Since pentadecane is a common hydrocarbon in diesel fuel, the team had to take extra precautions to avoid contamination from the ship itself. They made the captain turn the ship in the wind so that 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 pentadecan they found.

In addition, no one on deck was allowed to 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 a long time, but you paint every day. It’s like the Golden Gate Bridge: you start at one end and by the time you get to the other, it’s time to start over. “

Precautions worked, and the team collected pristine seawater samples. “When I stood in front of the gas chromatograph at Woods Hole after the 2017 expedition, it was clear the samples were clean and not a trace of diesel,” said co-lead author Love. Pentadecan was unmistakable and already showed clear oceanographic patterns, even in the first few samples [we] walked. “

Because of their vast numbers in the world’s oceans, Love continued, “only two types of marine cyanobacteria add up to 500 times more hydrocarbons to the ocean per year than the sum of all other types of petroleum that end up in the ocean, including natural oil. seeps, oil spills, fuel dumps and land runoff. ”These microbes collectively produce 300-600 million tons of pentadecane per year, an amount that dwarfs the 1.3 million tons of hydrocarbons released from all other sources.

While these amounts are impressive, they are a bit misleading. The authors point out that the pentadecan cycle occupies 40% or more of the Earth’s surface, and that more than a trillion-trillion pentadecan-laden cyanobacterial cells float in the sunlit region of the world’s ocean. However, the life cycle of those cells is usually less than two days. As a result, the researchers estimate that the ocean contains only about 2 million tons of pentadecan at any one time.

It’s a fast spinning wheel, Valentine explained, so the actual amount present at any point in time isn’t particularly great. “Every two days you produce and consume all of the pentadecan in the ocean,” he said.

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

After confirming the existence and magnitude of this biocarbon cycle, the team set out to address the question of whether its presence could stimulate the ocean to break down petroleum spills. The main question, Arrington explained, is whether these abundant pentadecan-consuming microorganisms serve as an asset during oil spill cleanups. To investigate this, they added pentane – a petroleum hydrocarbon similar to pentadecane – to seawater sampled at various distances from natural oil swallows in the Gulf of Mexico.

They measured the overall respiration in each sample to see how long it took pentan-eating microbes to multiply. The researchers hypothesized that if the pentadecan cycle really primed microbes to consume other hydrocarbons as well, all samples would bloom at similar rates.

But this was not the case. Samples from close to the oil spills quickly developed flowers. “Within about a week of adding pentane, we saw an abundant population emerge,” said Valentine. “And it gets slower and slower as you get further away, until when you’re out in the North Atlantic, you can wait months and never see a bloom.” In fact, Arrington had to stay behind after the expedition at the facility in Woods Hole, Massachusetts to continue the experiment with the samples from the Atlantic because it took so long for those flowers to emerge.

Interestingly, the team also found evidence that microbes belonging to another domain of life, Archaea, may also play a role in the pentadecan cycle. “We have learned that a group of mysterious microbes that are abundant worldwide – not yet to be domesticated in the laboratory – can be fed by pentadecan in the ocean at the surface,” said study co-lead author Arrington.

The results raise the question why the presence of a huge pentadecan cycle did not appear to have an effect on the breakdown of the petrochemical pentane. “Oil is different from pentadecan,” said Valentine, “and you have to understand what the differences are and what compounds oil actually consists of to understand how the microbes of the ocean react to it.”

Ultimately, the genes commonly used by microbes to consume pentane are different from those for pentadecane. “A microbe living in the clear waters off the coast of Bermuda will be much less likely to encounter petrochemical pentane compared to pentadecan produced by cyanobacteria, and therefore less likely to carry the genes for pentane consumption,” said Arrington.

Plenty of different microbial species can consume pentadecane, but this doesn’t mean they can consume other hydrocarbons, Valentine continued, especially given the diversity of hydrocarbon structures found in petroleum. There are less than a dozen common hydrocarbons that marine organisms produce, including pentadecane and methane. Petroleum meanwhile contains tens of thousands of different hydrocarbons. In addition, we now see that organisms that can break down complex petroleum products live in greater quantities near natural oil spills.

Valentine calls this phenomenon ‘biogeographic priming’ – when the microbial population of the ocean is conditioned to a particular energy source in a specific geographic area. “And what we’re seeing with this work is a distinction between pentadecan and petroleum,” he said, “that’s important to understand how different ocean regions will respond to oil spills.”

Low-nutrient gyres such as the Sargasso Sea account for as much as 40% of the Earth’s surface. But if you ignore the land, you still have 30% of the planet to explore other biocarbon cycles. Valentine thinks that processes in regions with higher productivity will be more complex, and may provide better preparation for oil consumption. He also pointed out that nature’s blueprint for biological hydrocarbon production holds great promise for efforts to develop the next generation of green energy.