From University of California, Santa Barbara
February 2,.2021 -- A team of
researchers at UC Santa Barbara and Woods Hole Oceanographic Institution
investigated this previously neglected area of oceanography for signs of an
overlooked global cycle. They also tested how its existence might impact the
ocean's response to oil spills.
"We demonstrated that there is a
massive and rapid hydrocarbon cycle that occurs in the ocean, and that it is
distinct from the ocean's capacity to respond to petroleum input," said
Professor David Valentine, who holds the Norris Presidential Chair in the
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 at the University of Cambridge published a study demonstrating that
the hydrocarbon pentadecane was produced by marine cyanobacteria in laboratory
cultures. The researchers extrapolated that this compound might be important in
the ocean. The molecule appears to relieve stress in curved membranes, so it's
found in things like chloroplasts, wherein tightly packed membranes require
extreme curvature, Valentine explained. Certain cyanobacteria still synthesize
the compound, while other ocean microbes readily consume it for energy.
Valentine authored a two-page commentary
on the paper, along with Chris Reddy from Woods Hole, and decided to pursue the
topic further with Arrington and Love. They visited the Gulf of Mexico in 2015,
then the west Atlantic in 2017, to collect samples and run experiments.
The team sampled seawater from a
nutrient-poor region of the Atlantic known as the Sargasso Sea, named for the
floating sargassum seaweed swept in from the Gulf of Mexico. This is beautiful,
clear, blue water with Bermuda smack in the middle, Valentine said.
Obtaining the samples was apparently a
rather tricky endeavor. Because pentadecane is a common hydrocarbon in diesel
fuel, the team had to take extra precautions to avoid contamination from the
ship itself. They had the captain turn the ship into the wind so the exhaust
wouldn't taint the samples and they analyzed the chemical signature of the
diesel to ensure it wasn't the source of any pentadecane they found.
What's more, no one could smoke, cook or
paint on deck while the researchers were collecting seawater. "That was a
big deal," Valentine said, "I don't know if you've ever been on a
ship for an extended period of 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
end it's time to start over."
The precautions worked, and the team
recovered pristine seawater samples. "Standing in front of the gas
chromatograph in Woods Hole after the 2017 expedition, it was clear the samples
were clean with no sign of diesel," said co-lead author Love.
"Pentadecane was unmistakable and was already showing clear oceanographic
patterns even in the first couple of samples that [we] ran."
Due to their vast numbers in the world's
ocean, Love continued, "just two types of marine cyanobacteria are adding
up to 500 times more hydrocarbons to the ocean per year than the sum of all
other types of petroleum inputs to the ocean, including natural oil seeps, oil
spills, fuel dumping and run-off from land." These microbes collectively
produce 300-600 million metric tons of pentadecane per year, an amount that
dwarfs the 1.3 million metric tons of hydrocarbons released from all other
sources.
While these quantities are impressive,
they're a bit misleading. The authors point out that the pentadecane cycle
spans 40% or more of the Earth's surface, and more than one trillion
quadrillion pentadecane-laden cyanobacterial cells are suspended in the sunlit
region of the world's ocean. However, the life cycle of those cells is
typically less than two days. As a result, the researchers estimate that the
ocean contains only around 2 million metric tons of pentadecane at any given
time.
It's a fast spinning wheel, Valentine
explained, so the actual amount present at any point in time is not
particularly large. "Every two days you produce and consume all the
pentadecane in the ocean," he said.
In the future, the researchers hope to
link microbes' genomics to their physiology and ecology. The team already has
genome sequences for dozens of organisms that multiplied to consume the
pentadecane in their samples. "The amount of information that's there is
incredible," said Valentine, "and I think reveals just how much we
don't know about the ecology of a lot of hydrocarbon-consuming organisms."
Having confirmed the existence and
magnitude of this biohydrocarbon cycle, the team sought to tackle the question
of whether its presence might prime the ocean to break down spilled petroleum.
The key question, Arrington explained, is whether these abundant
pentadecane-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 seeps in the Gulf of Mexico.
They measured the overall respiration in
each sample to see how long it took pentane-eating microbes to multiply. The
researchers hypothesized that, if the pentadecane cycle truly primed microbes
to consume other hydrocarbons as well, then all the samples should develop
blooms at similar rates.
But this was not the case. Samples from
near the oil seeps quickly developed blooms. "Within about a week of
adding pentane, we saw an abundant population develop," Valentine said.
"And that gets slower and slower the further away you get, 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 on the samples
from the Atlantic because those blooms took so long to appear.
Interestingly, the team also found
evidence that microbes belonging to another domain of life, Archaea, may also
play a role in the pentadecane cycle. "We learned that a group of
mysterious, globally abundant microbes -- which have yet to be domesticated in
the laboratory -- may be fueled by pentadecane in the surface ocean," said
co-lead author Arrington.
The results beg the question why the
presence of an enormous pentadecane cycle appeared to have no effect on the
breakdown 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's microbes are going to respond to it."
Ultimately, the genes commonly used by
microbes to consume the pentane are different than those used for pentadecane.
"A microbe living in the clear waters offshore Bermuda is much less likely
to encounter the petrochemical pentane compared to pentadecane produced by
cyanobacteria, and therefore is less likely to carry the genes for pentane
consumption," said Arrington.
Loads of different microbial species can
consume pentadecane, but this doesn't imply that they can also consume other
hydrocarbons, Valentine continued, especially given the diversity of
hydrocarbon structures that exist in petroleum. There are less than a dozen
common hydrocarbons that marine organisms produce, including pentadecane and methane.
Meanwhile, petroleum comprises tens of thousands of different hydrocarbons.
What's more, we are now seeing that organisms able to break down complex
petroleum products tend to live in greater abundance near natural oil seeps.
Valentine calls this phenomenon
"biogeographic priming" -- when the ocean's microbial population is
conditioned to a particular energy source in a specific geographic area.
"And what we see with this work is a distinction between pentadecane and
petroleum," he said, "that is important for understanding how
different ocean regions will respond to oil spills."
Nutrient-poor gyres like the Sargasso
Sea account for an impressive 40% of the Earth's surface. But, ignoring the
land, that still leaves 30% of the planet to explore for other biohydrocarbon
cycles. Valentine thinks the processes in regions of higher productivity will
be more complex, and perhaps will provide more priming for oil consumption. He
also pointed out that nature's blueprint for biological hydrocarbon production holds
promise for efforts to develop the next generation of green energy.
https://www.sciencedaily.com/releases/2021/02/210202101047.htm
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