After an oil spill, it is usually the smallest organisms that clean up the most. Surprisingly, scientists know very little about the tools available to these little cleaning crews.
But now, thanks to a new study, researchers have discovered a whole new cycle of natural hydrocarbon emissions and recycling made possible by a wide variety of tiny organisms – which could help us better understand how some microbes have the power to make a mess of a clean up oil. let leaves spill into the ocean.
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 entering the ocean, including natural oil seepage, oil spills, fuel dumping and land runoff ., ”Said earth scientist Connor Love of the University of California, Santa Barbara (UCSB).
But unlike the more famous human input of hydrocarbons into our ocean, this is not a one-way dump.
These hydrocarbons, mainly in the form of pentadecane (nC15), are spread over 40 percent of the Earth’s surface and other microbes feast on them. They are constantly cycled in such a way that Love and colleagues estimate that there are only about 2 million tons in the water at a time.
“Every two days you produce and consume all the pentadecan in the ocean,” Love explained.
(Luke Thompson, Chisholm Lab / Nikki Watson, MIT)
Above: a species of the globally distributed marine cyanobacteria, Prochlorococcus.
Today, humanity’s hydrocarbon footprints can be found in most aspects of our environment. We emit these molecules made up only of carbon and hydrogen atoms – most of it through extraction and use of fossil fuels, but also through plastic, cooking, candles, painting, and so on.
So it probably should come as no big surprise that traces of our own emissions drown out our ability to see the immense hydrocarbon cycle that occurs naturally in our oceans.
Love and colleagues took some effort to clearly identify this global cycle for the first time.
Far from most human sources of hydrocarbons, in the nutrient-poor North Atlantic subtropical waters, the team had to position the ship they sampled to defy the wind so that the diesel fuel that also contains pentadecan did not contaminate the seven research sites. No one was allowed to cook, smoke or paint on deck during the collection.
“I don’t know if you’ve ever been on a ship for any length of time, but you paint every day,” explains UCSB earth scientist David Valentine. “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.”
Back on land, the researchers were able to confirm with the help of a gas chromatograph that the pentadecan in their seawater samples was of biological origin.
Analyzing their data, they found that concentrations of pentadecane increased with greater abundance of cyanobacteria cells, and that the geographic and vertical distribution of the hydrocarbon was consistent with the ecology of this microbe.
Cyanobacteria Prochlorococcus and Synechococcus are responsible for about a quarter of the global conversion of sunlight energy into organic matter (primary production) and previous lab cultivation revealed that they produce pentadecane in the process.
Valentine explains that the cyanobacteria likely use pentadecan as a stronger component for highly curved cell membranes, such as those in chloroplasts (the organelle that photosynthesize).
The ocean pentadecan cycle also follows the diel cycle of these cyanobacteria – their vertical migration in the water in response to changes in light intensity over a day.
Together, these findings suggest that the cyanobacteria are indeed the source of the biological pentadecan, which is then consumed by other microorganisms that produce the carbon dioxide that the cyanobacteria then use to continue the cycle.
Earth’s natural hydrocarbon cycle. (David Valentine / UCSB)
Love’s team identified dozens of surface bacteria and archaea that flourished in response to the addition of pentadecan in their samples.
So they tested to see if the hydrocarbon-consuming microbes could also break down petroleum. The researchers added a petroleum hydrocarbon to samples that are moving closer and closer to areas of active oil seepage, in the Gulf of Mexico.
Unfortunately, only the sea samples from areas already exposed to non-biological hydrocarbons contained microbes that thrived in response to consuming these molecules.
DNA tests showed that genes believed to encode proteins capable of breaking down these hydrocarbons differed between the microbes, with a marked contrast between those who ate biological hydrocarbons and those who devoured the petroleum-derived hydrocarbons.
“We have shown that there is a massive and fast hydrocarbon cycle in the ocean, different from the ocean’s ability to respond to petroleum inputs,” Valentine said.
The researchers have begun to sequence the genomes of the microbes in their sample to better understand the ecology and physiology of the creatures involved in Earth’s natural hydrocarbon cycle.
“I think [these findings reveal] how much we don’t know about the ecology of many hydrocarbon-consuming organisms, ”said Love.
This research is published in Nature Microbiology.