A new tier to the ocean ecosystem: tiny packages of DNA

 A 20 liter laboratory culture of the marine cyanobacterium Prochlorococcus.
A 20 liter laboratory culture of the marine cyanobacterium Prochlorococcus.
Image courtesy of Steven Biller

For years, researchers studying phytoplankton abundant in seawater saw what looked like tiny little specks around the cells of interest. The researchers just called them “blebs.”

Sallie Chisholm, a biological oceanographer at the Massachusetts Institute of Technology, would show the images to colleagues and researchers who joined her laboratory to ask if they knew what they were—or were interested in finding out.

A few years ago, a post-doctoral scientist named Steven Biller took up the challenge. Biller had a background in classical microbiology, and he had a hunch: the blebs looked like vesicles, spherical packets that are known to pinch off from bacterial cells.

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Scanning electron micrograph of the marine cyanobacterium Prochlorococcus, with small vesicles visible near the cell surface.
courtesy of Steven Biller

Now, a careful examination has revealed that the blebs are indeed vesicles, containing DNA and proteins. The vesicles shed by Prochlorococcus, the most abundant organisms on Earth that generate energy from the sun, make up a whole new and previously unknown tier within the ocean ecosystem.

The discovery, reported Thursday in the journal Science, is the first known instance of vesicles being made by organisms in the ocean. They are likely to be an important source of the carbon found in seawater, an area of great interest because of efforts to understand how the ocean stores and cycles carbon. The vesicles may also play a wide variety of roles, ranging from a food source for other bacteria to decoys that help protect phytoplankton against pathogens.

It’s an insight into an ecosystem that is crucially important, but poorly understood. Prochlorococcus was only discovered in 1988. The possibility that it sheds vesicles has been startling to researchers, since it is doing so in a resource-constrained environment. That left researchers wrestling with what advantage shedding the vesicles would have for the organism.

“I think it’s just not something that oceanographers have thought about,” Chisholm said. “We didn’t set out to study them. Most of the things with Prochlorococcus we don’t set out to study.”

Collecting ocean water samples to look for vesicles in the Sargasso Sea, December 2012.
courtesy of Steven Biller

In order to confirm that the results they were getting in laboratory studies of the organism were indicative of what happened in the ocean, Biller analyzed seawater from Vineyard Sound off the coast of Massachusetts and from the Sargasso Sea. The vesicles were abundant at similar concentrations as bacteria.

In an accompanying piece in Science, David Scanlan of the University of Warwick, Coventry wrote that what makes the discovery so puzzling is the question of why the organism would shed valuable resources at all.

“Why would an organism like Prochlorococcus, which is clearly well adapted to the nutrient-poor open ocean, shed vesicles containing large amounts of carbon, nitrogen, and phosphorus that have been hard work to win?” Scanlan wrote.

Teasing out the answer to the question is the next big project for the MIT scientists, who have a number of strategies to see if any of their hypotheses will explain why the vesicles are being produced. For example, tagging DNA found in the vesicles could allow them to see whether it is used as a method of transferring genes to other microbes. They can also examine closely whether phytoplankton are protected from attacks by pathogens when there are vesicles present.

“It sort of sets the stage for solving a huge mystery,” Chisholm said. “What is the function of the vesicles in this ecosystem?”