Tiny marine organisms that are thought to play a crucial role in the planet's carbon and nutrient cycles are mysteriously shedding massive amounts of bacterial "buds," loaded with proteins and genetic information, into the world's oceans, according to a new study.
These so-called vesicles are spherical pouches containing DNA, carbon and nutrients that are being continually produced and released by Prochlorococcus, the most abundant type of cyanobacteria, which are miniscule photosynthesizing cells in the ocean that convert sunlight and carbon dioxide into oxygen and organic carbon. This puzzling discovery, reported online today (Jan. 9) in the journal Science, could lead to a new understanding of how carbon moves through the oceans, and possibly how genetic information is swapped between marine organisms, the researchers said.
Prochlorococcus is dominant in all of the world's open oceans, except at high latitudes, where the water is very cold, said Steve Biller, a postdoctoral researcher at MIT in Cambridge, Mass., and lead author of the new study. The oxygen exhaled by these photosynthesizing microbes helps nourish other organisms in the marine environment. [Extreme Life on Earth: 8 Bizarre Creatures]
"They're doing roughly 10 percent of all photosynthesis on the planet, so they play an important role at the base of the food web of the world's oceans," Biller told LiveScience.
The marine ecosystem
Biller began studying this type of cyanobacteria at MIT after a previous graduate student in his lab examined Prochlorococcus under a powerful electron microscope and was baffled by the presence of small, pimple-type specks around the cells.
"It was complete serendipity," said study co-author Sallie Chisholm, a professor of biology at MIT. "Anytime anyone new joined the lab, I would say, 'What do you think these are?' When Steve joined, he had classical training in microbiology, and thought they might be vesicles."
Other types of bacteria, such as E. coli, were previously known to produce vesicles, but this is the first time photosynthetic cells in the ocean have been shown to produce such extracellular structures, Chisholm said.
The vesicles were detected in laboratory cultures of cyanobacteria, and in samples of seawater taken from the nutrient-rich waters off the coast of New England and the more nutrient-sparse waters of the Sargasso Sea, a region in the middle of the North Atlantic Ocean.
The vesicles from the seawater were found to contain DNA from different types of bacteria — a discovery that suggests many other ocean microbes also may be capable of producing vesicles, Biller said. Furthermore, the researchers found that vesicles were being produced rapidly.
"We show that two to five vesicles are produced per cell per generation," Chisholm said. "This means that every time the cell divides into two, it produces two to five of these things. If you extrapolate that to global production, based on the growth rates of Prochlorococcus in the wild, it's a huge amount that they're shedding and putting out into the seawater." [50 Amazing Facts About Earth]
Biller estimates Prochlorococcus alone is releasing about a billion-billon-billion (a billion times a billion times a billion) vesicles per day, representing huge pools of carbon in the open oceans. Typically, bacteria grow to a certain size and then reproduce by dividing into two or more parts — a biological process known as fission. Under suitable conditions, bacteria can divide rapidly, with some populations capable of doubling in less than 10 minutes.
"It adds a whole other dimension to parts of the ocean that we need to better understand," Biller said. "For one, figuring out how carbon moves through the ocean has been something of a black box for a number of years. The idea that this could be a new mechanism for how some portion of that carbon moves around is pretty important."
An ocean of mysteries
Yet the discovery raises as many questions as it answers, he added. Most puzzling is why cyanobacteria would produce vesicles in the first place.
"If you have an organism eking out a living in a really dilute environment, where nutrients are extremely low, why would it cast things off into the environment that would limit its own growth?" Chisholm said. "We figure these vesicles have to have some important function."
Research in this area is preliminary, but the scientists have some intriguing hypotheses. For example, since the vesicles contain DNA, they could play a role in transferring genes and developing genetic diversity among populations of cyanobacteria in the oceans.
"They could be moving genetic information between cells in the ocean," Biller said. "We've also talked a little about their potential roles in helping to move nutrients around within the microbial food web. But the magnitude of these benefits to the cell is still beyond our understanding."
Other ideas include the production of vesicles as a defense mechanism against predators. Viruses have been shown to attach themselves to vesicles, injecting their DNA into the spherical structures. This effectively prevents the virus from being able to reproduce in a living cell.
As such, cyanobacteria could be deploying vesicles to use as decoys to deflect attacking viruses, said David Scanlan, a professor of marine microbiology at the University of Warwick in the United Kingdom. Scanlan, who was not involved in the new study, penned an accompanying editorial in the journal Science about the implications of the findings.
"It would be like thinking of these vesicles as anti-aircraft chaffs that planes use as decoys against missiles," Scanlan told LiveScience.
Yet, it is still unclear how these vesicles are produced and, in particular, how they come to contain genetic information, which is found in a cell's nuclei and mitochondria.
"If these vesicles are just budding off the outside of the cell, it's not really clear how DNA gets into them," Scanlan said. "It could be an interesting, and potentially novel, angle on how DNA and RNA can be moved between organisms."
In cells, RNA is a single-stranded molecule involved in the coding, regulation and expression of genes. Among its myriad functions, RNA works as an on-and-off switch for some genes.
Biller and his colleagues plan to investigate some of these ideas, but studying such tiny organisms remains challenging.
"It took about three years to get to this point, and it could take another five years to figure out why Prochlorococcus might be doing this," Chisholm said.