New research from the Massachusetts Institute of Technology reveals that marine cyanobacteria, whose body mass forms the base of the ocean food chain, also feed marine organisms in another way - they deliver "food parcels" packed with carbon and other nutrients.
The bacteria release lipid vesicles - spherical sacs containing proteins and genetic material in the form of DNA - and RNA, which the researchers suggest provide a means of gene transfer in bacterial communities and could also act as decoys for viruses.
Marine cyanobacteria are like tiny ocean plants in that they make their own carbon-rich food from photosynthesis, which uses sunlight and CO2. Thus, through their own body mass, they provide the ocean's food chain with organic compounds and also with oxygen as a byproduct of photosynthesis.
In a recent issue of Science, Sallie W. Chisholm, a professor in the Massachusetts Institute of Technology's (MIT's) department of civil and environmental engineering, and colleagues report how they discovered large numbers of extracellular vesicles - each measuring about 100 nanometers across - linked to the two most abundant types of cyanobacteria, Prochlorococcus and Synechoccocus.
First time extracellular vesicles linked to ocean bacteria
The knowledge that bacteria release extracellular vesicles has been around since the 1960s, but this is the first time it has been observed in ocean bacteria.
The team found the vesicles in cultures of cyanobacteria and also in samples taken from the nutrient-rich waters around the coast of New England, as well as the nutrient-sparse waters of the Sargasso Sea, in the middle of the North Atlantic Gyre.
When they tested them in the lab, they found the vesicles to be stable and able to last 2 weeks or more, offering enough carbon to sustain the growth of bacteria that do not use photosynthesis.
Discovery will change the way we think about ocean's food cycle
Finding these vesicles are so abundant in the oceans means we have to change the way we think about them and their role in the ocean's food cycle - a key message from the study.
We know little about how they contribute to the circulation and supply of dissolved organic carbon in marine ecosystems. They could be an important way that organisms in the sea exchange genes and other essential materials, energy and information.
When they analyzed the genetic material in the vesicles taken from the seawater, the team found DNA from a wide range of bacteria, suggesting many of them produce vesicles.
Just Prochlorococcus's global production amounts to some billion billion billion vesicles per day - contributing a significant amount of carbon-rich material to the sparse nutrient pool of the open seas, they note.
What is the evolutionary advantage of giving away food in vesicles?
But why is a bacterial cell prepared to release a packet one-sixth of its own size every day, especially in the nutrient-sparse environment of the open seas? The researchers wondered why they would take such a risk.
Prof. Chisolm says:
"Prochlorococcus is the smallest genome that can make organic carbon from sunlight and carbon dioxide and it's packaging this carbon and releasing it into the seawater around it. There must be an evolutionary advantage to doing this. Our challenge is to figure out what it is."
One explanation might lie in the fact Prochlorococcus relies on non-photosynthetic bacteria to break down chemicals that are toxic to it - it has lost the ability to do it for itself.
So perhaps, by sending tasty little snack parcels to its non-photosynthetic neighbours, Prochlorococcus is keeping the relationship mutually beneficial.
Another idea the researchers suggest is that the vesicles act as a decoy for predators. Under electron microscopes they could see how phages - viruses that attack bacteria - became attached to vesicles.
Once a phage injects its DNA into a vesicle, it is effectively disarmed and rendered ineffective - it cannot then reproduce itself in a living cell. It is as though the bacteria release the vesicles in a similar way to fighter jets that release chaff to divert missile attacks.
The MIT Energy Initiative, along with grants from the Gordon and Betty Moore Foundation and the National Science Foundation's Center for Microbial Oceanography, helped finance the study.
Meanwhile, scientists from the University of Copenhagen have reported a study where they showed how marine bacteria can help fight tough infections, such as those caused by Staphylococci.