Ocean bacteria deliver food parcels to marine organisms

Date: Jan-14-2014New 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.

Written by Catharine Paddock PhD

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