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Hidden diversity floating on top of the sea

Artist’s rendering of the diversity of Prochlorococcus in a drop of seawater.
[Credit: Carly Sanker (MIT)]

Individual cells of the tiny ocean bacteria Prochlorococcus – perhaps the most plentiful photosynthetic creature on Earth – are more diverse from one cell to the next than previously thought, a new study in the 25 April issue of the journal Science reports.

Prochlorococcus are photosynthetic, and so give off oxygen. They are responsible for generating at least 50 percent of oxygen on Earth, and they also play a key role in the global carbon cycle by absorbing a lot of carbon dioxide from the air.

The Prochlorococcus genus contains distinct types of ocean bacteria adapted to different light as it changes with season, or with geography.

Even though little adjustments are constantly happening within floating Prochlorococcus communities, the overall population density of these tiny bacteria stays about the same -- at about 100,000 cells per millimeter in most surface waters.

Now, MIT's Nadav Kashtan and colleagues have shown that, though the overall genetic diversity of a Prochlorococcus group is maintained, the amounts of specific genotypes within that group change.

To make this discovery, the researchers analyzed genome sequences of single cells from Prochlorococcus samples collected at three different times of the year from the Sargasso Sea (a region of water in the North Atlantic Ocean). They uncovered an array of diverse genotypes, leading them to conclude that populations of Prochlorococcus are made of hundreds of little subpopulations with unique 'genomic backbones' that offer flexible gene variations.

These genes are expressed differently between types of bacteria, even those in the same community.

The researchers tried to figure out the forces that maintain the great diversity they found. The diversity appears to be driven by temperature, light, and nutrients, and by biological phenomena, like the bacteria's need to resist certain predators.

These factors appear to select the strongest and best-suited genotypes from the Prochlorococcus community, likely according to how fit they are in changing environmental conditions. That's why some genotypes are more abundant at certain times of year, depending on the environmental conditions at that time.