This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The oxygen in about every other breath we take comes from microscopic cells—phytoplankton—drifting in the surface of the ocean. Among them is Prochlorococcus, the smallest and most abundant photosynthetic organism in the seas.
It single-handedly accounts for roughly 10% of all the carbon that the land and the oceans pull from the atmosphere each year. Since its discovery decades ago by MIT Institute Professor Sallie (Penny) Chisholm and colleagues, Prochlorococcus has been known as the loner microbe, free-floating along and drifting through sunlit ocean waters. Because it lives and dies quickly near the water's surface, Prochlorococcus was thought to contribute very little to the ocean's biological pump, the natural process of moving carbon into the deep ocean and storing it there.
Now, a team of MIT researchers in the Chisholm lab has found that picture is incomplete. In an open-access study published in Nature Communications, they demonstrate that Prochlorococcus can also form biofilms, sticky communities of cells attached to surfaces. The finding suggests that the tiny organism has another way of life, and one that could play a much larger role in carbon sequestration.
"We can now think of Prochlorococcus as existing in two very different—but connected—worlds," says lead author Maya Anjur-Dietrich, a postdoc in the MIT Department of Civil and Environmental Engineering (CEE). "When these cells are planktonic, they're drifting through nutrient-poor water and could be hundreds of cell lengths from their nearest neighbors. On a particle, they experience a totally different physical and chemical world." The research team also found evidence that attached Prochlorococcus is common in the ocean, from the sunlit surface down to 1,000 meters (3,300 feet).
The lab had been noticing hints of stickiness for years. There would be clumps of cells at the bottom of tubes with old cultures, and an earlier study from the lab showed that some strains attach to chitin particles. When Anjur-Dietrich joined the lab, she decided to study the "attachment phenomenon" in detail in diverse "ecotypes" of Prochlorococcus that had been isolated from different ocean regions.
The researchers tested strains from every major branch of the Prochlorococcus family tree. "We saw the biofilm-formation phenotype across every Prochlorococcus strain we've tested," she says. "It was a huge surprise!" In a typical culture, about 6% of cells stuck to the walls of the tube.
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