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 percent 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 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.
A sticky surprise
The lab was 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 Prochlorococcusfamily 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 percent of cells stuck to the walls of the tube. But if only the cells that stuck were transferred into fresh media, repeated many times over many generations, eventually roughly 85 percent of the cells in these populations were attached.
Same instructions, different performance
The sticky characteristic passed from parent cells to their offspring, but over time, and with selection, sticky populations could return to a planktonic state. When the team sequenced the genomes of cells in the two different states, they found no mutation responsible for adherence. Genetically identical cells can exist in either state, and populations can flip from mostly attached to mostly free-floating and back again.
"These cells all share the same genome, or set of instructions, but they're changing what parts of the instructions are read out at specific points," says Anjur-Dietrich. "You could think of it like different musicians all working from the same sheet music - the notes are not going to change, but each person decides whether to play loudly or softly, quickly or slowly."
"What's cool about this is that it means, in principle, any cell could be in either state, depending on gene expression, and this could change over the course of a cell's lifetime and that of its descendants," she says. "To go back to the analogy, the same musician could play the same piece very slowly and loudly, and then 10 minutes later go back and decide to play very quickly and softly."
Life in a biofilm changed how the cells behave. Attached cells produced more chlorophyll and grew more slowly than free-floating cells, but they released almost twice as much organic carbon into the surrounding water. Cells also didn't stay stuck forever. Some broke away from their biofilm and went on to start new free-floating populations.
Why would a microbe live both ways?
"In general, diversity in a population probably ensures survival in a variable environment," says Anjur-Dietrich. "Environmental and nutrient conditions in the ocean can be unpredictable, and having multiple lifestyles could mean that some part of the population could always be thriving."
Free-floating cells likely have better access to sunlight and can spread farther, she explains. Attached cells might gain nutrients made by neighboring microbes and protection from predators, but increase their probability of eventually sinking out of the sunlit zone.
A new piece of the carbon puzzle
To find out whether attachment also occurs in nature, the team analyzed DNA from seawater samples filtered by particle size, combining their own data with the global Tara Oceans Project database. They found particle-attached Prochlorococcus in every sample. In sunlit waters at Station ALOHA research site in Hawaii, for example, about 12 percent of the microbial community on particles was made up of Prochlorococcus.
If Prochlorococcus attaches to sinking particles out of the sunlit zone, the researchers estimate it could carry about half a billion metric tons of carbon into deeper ocean waters each year.
"The ocean is a critical part of Earth's climate system," says Anjur-Dietrich. "The vast majority of carbon fixed from atmospheric carbon dioxide (by organisms like Prochlorococcus!) is recycled by being consumed by organisms along the food chain. Eventually, that carbon will return to the atmosphere as a result of processes that break down organic carbon compounds during digestion and respiration."
Some of that carbon, however, sinks to the seafloor, where it can stay locked away for hundreds of years. Estimates of how much carbon is stored this way are based on assumptions about which organisms sink and how nutrients are recycled on the way down.
Anjur-Dietrich stresses that the team's number is a starting point. "Our estimate about Prochlorococcus is a very rough, first-order estimate that depends on how much Prochlorococcus is present at the surface, how much sticks to particles, and how it might be consumed on the way to the seafloor," she says. "But, what's really exciting is that a lot of these factors are dependent on microbial biology, and we've figured out that Prochlorococcus has a role in those processes."
For Chisholm, the senior author, the findings confirm a long-held suspicion. "We have been wondering about this possibility for some time. There were hints in the way our cultures behaved," she says. "It is wonderful to finally begin to understand the phenomenon, and it gives us more license to speculate about the impact 'attachment' has on carbon fluxes in the oceans."
"The fact that Prochlorococcus can attach to surfaces puts it front and center in terms of playing a key role in 'pumping' carbon from the atmosphere to the deep sea," Chisholm adds.
More to learn
Building on their finding that Prochlorococcus is attached to particles in the ocean, the team has already followed up with a study that estimates what share of the total population - meaning both free-floating and attached cells - is particle-associated and how that fraction might change across ocean basins, again leveraging their own data and archived genetic data. "If we knew that, we could make huge improvements in predicting how much carbon from Prochlorococcus is sequestered," says Anjur-Dietrich.
She is also curious about how Prochlorococcus affects its neighbors on a particle. While still in the sunlit ocean, attached cells keep photosynthesizing and releasing carbon. That could give other microbes an easier meal than the particle itself.
As she puts it, "maybe Prochlorococcus is providing the other microbes with cake when otherwise they'd have to eat cardboard."
If so, particles carrying Prochlorococcus might break down more slowly as they sink. She has begun early experiments with chitin particles, Prochlorococcus, and a chitin-eating microbe to test the idea.
The more researchers learn about Prochlorococcus, the more complex its existence appears; the tiny cell can drift alone, join a community, switch between lifestyles, and potentially attach to particles that sink toward the deep ocean.
Chisholm expects the tiny cell still holds many more secrets. "I'd love to be able to check back in 50 years and see what they are," she says.
Additional MIT CEE co-authors are Katelyn G. Jones, James I. Mullet, Nhi N. Vo, Kurt G. Castro, and Sierra M. Parker, and builds on the work of former MIT postdocs Nikolai Radzinski and Gio Capovilla. The work was funded by Simons Foundation grants and the Burroughs Wellcome Fund.
