Prochlorococcus, the ocean’s top microbe, can also live stuck to particles
Longtemps décrit comme un solitaire flottant, le microbe photosynthétique le plus abondant des mers peut aussi former des communautés collées à des surfaces.
By
Nathan Lieu
In brief
- Every Prochlorococcus strain tested formed biofilms, so the cell is not strictly a loner.
- Genetically identical cells can be attached or free-floating, with no mutation behind the switch.
- Attached cells were found in every ocean sample analysed, down to 1,000 meters.
- An estimate of about half a billion metric tons of carbon a year is described as very rough.
Textbooks have long suggested that the tiniest and most plentiful photosynthetic microbe in the ocean lives alone, but it is less of a loner than that. In an open-access paper in Nature Communications, researchers in the Chisholm lab at MIT report that Prochlorococcus can also form sticky communities on surfaces. This habit could shape how carbon makes its way to the deep sea, according to MIT News.
A tiny cell with a large climate footprint
Each year, about 10 percent of the carbon that land and oceans draw out of the atmosphere is taken up by Prochlorococcus. Even so, it has been viewed as a free-floating loner that lives and dies quickly close to the surface. By that reasoning, it added very little to the biological pump, the natural process that transports carbon to the deep ocean and holds it there.
For years the Chisholm lab had seen signs that the cells were sticky. Cells clustered at the bottom of tubes containing aged cultures, and previous work from the lab found that certain strains cling to particles of chitin. Senior author Sallie (Penny) Chisholm, an MIT Institute Professor, says: “We have been wondering about this possibility for some time. There were hints in the way our cultures behaved.”
What the culture experiments showed
Maya Anjur-Dietrich, the lead author and a postdoc in the Department of Civil and Environmental Engineering at MIT, examined strains drawn from each major branch of the Prochlorococcus family tree. She says: “We saw the biofilm-formation phenotype across every Prochlorococcus strain we’ve tested.”
| Observation | Result |
|---|---|
| Cells stuck to tube walls in a typical culture | About 6 percent |
| Attached share after repeatedly transferring only stuck cells | Roughly 85 percent |
| Genetic basis for sticking | No responsible mutation found in sequencing |
| Chlorophyll in attached cells | Higher than in free-floating cells |
| Growth of attached cells | Slower than free-floating cells |
| Organic carbon released by attached cells | Almost twice as much as free-floating cells |
Since all the cells share a single genome, a population can shift from largely attached to largely free-floating, then return again. Anjur-Dietrich compares it to musicians working from identical sheet music, each deciding how loudly or quickly to play. Cells do not remain stuck permanently: some detach from a biofilm and found new free-floating populations.
In her interpretation, each lifestyle comes with its own trade-offs. Cells drifting freely probably receive more sunlight and can travel greater distances. Cells that attach might benefit from nutrients produced by their neighbors and from shelter against predators, yet they are more likely to sink below the sunlit zone.
Evidence from the open ocean
To test whether the behavior occurs beyond the lab, the team studied DNA taken from seawater that had been filtered by particle size, merging its own measurements with the worldwide Tara Oceans Project database. Prochlorococcus attached to particles turned up in every sample, from surface waters reached by sunlight to a depth of 1,000 meters. At Station ALOHA in Hawaii, Prochlorococcus made up roughly 12 percent of the microbes living on particles in sunlit waters.
How much carbon could be involved
Should Prochlorococcus attach to particles that sink below the sunlit layer, the researchers calculate that it might transport around half a billion metric tons of carbon to deeper water every year.
Most of the carbon captured by organisms such as Prochlorococcus is recycled while moving through the food chain, and it ultimately goes back to the atmosphere through digestion and respiration. A portion falls to the ocean floor, where it may remain sequestered for hundreds of years. Calculations of that storage are built on assumptions regarding which organisms descend and how nutrients get recycled during the fall.
“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,”
Sallie (Penny) Chisholm, MIT Institute Professor
Chisholm adds that the findings give her more license to speculate about the impact that attachment has on carbon fluxes in the oceans.
Next steps in the lab
- The group has since completed a follow-up study that estimates the share of the entire population, both free-floating and attached, that is linked to particles, and how this share could differ among ocean basins. Anjur-Dietrich says that having this knowledge would let researchers make huge improvements in predicting how much carbon from Prochlorococcus is sequestered.
- She has started early experiments combining chitin particles, Prochlorococcus and a microbe that feeds on chitin. The goal is to test whether particles carrying Prochlorococcus degrade more slowly while sinking, an idea that remains a hypothesis.
Funding came from the Burroughs Wellcome Fund as well as grants from the Simons Foundation. Katelyn G. Jones, James I. Mullet, Nhi N. Vo, Kurt G. Castro and Sierra M. Parker are co-authors, and the study builds on earlier work by former MIT postdocs Nikolai Radzinski and Gio Capovilla.
Photo: ArtHouse Studio / Pexels
