One of the smallest and most powerful organisms on the planet is a plant-like bacterium known to marine biologists as Prochlorococcus. The green-tinted microbe measures less than a micron in diameter and its populations spread throughout the upper layers of the ocean, where a single teaspoon of seawater can contain millions of tiny organisms.
Prochlorococcus grows by photosynthesis, using sunlight to convert atmospheric carbon dioxide into organic carbon molecules. The microbe is responsible for 5 percent of the world’s photosynthetic activity, and scientists have assumed that photosynthesis is the microbe’s primary strategy for acquiring the carbon it needs to grow.
But a new MIT study in Nature Microbiology today found that Prochlorococcus relies on another carbon-feeding strategy, more so than previously thought.
Organisms that use a mixture of strategies to provide carbon are known as mixotrophs. Most marine plankton are mixotrophic. And while Prochlorococcus is known to occasionally dabble in mixotrophy, scientists have assumed the microbe lives primarily a phototrophic lifestyle.
The new MIT study shows that Prochlorococcus may in fact be more mixotrophic than it lets on. The microbe can obtain up to a third of its carbon through a second strategy: consuming the dissolved remains of other dead microbes.
The new estimate may have implications for climate models, as the microbe is a major force in capturing and “fixing” carbon in Earth’s atmosphere and ocean.
“If we want to predict what will happen to carbon fixation in a different climate, or predict where Prochlorococcus will or won’t live in the future, we probably won’t get it right if we miss a process that accounts for a… third of the world’s carbon supply population,” says Mick Follows, a professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS) and its Department of Civil and Environmental Engineering.
Co-authors of the study include first author and MIT postdoc Zhen Wu, along with collaborators from the University of Haifa, the Leibniz Institute for Baltic Sea Research, the Leibniz Institute for Water Ecology Sweet and continental fishing and the University of Potsdam.
Persistent plankton
Since Prochlorococcus was first discovered in the Sargasso Sea in 1986 by MIT Institute professor Sallie “Penny” Chisholm and others, the microbe has been observed in the world’s oceans, inhabiting the upper layers of the ·lit by the sun that go from the surface up to about 160 meters. Within this range, light levels vary, and the microbe has evolved several ways to photosynthesize carbon even in low-light regions.
The organism has also developed ways to consume organic compounds such as glucose and certain amino acids, which could help the microbe survive for limited periods of time in the dark regions of the ocean. But surviving on only organic compounds is a bit like eating only junk food, and there is evidence that Prochlorococcus will die after a week in regions where photosynthesis is not an option.
However, researchers such as Daniel Sher of the University of Haifa, who is a co-author of the new study, have observed healthy populations of Prochlorococcus persisting deep in the sunlit zone, where light intensity should be too low. to maintain a population. . This suggests that microbes must switch to a mixotrophic, non-photosynthesizing lifestyle to consume other organic carbon sources.
“It appears that at least some Prochlorococcus are using existing organic carbon in a mixotrophic way,” says Follows. “That spurred the question: How much?”
What light cannot explain
In their new paper, Follows, Wu, Sher and their colleagues sought to quantify how much carbon Prochlorococcus consumes through processes other than photosynthesis.
The team first looked at measurements taken by Sher’s team, who previously took ocean samples at various depths in the Mediterranean Sea and measured the concentration of phytoplankton, including Prochlorococcus, along with the associated light intensity and the concentration of nitrogen, an essential nutrient that is abundantly available in the deepest layers of the ocean and that plankton can assimilate to make protein.
Wu and Follows used these data and similar information from the Pacific Ocean, along with previous work from Chisholm’s lab, which established the rate of photosynthesis that Prochlorococcus could carry out at a given light intensity.
“We converted this light intensity profile into a potential growth rate—how fast the Prochlorococcus population could grow if it acquired all of its carbon through photosynthesis, and light is the limiting factor,” Follows explains.
The team then compared this calculated rate with growth rates previously observed in the Pacific Ocean by several other research teams.
“These data showed that, below a certain depth, there’s a lot of growth that photosynthesis simply can’t account for,” says Follows. “Some other process must be operating to make up for the difference in carbon supply.”
The researchers inferred that in the deepest, darkest regions of the ocean, Prochlorococcus populations can survive and thrive by resorting to mixotrophy, including consumption of organic carbon from detritus. Specifically, the microbe may be performing osmotrophy, a process by which an organism passively absorbs organic carbon molecules through osmosis.
Judging by how quickly the microbe is estimated to grow below the sunlit zone, the team estimates that Prochlorococcus obtains up to a third of its carbon diet through mixotrophic strategies.
“It’s like going from a specialist lifestyle to a generalist,” says Follows. “If I just eat pizza, then if I’m 20 miles from a pizza place, I’m in trouble, whereas if I eat hamburgers too, I might go to the nearest McDonald’s. People had thought of Prochlorococcus as a specialist, where they do this thing (photosynthesis ) very well. But it turns out they may have a more generalist lifestyle than we previously thought.”
Chisholm, who literally and figuratively wrote the book on Prochlorococcus, says the group’s findings “expand the range of conditions under which its populations can not only survive, but thrive. This study changes the way we think about the role of Prochlorococcus in the microbial food web”.
This research was supported in part by the Israel Science Foundation, the US National Science Foundation, and the Simons Foundation.
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