Newswise — Across the ocean, countless plant-like microorganisms form an unseen, drifting woodland, numbering in the billions. Using sunlight as their source of energy, these minuscule creatures extract carbon dioxide from the atmosphere. Together, these photosynthetic plankton, also known as phytoplankton, absorb nearly as much CO2 as the terrestrial forests of the planet. A measurable portion of their carbon-absorbing power originates from Prochlorococcus - a green-tinted, free-floating organism that is currently the most prevalent type of phytoplankton in the world's oceans.
However, Prochlorococcus was not always found in the open ocean. The microbe's predecessors probably remained closer to the coasts, where nutrients were abundant, and organisms survived in communal microbial mats on the seafloor. So, how did the descendants of these coastal inhabitants evolve to become the dominant photosynthetic organisms in the open oceans today?
According to a recent study by MIT scientists, rafting may have been the crucial factor. They suggest that the predecessors of Prochlorococcus developed the ability to attach themselves to chitin - the decomposed remains of ancient exoskeletons. By hitching a ride on floating chitin particles, the microorganisms were able to venture farther out into the ocean. These chitin rafts may have also provided vital nutrients, supporting and sustaining the microbes during their voyage.
With these nutrients, successive generations of microorganisms may have had the chance to develop new adaptations to thrive in the open ocean. Gradually, they evolved to the point where they could detach themselves from the chitin rafts and survive as the independent, free-floating denizens of the open seas that exist today.
Rogier Braakman, a research scientist in MIT's Department of Earth, Atmospheric, and Planetary Sciences (EAPS), remarks that "If Prochlorococcus and other photosynthetic organisms had not colonized the ocean, we would be looking at a very different planet. It was their ability to attach to chitin rafts that allowed them to establish a presence in a completely new and vast region of the planet's biosphere, in a way that transformed the Earth permanently."
The hypothesis of the "chitin raft," along with experiments and genetic analyses that back up the idea, is presented by Braakman and his colleagues in a study published this week in the Proceedings of the National Academy of Sciences (PNAS).
The MIT co-authors of the study include Giovanna Capovilla, Greg Fournier, Julia Schwartzman, Xinda Lu, Alexis Yelton, Elaina Thomas, Jack Payette, Kurt Castro, Otto Cordero, and MIT Institute Professor Sallie (Penny) Chisholm, along with researchers from various institutions, including the Woods Hole Oceanographic Institution.
A strange gene
Picocyanobacteria is a class of the smallest photosynthetic organisms on earth, of which Prochlorococcus is one of the two main groups. The other group is Synechococcus, a closely related microbe that can be found in large numbers in both ocean and freshwater systems. Both microorganisms survive by means of photosynthesis.
Interestingly, some strains of Prochlorococcus have been discovered to have the ability to adapt to alternative lifestyles, especially in regions with low light where photosynthesis becomes challenging to sustain. These microorganisms are called "mixotrophic" as they use a combination of various carbon-capturing strategies to thrive.
The researchers in Chisholm's laboratory stumbled upon a common gene in multiple modern strains of Prochlorococcus while searching for signs of mixotrophy. This gene was responsible for breaking down chitin, a carbon-rich substance derived from the discarded shells of arthropods like crustaceans and insects.
Capovilla found the discovery to be peculiar, and as a postdoctoral researcher who had recently joined the lab, she decided to investigate the finding further.
To investigate the discovery further, Capovilla conducted experiments as part of the new study to determine if Prochlorococcus could effectively break down chitin. Prior work carried out by the lab had shown that the gene responsible for breaking down chitin was present in strains of Prochlorococcus that resided in regions with low light and in Synechococcus, but was not present in Prochlorococcus strains that lived in sunlit regions.
To test the hypothesis further, Capovilla conducted experiments in the lab using samples of low-light and high-light Prochlorococcus strains, introducing chitin particles into them. The results showed that only the microbes that contained the gene were able to break down chitin, and only the low-light-adapted strains of Prochlorococcus appeared to benefit from this breakdown, as they grew faster as a result. The microbes were also observed to be able to attach themselves to chitin flakes, which caught the interest of Braakman due to his focus on the evolution of metabolic processes and their impact on the planet's ecology.
Braakman realized that chitin particles could have served as rafts for Prochlorococcus and other microbes, allowing them to be transported to new locations in the ocean. This could have enabled the microbes to adapt to new environments, and eventually evolve into the free-floating ocean dwellers that exist today.
Braakman raised the question of whether the chitin-degrading gene could have existed in the ancestors of Prochlorococcus, enabling coastal microbes to attach to chitin and utilize it as a food source, allowing them to ride the flakes out to sea.
It’s all in the timing
To validate the "chitin raft" hypothesis, the team enlisted the help of Fournier, an expert in tracing the history of genes across microbial species. In 2019, Fournier's lab created an evolutionary tree of microbes with the chitin-degrading gene and observed a pattern: these microbes began to use chitin only after the abundance of arthropods in a particular ecosystem increased.
In order for the chitin raft theory to be valid, the gene must have been present in Prochlorococcus ancestors shortly after the colonization of marine environments by arthropods.
The researchers turned to the fossil record and discovered that marine arthropods became prevalent during the early Paleozoic era, approximately 500 million years ago. As per Fournier's evolutionary tree, this also corresponds to the period when the chitin-degrading gene emerged in the shared ancestors of Prochlorococcus and Synechococcus.
Fournier remarked that the timing of events is compelling. As he explained, "Marine ecosystems were being inundated with chitin, a new type of organic carbon, just as genes for utilizing this carbon were spreading among different microbe types. The transport of chitin particles created an opportunity for microbes to disperse to the open ocean."
The emergence of chitin may have been particularly advantageous for microbes residing in low-light environments, such as those along the coastal seafloor, where it is believed that early picocyanobacteria thrived. For these microbes, chitin would have served as a valuable energy source, as well as an escape route from their communal, coastal habitat.
According to Braakman, once the chitin-rafting microbes made it out to sea, they could have developed additional adaptations to ocean life. Over millions of years, these organisms eventually evolved into the free-floating Prochlorococcus that exist today.
Braakman believes that this discovery is about the co-evolution of ecosystems. The chitin rafts helped both arthropods and cyanobacteria to expand into the open ocean, resulting in the rise of modern marine ecosystems. He also thinks that this research shows the importance of studying the history of life and the way that metabolic processes have evolved and shaped the planet.
This research received support from the Simons Foundation, the EMBO Long-Term Fellowship, and the Human Frontier Science Program. The paper is also a contribution from the Simons Collaboration on Ocean Processes and Ecology (SCOPE).
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Journal Link: Proceedings of the National Academy of Sciences
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