Member Highlight: Ubiquitous Marine Organism Co-evolved With Other Microbes, Promoting More Complex Ecosystems

2017-04-10T16:24:12+00:00 March 30, 2017|
Vibrio alginolyticus. (Credit: Dr. Gary Gaugler/Visuals Unlimited, Inc.)

(Click to enlarge) A new study shows a tiny bacterium’s metabolic evolution holds clues to the evolution of large, complex ecosystems. Photo: Vibrio alginolyticus. (Credit: Dr. Gary Gaugler/Visuals Unlimited, Inc.)

William Blake may have seen a world in a grain of sand, but for scientists at MIT the smallest of all photosynthetic bacteria holds clues to the evolution of entire ecosystems, and perhaps even the whole biosphere.

(From / by David L. Chandler) — The key is a tiny bacterium called Prochlorococcus, which is the most abundant photosynthetic life form in the oceans. New research shows that this diminutive creature’s metabolism has evolved in a way that may have helped trigger the rise of other organisms, to form a more complex marine ecosystem. Its evolution may even have helped to drive global changes that made possible the development of Earth’s more complex organisms.

The research also suggests that the co-evolution of Prochlorococcus and its interdependent co-organisms can be seen as a microcosm of the metabolic processes that take place inside the cells of much more complex organisms.

The new analysis is published this week in the journal Proceedings of the National Academy of Sciences, in a paper by postdoc Rogier Braakman, Professor Michael Follows, and Institute Professor Sallie (Penny) Chisholm, who was part of the team that discovered this tiny organism and its outsized influence.

“We have all these different strains that have been isolated from all over the world’s oceans, that have different genomes and different genetic capacity, but they’re all one species by traditional measures,” Chisholm explains. “So there’s this extraordinary genetic diversity within this single species that allows it to dominate such vast swaths of the Earth’s oceans.”

Because Prochlorococcus is both so abundant and so well-studied, Braakman says it was an ideal subject for trying to figure out “within all this diversity, how do the metabolic networks change? What drives that, and what are the consequences of that?”

They found a great amount of variation in the bacteria’s “metabolic network,” which refers to the ways that materials and energy pass in and out of the organism, along its phylogeny. The fact that such significant changes have taken place over the course of Prochlorococcus evolution “tells you something quite dramatic,” he says, because these metabolic processes are so fundamental to the organism’s survival that “it’s like the engine of the system. So imagine trying to change the engine of your car while you’re driving. It’s not easily done, so if something is changing, it’s telling you something significant.”

The variations form a kind of layered structure, with more ancestral variants living deeper in the water column and more recent variants living near the surface. The team found that as Prochlorococcus started out living in the top layers of the ocean, where light is abundant but food is relatively scarce, it developed a higher and higher rate of metabolism. It took in more solar energy and used that to power a stronger uptake of scarce nutrients from the water—in effect, creating a more powerful vacuum cleaner but in the process also generating more waste, Braakman says.

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