Public Release: 

Tiny bacterium provides window into whole ecosystems


Massachusetts Institute of Technology


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.



"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 carbon compounds that make up Prochlorococcus' waste in turn provided nutrients that drove the evolution of another kind of bacteria, known as SAR11, whose own waste products were useful to Prochlorococcus, thus forming a cooperative system that benefited both organisms. The mutual recycling of waste reinforces the collective maximization of metabolic rate. "It looks like the system is in fact evolving to maximize the total throughput" of energy, not just that of individual organisms, Braakman says.

"As they optimize their ability to acquire nutrients, cells produce more organic carbon and end up promoting greater levels of mutualism," Follows adds.


Other features of the two systems are also very similar, including their photosynthetic pigments and how they deal with the detoxification of hydrogen peroxide. This suggests parallel evolutionary processes produced the same outcome in very different environments. "Plant cells really look like microscopic ocean microbial ecosystems," he explains.

Partly because of those parallels, Braakman says this dynamic could potentially describe the evolution of the biosphere more generally. He suggests that the mathematical descriptions of Prochlorococcus evolution, which he and Follows developed together, emerge from basic principles of kinetics and thermodynamics and so could provide some insights into other systems as well. "It could be a universal kind of dynamic," he says.

"This framework can also help us model the interactions of life, sunlight, and ocean chemistry at the ocean scale," Follows says.


What this analysis suggests, he says, "is what looks like a directional evolutionary process, which is steadily marching toward a direction where it's increasing the energy flux through the system. One of the consequences of that is that then oxygen ended up rising in the atmosphere, and the complexity of the ecosystem increased."

A lot of evolutionary theory emphasizes competition, Braakman says, where "there are limited resources and we're all fighting for them. But what this evolutionary dynamic is saying is that it's a way of increasing the resources for the whole system, so everyone is better off. It increases total system resources."

This work, Chisholm says, demonstrates that "you really have to think about evolution at all these scales, to understand it. It's not just about a bunch of selfish genes jumping around. If you want to understand life in all its dimensions, you have to look at the genes, but also all the way up to the ecosystems. None of it will make sense if you don't look at it at all those scales."




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