Climate change could transform key bacterial interactions in the ocean by 2100

As global warming yields warmer, more acidic ocean waters, scientists worry interactions between common ocean bacteria could be altered, disrupting entire food chains and ecosystems.

During lab tests at Columbia University's Lamont-Doherty Earth Observatory, scientists found rising CO2 levels can breakup the partnership between a pair of common ocean bacteria, Prochlorococcus and Alteromonas.

"This is a breakthrough that will help scientists do a better job of modeling the ocean ecosystem of the future," Gwenn Hennon, postdoctoral researcher at the observatory, said in a news release.

Previous studies have suggested the ocean's biochemistry could be significantly altered by global warming, but the latest -- published this week in the ISME Journal -- is the first to show exactly how interactions between common bacteria are likely to breakdown.

"What's striking about Gwenn's study is that it's the first time we've been able to show mechanistically how elevated carbon dioxide influences the relationship between these microbes," said Sonya Dyhrman, microbial oceanographer at the observatory. "We know Prochlorococcus needs helper bacteria or it doesn't grow well, but now we are able to see how this partnership breaks down in future ocean conditions."

Earth's oceans are filled with Prochlorococcus, the smallest, most plentiful photosynthetic organism on the planet. The microbes serve as a vital food source for slightly larger microorganisms. They also help trap CO2 and carry it to the bottom of the ocean, assisting Earth's carbon cycle.

The abundance of Prochlorococcus, which can occupy vast expanses of nutrient-poor, mostly lifeless open ocean, is made possibly by Alteromonas. As lab tests revealed, the bacterial sidekick performs several tasks that Prochlorococcus cannot, like cleaning up excess hydrogen peroxide.

Prochlorococcus lacks the gene needed to produce the enzyme that can prevent a buildup of free radicals like hydrogen peroxide.

When researchers upped CO2 levels in the lab to the atmospheric amount expected by 2100, they measured an increase in the mortality rate of Prochlorococcus. They also measured an uptick in the number of free radicals invading the ecosystem.

"Under higher levels of carbon dioxide, Alteromonas doesn't provide the same level of ecosystem services," Hennon said. "It begins to have a more antagonistic relationship with Prochlorococcus."

Genetic analysis showed as CO2 levels rise, Alteromonas turns off the genes responsible for producing the enzymes that breakdown free radicals. Alteromonas may also begin eating dying Prochlorococcus cells.

If Alteromonas indeed abandons Prochlorococcus as carbon levels continue to rise, the base of the ocean's food chains could be dramatically altered. Such a drastic change could disrupt entire ecosystems.

"This study is really a wake-up call," Hennon said. "We need to do a better job in including information like this in models to understand how the global carbon cycle, ocean ecosystems, and fisheries might change in the future. If we don't do this work now, we'll be blindsided in the future by these ecological changes."

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