Marine microbes, like bacteria and phytoplankton, form the foundation of the ocean’s food web, providing sustenance for a whole host of marine creatures – from tiny zooplankton to ocean giants like blue whales, and even the fisheries that feed billions of people worldwide.

Until now though, ecologists have devoted very little attention to how they behave. 

A new six-year analysis of marine microbes in coastal Californian waters has overturned long-held assumptions about how these organisms interact. 

The study, from researchers at UC San Diego’s Scripps Institution of Oceanography, found that marine microbes have a positive impact on their fellow microbes – for instance where one microbe’s growth promoted another’s. 

In fact, roughly 78% of microbes have a net positive effect on their neighbours. The study didn’t reveal the mechanisms behind these positive interactions, but Ewa Merz, a postdoctoral researcher at Scripps and the study’s lead author, said a potential example could be one organism releasing waste that another species uses as nutrients.

Interestingly, the team also found that levels of positive interactions increased during periods of stress for the microbes, like elevated ocean temperatures.

Researchers analysed seawater samples that had been collected twice weekly from Scripps Pier in San Diego since 2018. Most prior research on this topic took place in the lab, so this dataset gave the scientists a powerful, long-term insight into the understudied behaviour of these microbes. 

Jeff Bowman, a microbiologist at Scripps who started the data collection project said, “Scripps Pier is also unique in that the water we collect there is very similar to the seawater five miles offshore. Being able to get open ocean data without a ship is huge.”

The findings suggest that warming oceans may do more than shift which microbes live where; it could fundamentally alter how marine microbial communities interact and function. 

“Marine ecologists have focused on competitive and predatory interactions while neglecting positive interactions,” said Andrew Barton, a marine ecologist at Scripps and the study’s senior author. 

This means predictions of how ocean ecosystems will respond to warming may be missing critical dynamics. 

Because marine microbes regulate carbon sequestration and support the fisheries that humanity depends on, these unseen shifts could have far-reaching consequences. As ocean temperatures continue to rise, understanding how the microscopic foundation of marine life responds will be essential for anticipating changes to the ecosystems and services that depend on it.

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