Ocean Warming Threatens Microbe That Makes Nearly a Third of Earth’s Oxygen

Photosynthesis plays a crucial role in sustaining life on Earth, as it captures solar energy and serves as the primary driver of most food chains. A diverse range of organisms, including plants, algae, and cyanobacteria, are integral to this process. Among them, Prochlorococcus stands out as the most prolific photosynthetic organism on our planet. This diminutive marine microbe, despite its size, has an immense impact, providing nearly a third of Earth’s oxygen and serving as a foundational element in marine ecosystems.

However, a recent study has raised concerns regarding the resilience of Prochlorococcus and its associated ecosystems in the face of rising ocean temperatures, revealing vulnerabilities that were not fully appreciated before.

Related: The Ocean Surface Is Warming Over 400% Faster Than in The 1980s

Typically found in more than 75 percent of the sunlit surface waters, Prochlorococcus thrives predominantly in tropical regions, where it is well-adapted to warm, nutrient-scarce environments. Lead author, François Ribalet, an oceanographer from the University of Washington, notes that the clarity of tropical waters is largely due to the dominance of this microorganism, which thrives in areas nearly devoid of nutrients.

While some experts previously speculated that Prochlorococcus would continue to flourish as ocean temperatures rise due to climate change, the findings of this study introduce a more nuanced perspective. The researchers found that, despite their preference for warmer waters, Prochlorococcus may indeed struggle when water temperatures surpass their optimal range.

The study identified the ideal temperature range for Prochlorococcus growth as between 19 to 28 degrees Celsius (66 to 82 degrees Fahrenheit). Alarmingly, many regions currently inhabited by these microbes are projected to exceed this upper threshold within the next 75 years. Ribalet’s research indicates that the species may face significant challenges at temperatures even slightly above this range.

“For years, we’d thought Prochlorococcus would adapt well to climate change,” Ribalet states. “Yet, new evidence shows that as temperatures rise, their growth slows, which in turn means less food for the talented array of marine life that relies on them.”

This comprehensive study, which analyzed a staggering 800 billion cells of wild Prochlorococcus from a span of 90 research voyages over 13 years, aimed to provide insights into the actual performance of these microbes in their natural environments, as opposed to the previous reliance on laboratory-grown samples. Ribalet’s fundamental inquiry was whether these organisms prospered during warmer conditions or suffered.

Employing advanced technology, including a flow cytometer designed to detect exceptionally small phytoplankton, the researchers meticulously measured the microbe’s growth rates. Surprisingly, they discovered that while Prochlorococcus performs well in slightly warmer waters, any increase beyond approximately 30 degrees Celsius proved detrimental, reducing their cell division rates significantly.

As tropical water temperatures typically remain high and nutrient-poor, the study illustrates that extensive adaptations in Prochlorococcus, such as their minimalistic genome, may come at a cost—potentially compromising their recovery mechanisms in increasingly hostile environments.

This situation may provide opportunities for other cyanobacterial species, specifically Synechococcus, to take their place. Although Synechococcus is more tolerant of warmer temperatures, it requires higher nutrient levels. Future shifts in dominance from Prochlorococcus to Synechococcus could yield unpredicted consequences for marine food webs.

Predictions from the study indicate that by the end of this century, productivity of Prochlorococcus could diminish by 17 percent under moderate warming conditions and by as much as 51 percent in extreme scenarios. Globally, corresponding drops could see declines of 10 percent and 37 percent, respectively, while their habitats may migrate poleward to adapt.

The researchers caution that this study has its limitations, including potential oversight of rare heat-resistant strains. While encompassing various oceanic regions, significant tropical areas remain unexamined.

“This is the most straightforward interpretation of our current data,” Ribalet acknowledges. “While evidence of heat-tolerant strains could offer renewed hope for these critical organisms, we eagerly await further discoveries.”

The full findings of this important research are published in Nature Microbiology.