A recent study reveals that rising ocean temperatures may pose a significant threat to the microbe Prochlorococcus, known for contributing nearly a third of Earth’s oxygen. This tiny marine organism plays a crucial role in photosynthesis, supporting marine food webs across the globe. Researchers from the University of Washington, led by oceanographer François Ribalet, conducted extensive field research to assess how increased temperatures could impact the health and productivity of these vital microbes.
Prochlorococcus thrives in more than 75 percent of sunlit surface waters, particularly in tropical regions where conditions are typically warm and nutrient-poor. Ribalet notes that the vibrant blue waters in these areas are largely due to the dominance of Prochlorococcus. While some anticipated that these organisms would adapt well to warmer waters due to their heat affinity, the latest findings suggest otherwise.
According to the study, Prochlorococcus has an optimal temperature range of 19 to 28 degrees Celsius (66 to 82 degrees Fahrenheit). However, many tropical and subtropical waters are projected to exceed this upper limit within just 75 years. Ribalet emphasizes the implications of this temperature increase: “For a long time, scientists thought Prochlorococcus was going to do great in the future, but in the warmest regions, they aren’t doing that well, which means that there is going to be less carbon – less food – for the rest of the marine food web.”
To gather data, Ribalet and his colleagues analyzed around 800 billion Prochlorococcus-sized cells during 90 research voyages over a span of 13 years. Utilizing a specialized flow cytometer, co-developed by Ribalet, the team measured microbial growth rates with minimal disturbance. They found that while these microbes thrive in their optimal temperature range, their growth rates significantly decline beyond 30 degrees Celsius. In fact, cell division rates dropped to one-third of the rates observed at the low end of their tolerance range.
The study highlights the challenges faced by Prochlorococcus in nutrient-poor tropical seas. Their small size and simplified genome have allowed them to adapt, but this may have come at a cost, limiting their ability to cope with rising temperatures. If Prochlorococcus declines further, it may open the door for Synechococcus, another group of cyanobacteria that can tolerate higher temperatures but requires more nutrients. However, it remains uncertain how this shift would affect existing marine food webs.
The implications of this research are substantial. Under a moderate warming scenario, Prochlorococcus productivity could decrease by 17 percent in tropical regions and by 51 percent with more severe temperature increases. Globally, productivity could drop 10 percent under moderate warming and 37 percent under extreme scenarios. Ribalet warns, “Their geographic range is going to expand toward the poles, to the north and south. They are not going to disappear, but their habitat will shift.”
While the study provides critical insights, the authors acknowledge its limitations. The methodology may overlook rare heat-resistant strains, and important tropical regions remain underrepresented in the data. Ribalet concludes, “This is the simplest explanation for the data that we have now. If new evidence of heat-tolerant strains emerges, we’d welcome that discovery. It would offer hope for these critical organisms.” The findings were published in Nature Microbiology, shedding light on the urgent need to understand how climate change impacts ocean ecosystems.
