New research indicates environmental change is putting Prochlorococcus, the ocean’s most abundant photosynthesizing organism, in peril. With rising ocean temperatures, its very existence is now under threat. This small scale microscopic powerhouse of a cyanobacterium, responsible for 5% of all global photosynthesis, flourishes in warm tropical and subtropical waters. Climate models predict that temperatures in these areas could exceed 86 degrees Fahrenheit within 75 years. Such a major jump would blow the already fragile balance of our oceans and marine ecosystems.
Prochlorococcus grows best in warm waters — 66 to 84 degrees Fahrenheit is the sweet spot. When high temperatures exceed 30°C (86°F), division rates fall off an absolute cliff. They plummet to just one-third of the usual bustling scene at lower temps. The survival of this essential microorganism is critical as over 75% of the ocean’s surface waters are home to Prochlorococcus, making it a key player in marine food webs.
The Role of Prochlorococcus in Marine Ecosystems
Prochlorococcus is no ordinary microbe. It is the foundation of the entire marine food web. This microscopic organism has a typical diameter of only 500 nanometers, far thinner than a human hair. That’s because it has mastered the art of thriving even in the worst waters. Over the course of millions of years, it’s shed unnecessary genes, perfecting its efficiency in settings where vital nutrients like iron don’t exist.
“This is the simplest explanation for the data that we have now,” said François Ribalet, a researcher involved in the study. His team employed the cutting edge SeaFlow flow cytometer aboard the UW operated research vessel Thomas G. Thompson. They measured an enormous circumference of 150,000 miles around the globe, analyzing up to about 800 billion Prochlorococcus-sized cells.
The consequences of reduced Prochlorococcus populations go far beyond the microbe in question. As Ribalet pointed out, “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.”
Impacts of Climate Change on Prochlorococcus Populations
Recent research has shown that even under moderate climate scenarios, Prochlorococcus populations may be reduced by 10%. Under warmer predictions, this increase could go as high as 37%. Scenarios with dramatic reductions have the potential to accelerate cascading impacts to marine ecosystems. Prochlorococcus is incredibly important in the global carbon cycle, as well as the source of energy to valuable open ocean marine life.
Ribalet was especially concerned about the potential encroachment of harmful cyanobacteria, such as Synechococcus. He noted that these organisms may not interact with marine biota as profoundly as Prochlorococcus has for hundreds of millions of years. “If Synechococcus takes over, it’s not a given that other organisms will be able to interact with it the same way they have interacted with Prochlorococcus for millions of years,” he stated.
The Prochlorococcus’s geographic range is set to change, too, as ocean waters heat up. “Their geographic range is going to expand toward the poles, to the north and south,” Ribalet explained. “They are not going to disappear, but their habitat will shift.” Gauging how these dynamics change our oceans is critical to imagining the future state of marine biodiversity and worn ecosystems.
Adaptation and Future Prospects
Prochlorococcus has proven exceptional resilience through tens of thousands of years. Scientists are still optimistic about future finds that could provide salvation. “If new evidence of heat-tolerant strains emerges, we’d welcome that discovery. It would offer hope for these critical organisms,” Ribalet noted.
Even this tough little guy has its limits, and the new research shows that Prochlorococcus’s “burnout temperature” is significantly lower than once realized. The organism’s continued survival truly rests on the ability to retain (or recreate) the best possible conditions within its ecosystem. As ocean temperatures continue to rise, the critical question persists: “Are they happy when it’s warm? Or are they not happy when it’s warm?” Ribalet pondered.