Ocean warming poses a significant threat to Prochlorococcus, a marine microbe vital for global oxygen production. This tiny organism contributes nearly a third of Earth’s oxygen and plays a crucial role in marine food webs. A recent study reveals that as ocean temperatures rise, Prochlorococcus may be more vulnerable than previously believed, potentially disrupting ecosystems that rely on it.
Prochlorococcus thrives in more than 75 percent of sunlit surface waters, particularly in tropical regions where conditions are warm and nutrient-poor. According to lead author François Ribalet, an oceanographer at the University of Washington, “Offshore in the tropics, the water is this bright, beautiful blue because there’s very little in it, aside from Prochlorococcus.” While some scientists anticipated that these microbes would adapt well to rising temperatures, the study suggests otherwise.
The research indicates that Prochlorococcus prefers temperatures between 19 and 28 degrees Celsius (66 to 82 degrees Fahrenheit). Many tropical and subtropical waters are projected to exceed this upper limit within the next 75 years due to climate change and fossil fuel emissions. Ribalet states, “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 assess the health of wild Prochlorococcus populations, Ribalet and his team analyzed data from 800 billion cells collected during 90 research voyages over a span of 13 years. They used a specialized flow cytometer co-developed by Ribalet, which allows for precise detection of small phytoplankton like Prochlorococcus. The study found that while these microbes perform optimally in warmer waters, their growth rate significantly declines at temperatures above 30 degrees Celsius. In fact, cell division rates dropped to one-third of those in cooler waters.
Tropical seas are inherently nutrient-poor, limiting the upward cycling of nutrients from the depths. Prochlorococcus has adapted to these conditions by evolving a minimal genome, shedding non-essential genes. However, this adaptation might also have compromised its ability to withstand rising temperatures. As Prochlorococcus faces potential decline, the cyanobacteria group Synechococcus could seize the opportunity to dominate. Unlike Prochlorococcus, Synechococcus can tolerate warmer water but requires more nutrients for survival. The implications of this shift for marine ecosystems remain uncertain. Ribalet notes, “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.”
The study predicts that by the end of this century, Prochlorococcus productivity could decrease by 17 percent in the tropics under a moderate warming scenario and by as much as 51 percent under severe warming. Globally, the decline could reach 10 percent and 37 percent in the respective warming scenarios. Ribalet explains, “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.”
Despite its findings, the study acknowledges certain limitations. The methodology may overlook rare heat-resistant strains, and several critical tropical regions were not included in the data collection. Ribalet emphasizes the need for further research: “If new evidence of heat-tolerant strains emerges, we’d welcome that discovery. It would offer hope for these critical organisms.”
This research was published in the journal Nature Microbiology, highlighting the urgent need to understand the impacts of climate change on vital marine life.
