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Among the smallest organisms in the ocean exists a category of single-celled microbes known as Prochlorococcus. They are cyanobacteria, commonly referred to as blue-green algae, and they provide essential nutrients for fauna throughout the food web. More than 75% of surface waters are inhabited by Prochlorococcus, yet as oceanic temperatures increase, researchers are concerned that the waters may become excessively warm to sustain their populations.

Prochlorococcus is the predominant photosynthesizing species in the ocean, contributing 5% of global photosynthesis. Since Prochlorococcus thrive in tropical regions, scientists assumed they would adapt well to climate change. However, recent findings indicate that Prochlorococcus favors water temperatures between 66 and 86 degrees, unable to withstand much higher conditions. Climate projections suggest that subtropical and tropical ocean temperatures will surpass this range within the next 75 years.

“For years, researchers believed Prochlorococcus would prosper in the future, but in the warmest areas, they aren’t thriving, which implies less carbon — less nourishment — available for the broader marine food network,” stated François Ribalet, a research associate professor in oceanography at the University of Washington, who spearheaded the study.

Their findings were published in Nature Microbiology on September 8.

Over the past decade, Ribalet and his team have participated in nearly 100 research cruises focusing on Prochlorococcus. Their team has examined roughly 800 billion Prochlorococcus-sized cells over 150,000 miles to understand their condition and adaptability.

“I had very fundamental inquiries,” Ribalet remarked. “Are they content in warmer conditions? Or do they suffer when it’s too warm?” Most of the data has been gathered from cells cultured in a laboratory environment, yet Ribalet aimed to observe them in their natural surroundings. Utilizing a continuous flow cytometer — known as SeaFlow — they directed a laser through the water to analyze cell type and size. Subsequently, they created a statistical model to track cell proliferation in real time without disturbing the microbes.

The findings revealed that the rate of cell division fluctuates with latitude, potentially due to nutrient availability, sunlight, or temperature. The researchers excluded nutrient levels and sunlight before focusing on temperature. Prochlorococcus reproduces most effectively in waters ranging from 66 to 84 degrees, but above 86, the division rates sharply declined to merely a third of the rate noted at 66 degrees. Abundance of cells exhibited the same pattern.

In the marine environment, mixing draws nutrients from the depths to the surface. This process occurs more slowly in warm waters, and surface layers in the hottest regions of the ocean are deficient in nutrients. Cyanobacteria are among the rare microbes that have learned to survive in these conditions.

“Far from the shore in the tropics, the water appears this vibrant blue due to the lack of content, except for Prochlorococcus,” Ribalet explained. The microbes thrive in these areas as they require minimal nourishment, being so diminutive. Their activity sustains most of the marine food web, from tiny herbivorous creatures to whales.

Over millions of years, Prochlorococcus has mastered the art of maximizing efficiency by discarding unnecessary genes and retaining only those crucial for survival in nutrient-limited tropical waters. This approach has proven extraordinarily successful, yet now, with ocean temperatures rising faster than ever, Prochlorococcus faces limitations imposed by its genome. It cannot retrieve stress response genes that were shed long ago.

“Their burnout temperature is much lower than we previously believed,” Ribalet noted. Earlier models presumed that the cells would divide at a rate that they can no longer maintain, as they currently lack the cellular mechanisms to manage heat stress.

Prochlorococcus is one of the two cyanobacteria that dominate tropical and subtropical waters. The other, Synechococcus, is larger and possesses a less efficient genome. The researchers discovered that while Synechococcus can endure warmer waters, it requires higher nutrient levels to thrive. If Prochlorococcus populations decline, Synechococcus might fill the void, though it remains uncertain how this would influence the food web.

“If Synechococcus replaces Prochlorococcus, it’s not guaranteed that other organisms will interact with it as they have with Prochlorococcus over millions of years,” Ribalet cautioned.

Climate forecasts estimate ocean temperatures based on trends in greenhouse gas emissions. In this research, the scientists evaluated how Prochlorococcus might perform in scenarios of moderate and high warming. In tropical regions, slight warming could decrease Prochlorococcus productivity by 17%, while more severe warming could annihilate it by 51%. Globally, the moderate scenario forecasted a 10% reduction, and warmer predictions indicated a 37% decline in Prochlorococcus.

“Their geographical range is projected to shift towards the poles, both north and south,” Ribalet remarked. “They will not vanish, but their habitat will relocate.” This transition, he added, could have significant ramifications for subtropical and tropical ecosystems.

Nonetheless, the researchers recognize the constraints of their study. They were unable to scrutinize every cell or sample every body of water. Their
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Measurements are derived from combined samples, which could obscure the identification of a heat-resistant variant.

“This is the most straightforward interpretation of the data we currently possess,” remarked Ribalet. “Should new findings regarding heat-resistant variants surface, we would embrace that revelation. It would provide optimism for these essential organisms.”

Co-authors comprise E. Virginia Armbrust, a professor of oceanography at UW; Stephanie Dutkiewicz, a lead research scientist at the Center for Sustainability Science and Strategy at MIT; and Erwan Monier, co-director of the Climate Adaptation Research Center, as well as an associate professor in the Department of Land, Air, and Water Resources at UC Davis.

This study received funding from the Simons Foundation along with additional governmental, foundation, and industrial supporters of the MIT Center for Sustainability Science and Strategy.

For further inquiries, reach out to Ribalet at [email protected].

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