Ocean’s Most Abundant Microbe Approaches Breaking Point

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As we continue to grapple with the realities of climate change, the intricate web of life beneath the ocean’s surface is drawing increasing attention. Amongst the smallest yet most vital organisms inhabiting our seas are Prochlorococcus, a group of single-celled microbes that play a crucial role in marine ecosystems. These cyanobacteria, often referred to as blue-green algae, are not just tiny specks in the vast ocean; they are instrumental in sustaining the entire marine food chain.

Research has shown that Prochlorococcus thrives in over 75% of the ocean’s surface waters, contributing to approximately 5% of the global photosynthesis. This remarkable organism has adapted to flourish in tropical waters, where temperatures range between 19°C to 30°C (66°F to 86°F). However, recent studies have raised alarm bells among scientists, indicating that rising ocean temperatures due to climate change may push these vital microbes beyond their thermal limits, threatening the delicate balance of marine ecosystems.

François Ribalet, a research associate professor of oceanography at the University of Washington and the lead author of a recent study published in Nature Microbiology, expresses concern: “For a long time, scientists thought Prochlorococcus would thrive in a warming world. However, in the warmest regions, they aren’t performing as expected, which means there could be less carbon—and consequently, less food—for the rest of the marine food web.”

Over the past decade, Ribalet and his team have embarked on nearly 100 research cruises, gathering data on approximately 800 billion Prochlorococcus-sized cells across an astonishing 150,000 miles globally. Their mission? To understand how these microbes are coping with changing environmental conditions. Ribalet’s curiosity led him to investigate a fundamental question: “Are they happy when it’s warm? Or are they struggling?”

Utilising advanced technology, including a continuous flow cytometer known as SeaFlow, Ribalet’s team measured cell type and size in situ, allowing them to observe the microbes in their natural habitat without causing disturbance. Their findings revealed that the rate of cell division varies with latitude, which likely corresponds to the availability of nutrients, sunlight, and temperature. Having ruled out nutrient levels and sunlight as factors, they pinpointed temperature as the critical element influencing Prochlorococcus growth.

The study showed that these tiny organisms multiply most efficiently in water temperatures between 19°C and 29°C (66°F to 84°F). However, as temperatures exceed 30°C (86°F), their cell division rates dramatically decline, dropping to a mere third of the rate observed at cooler temperatures. This decline in cell abundance poses a significant threat to marine life, given that Prochlorococcus is a foundational element of the oceanic food web.

Warm waters inhibit the vertical mixing of the ocean, which is essential for transporting nutrients from deep waters to the surface. As a result, the nutrient-poor conditions prevalent in the warmest ocean regions present a formidable challenge for Prochlorococcus. Ribalet remarks, “Offshore in the tropics, the water appears bright blue due to its scarcity of nutrients, aside from Prochlorococcus.” These microbes have evolved to thrive in such nutrient-deficient environments, requiring minimal sustenance while supporting a vast array of marine life, from tiny herbivorous fish to majestic whales.

Throughout millions of years of evolution, Prochlorococcus has honed its ability to survive in challenging conditions by shedding unnecessary genes and retaining only those essential for life in tropical waters. However, with the rapid warming of our oceans, Ribalet warns that Prochlorococcus may be constrained by its own genetic makeup, as it cannot retrieve stress response genes that were discarded long ago. “Their burnout temperature is much lower than we previously thought,” he explains, highlighting the limitations of earlier models that assumed these cells could continue dividing even under increasing heat stress.

In addition to Prochlorococcus, another cyanobacterium named Synechococcus also dominates tropical and subtropical waters. While Synechococcus can tolerate warmer temperatures, it requires a greater quantity of nutrients for survival. Should Prochlorococcus populations decline, Synechococcus could potentially fill the ecological gap; however, the implications of this shift on the marine food web remain uncertain. Ribalet cautions, “If Synechococcus takes over, it’s not guaranteed that other organisms will interact with it in the same way they have with Prochlorococcus for millions of years.”

Climate models estimate future ocean temperatures based on current greenhouse gas emission trends. The research team investigated how Prochlorococcus might respond in both moderate and high warming scenarios. In a modest warming scenario, it is estimated that Prochlorococcus productivity could decrease by 17%. However, under more severe warming conditions, productivity could plummet by as much as 51%. Globally, projections indicate a 10% decline in productivity under moderate warming, escalating to a 37% reduction in more extreme forecasts.

Ribalet notes that while Prochlorococcus may not disappear entirely, their geographic range is likely to shift poleward, both to the north and south. This habitat alteration could have profound consequences for subtropical and tropical ecosystems, as the dynamics of these regions are intrinsically linked to the presence and health of Prochlorococcus populations.

Despite the alarming findings, Ribalet and his colleagues acknowledge the limitations of their research. They were unable to study every individual cell or sample every water body, relying instead on pooled samples that may overlook the potential existence of heat-tolerant strains of Prochlorococcus. “This is the simplest explanation for the data we have now,” Ribalet states. “If we discover evidence of heat-tolerant strains in the future, we would welcome that as it would provide hope for these crucial organisms.”

The study was co-authored by esteemed researchers including E. Virginia Armbrust, a professor of oceanography at the University of Washington; Stephanie Dutkiewicz, a senior research scientist at MIT’s Centre for Sustainability Science and Strategy; and Erwan Monier, co-director of the Climate Adaptation Research Center at UC Davis. The research was supported by the Simons Foundation alongside various governmental and industrial contributors to the MIT Centre for Sustainability Science and Strategy.

As we navigate the complex challenges posed by climate change, understanding the resilience and vulnerabilities of marine organisms like Prochlorococcus is essential. Their fate serves as a barometer for the health of our oceans and, by extension, the planet. Continued research and monitoring are crucial to inform conservation efforts and mitigate the impacts of climate change on these vital ecosystems.

Stay informed and engaged as we continue to explore the intersection of climate science and marine biology, striving towards a sustainable future for our oceans and the world at large.