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Earth's oxygen balance and much of the ocean food web depend on microscopic photosynthetic organisms. Among them, Prochlorococcus — a tiny marine cyanobacterium — is one of the planet's most abundant photosynthesizers, responsible for an estimated nearly one-third of global marine oxygen production and forming a foundational food source in sunlit surface waters.
A new multi-year ocean survey indicates that rising sea surface temperatures could reduce Prochlorococcus productivity more than earlier laboratory-based studies suggested, with cascading implications for marine ecosystems and global biogeochemical cycles.
Scientific background and field methods
Prochlorococcus inhabits over 75% of the world’s sunlit surface ocean, especially in tropical and subtropical waters adapted to warm, nutrient-poor conditions. Because these seas are warm, they limit vertical nutrient mixing; Prochlorococcus has evolved a streamlined, minimalist genome and very small cell size to thrive under those constraints. However, genome streamlining may have eliminated some ancient stress-response genes, potentially limiting thermal resilience.
To assess wild populations in situ, researchers analyzed roughly 800 billion Prochlorococcus-sized cells collected over 90 research cruises spanning 13 years. The team used a shipboard flow cytometer co-developed for detecting tiny phytoplankton. A laser-based measurement system counted and characterized cells with minimal disturbance; the researchers then applied statistical growth models derived from established methods to estimate cell division rates across latitudes and temperature regimes.
Key findings and temperature sensitivity
Field data show Prochlorococcus performs best between about 19 and 28 °C (66–82 °F). Contrary to expectations that heat-adapted microbes would gain from warming, the study found cell division rates decline sharply above this window. At temperatures above ~30 °C, division slowed to roughly one-third of rates observed near the lower end of their tolerance range. Lead author François Ribalet of the University of Washington summarized this as a lower-than-expected 'burnout temperature' for Prochlorococcus.
Because many tropical and subtropical surface waters are projected to exceed the upper optimum within decades under climate-change scenarios, the authors estimate substantial productivity losses: by century's end, tropical Prochlorococcus productivity could fall ~17% under a moderate warming scenario and ~51% under a high-warming scenario. Globally, declines are projected at roughly 10% (moderate) to 37% (severe).
Ecological implications and competitors
A reduction in Prochlorococcus productivity could mean less primary production and less carbon available to higher trophic levels in tropical oceans. One possible ecological response is niche replacement by Synechococcus, a different cyanobacterial group that tolerates higher temperatures but requires more nutrients. If Synechococcus expands where Prochlorococcus declines, the functional consequences for grazers, nutrient cycling, and food-web structure are uncertain: species interactions shaped over millions of years with Prochlorococcus may not transfer directly to Synechococcus.
The study also projects a poleward shift in Prochlorococcus geographic range rather than total disappearance; as tropical habitats warm, suitable conditions are predicted to move toward higher latitudes.
Limitations and future research
The authors note methodological limits: shipboard sampling and statistical models may miss rare heat-tolerant strains, and some tropical regions were under-sampled. If previously undocumented thermotolerant ecotypes exist, they could moderate projected declines. Continued targeted sampling, genomic surveys, and long-term monitoring will be necessary to refine projections and detect adaptive responses.
Expert Insight
Dr. Maya Ortega, marine microbial ecologist: 'This study underlines the value of measuring microbes in their natural environment. Laboratory strains provide controlled insight, but the ocean mixes physics, chemistry, and biology in ways that reveal vulnerabilities not always visible in culture. If Prochlorococcus declines as projected, the effects will ripple through food webs and influence regional carbon cycling.'
Conclusion
Field observations show Prochlorococcus has a narrower thermal comfort zone than previously assumed, with productivity falling sharply above ~30 °C. Projected ocean warming could reduce tropical and global Prochlorococcus productivity substantially by 2100 under high-emissions scenarios, prompting range shifts and potential ecological reshuffling with other cyanobacteria such as Synechococcus. While the organisms are unlikely to vanish, their changing abundance and distribution present important questions for marine ecosystems, oxygen production, and carbon cycling that merit continued monitoring and research.
Source: sciencealert
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