Study Predicts 50% Decline in Key Phytoplankton Due to Rising Ocean Temperatures

A recent study published in Nature Microbiology on September 8, 2025, reveals that Prochlorococcus, the smallest and most abundant phytoplankton species, is projected to decline by up to 50% in tropical oceans over the next 75 years due to rising sea surface temperatures. This unexpected vulnerability challenges previous assumptions and signals profound implications for global marine ecosystems.

Prochlorococcus, a microscopic cyanobacterium, is pivotal in producing approximately one-fifth of Earth's oxygen and forms the foundation of many oceanic food webs. The study, led by François Ribalet from the University of Washington, indicates that these organisms struggle to survive in waters exceeding 82°F (27.8°C), with projections showing temperatures surpassing 86°F (30°C) in the coming decades. This decline could disrupt marine biodiversity and global fisheries, underscoring the urgency of addressing climate change.

The research involved a decade of measurements using a custom SeaFlow device across over 100 oceanic cruises, analyzing approximately 800 billion individual cells. This extensive data collection allowed researchers to observe the temperature sensitivity of Prochlorococcus in natural settings, challenging previous assumptions based on limited laboratory data.

Prochlorococcus plays a vital role in marine ecosystems by forming the base of many oceanic food webs and contributing about one-fifth of the planet's oxygen through photosynthesis. A significant decline in their populations could disrupt marine biodiversity and global fisheries. While other phytoplankton species might partially compensate for the loss, they are unlikely to fully replace the ecological role of Prochlorococcus.

Paul Berube, a research scientist at the Massachusetts Institute of Technology who studies Prochlorococcus but was not involved in the work, emphasized the importance of these microorganisms:

"They're at the very base of the food web, and they feed everything else—the fish eat the things that eat the phyt ... , and we eat the fish. When changes are being made to the ... ."

Steven Biller, an associate ... highlighted the broader implications:

"Half of all photosynthesis ... , and Prochlorococcus is a ... . The magnitude of the potential impact is ... ."

The findings underscore the urgency of addressing climate change to prevent disruptions in biodiversity and global fisheries. Scientists stress the importance of reducing greenhouse gas emissions to mitigate the projected decline of Prochlorococcus and the cascading effects on marine ecosystems.

This study highlights the interconnectedness of climate change and marine health, emphasizing the need for immediate action to preserve the delicate balance of oceanic ecosystems.