Princeton University and Xiamen University researchers report that in tropical and subtropical oligotrophic waters, ocean acidification reduces primary production, the process of photosynthesis in phytoplankton, where they take in carbon dioxide (CO2), sunlight, and nutrients to produce organic matter (food and energy).
A six-year investigation found that eukaryotic phytoplankton decline under high CO2 conditions, while cyanobacteria remain unaffected. Nutrient availability, particularly nitrogen, influenced this response.
Results indicate that ocean acidification could reduce primary production in oligotrophic tropical and subtropical oceans by approximately 10%, with global implications. When extrapolated to all affected low-chlorophyll ocean regions, this translates to an estimated 5 billion metric tons loss in global oceanic primary production, which is about 10% of the total carbon fixed by the ocean each year.
The research is published in the journal Proceedings of the National Academy of Sciences.
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Results showed a consistent decline in primary production under acidified conditions in the North Pacific Subtropical Gyre and the South China Sea. No significant changes were observed in the North Pacific Transition Zone. Small eukaryotic phytoplankton exhibited a significant reduction in abundance, particularly in the North Pacific Subtropical Gyre during summer (30%) and winter (15%).
Cyanobacteria, including Prochlorococcus and Synechococcus, showed no substantial change, and in some cases, Synechococcus abundance increased. The correlation between declining eukaryotic phytoplankton populations and decreased primary production suggests that these organisms are key drivers of carbon fixation in nutrient-poor regions.
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In two additional nutrient-enrichment experiments, the addition of nitrate alleviated the negative effects of acidification on eukaryotic phytoplankton growth and increased community diversity. This finding suggests that nutrient availability modulates the impact of ocean acidification on primary production.
Extrapolating results to global oligotrophic tropical and subtropical oceans, researchers estimate that acidification could reduce primary production by approximately 10%. This would weaken the ocean’s ability to support life, reducing the amount of carbon that phytoplankton process into food by about 5 billion metric tons per year.
These findings suggest that ongoing ocean acidification may significantly alter carbon cycling in nutrient-depleted marine ecosystems, potentially impacting global fisheries and food webs, with long-term implications for accelerating climate change through reduced ocean sequestration capacity.
Justin Jackson, Phys.org, 12 March 2025. Press release.
