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.

Increasing anthropogenic CO₂ has led the world’s oceans to absorb approximately 3.3 billion metric tons of carbon, about 30% of annual human-caused emissions. Increases in carbon decreases seawater pH through ocean acidification. This acidification process affects marine species and ecosystem functions. Laboratory studies on phytoplankton have shown variable responses, with some species benefiting from CO₂-driven efficiencies while others suffer from disrupted pH homeostasis and altered nutrient bioavailability.

Most field research on ocean acidification has focused on mid- to high-latitude regions where diatoms, coccolithophores, and other large phytoplankton predominate. Limited data exists on ultraoligotrophic tropical and subtropical waters, which contribute approximately 20% of global oceanic primary production and are dominated by small phytoplankton species.

In the study, titled “Eukaryotic phytoplankton drive a decrease in primary production in response to elevated CO₂ in the tropical and subtropical oceans,” researchers conducted a series of microcosm experiments to examine eukaryotic phytoplankton community responses to ocean acidification.

A total of 48 onboard CO₂ enrichment experiments were performed at 45 stations in the North Pacific Subtropical Gyre, the northern South China Sea, and the North Pacific Transition Zone. Each experiment used microcosms ranging from 10 to 20 liters, incubated for approximately three days under controlled CO₂ conditions. Carbon dioxide concentrations were set at 400 µatm to simulate present-day ambient levels and 700 µatm to reflect projected acidification under moderate-emission scenarios.

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.

Nitrogen limitation exacerbated the effects of ocean acidification on eukaryotic phytoplankton. Stations with deeper nitraclines (depth at which nitrate levels in the ocean begin to rise) exhibited stronger primary production declines.

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.

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