High-CO₂ Levels Rather than Acidification Restrict Emiliania huxleyi Growth and Performance

Area of Science:

• Marine biology
• Oceanography
• Biogeochemistry

Background:

• Ocean acidification (OA) presents complex challenges for marine calcifiers.
• Differentiating the effects of high CO2 and decreased pH on phytoplankton is crucial but controversial.
• Emiliania huxleyi, a key coccolithophore, exhibits varied responses to changing ocean conditions.

Purpose of the Study:

• To investigate whether acclimation to OA in Emiliania huxleyi is primarily mediated by CO2 or H+.
• To assess the distinct impacts of high CO2 and low pH on cell growth, calcification, and physiological stress.
• To differentiate the effects of CO2 enrichment versus acid addition on coccolithophore physiology.

Main Methods:

• Batch culturing of a heavily calcified Emiliania huxleyi type A strain from the Norwegian Sea.
• Experimental manipulation of pCO2 and pH using CO2-enriched aeration and HCl addition.
• Analysis of cell growth rates, viability, reactive oxygen species (ROS) accumulation, carbon uptake affinity, photochemistry, and calcification (POC, PIC).

Main Results:

• High CO2 (1200 µatm) negatively impacted cell viability and increased ROS, indicating metabolic stress, unlike low pH alone (pH 7.60).
• Growth rates were more negatively affected by high pCO2 than by low pH without CO2 enrichment.
• Calcification patterns (POC, PIC, PIC:POC ratio) varied with culture phases and were not directly correlated with cell stress; HCl acidification led to more deformed coccoliths than CO2 enrichment.
• Carbon uptake affinity was reduced by both low pH and high CO2, while photochemistry was only marginally affected.

Conclusions:

• High CO2 concentrations, rather than acidification itself, appear to be the primary driver of metabolic stress in Emiliania huxleyi.
• The physiological stress response to high CO2 suggests a potential metabolic imbalance.
• Coccolithophore calcification is complex and not solely dictated by the level of physiological stress or acidification method.