Interactive effects of CO2 concentration and temperature on photosynthesis and growth of cultivated Ulva prolifera germling clusters

Terrestrial aquaculture of the economically important green seaweed Ulva prolifera offers potential for stable production, but requires optimization due to high operational costs. While CO ₂ enrichment can enhance growth, its interaction with temperature is not fully quantified for the germling cluster cultivation method. Methods: This study investigated the interactive effects of dissolved CO ₂ concentration (ambient to 150 mg L ^− 1 ) and temperature (10, 20, and 30°C) on the net photosynthetic rate (NPR) and relative growth rate (RGR) of cultivated U. prolifera germling clusters under controlled laboratory conditions (150 to 200 µmol photons m ^− 2 s ^− 1 ). NPR was measured via oxygen evolution in short-term incubations, and RGR was determined from wet weight changes over 6-day batch cultures with daily medium exchange. Responses were analyzed using a hierarchical generalized additive model (GAM). Results: RGR peaked under different CO ₂ concentrations that varied with temperature, reaching maxima at 4.8 mg L ^− 1 CO ₂ (pH 7.5) at 10°C, 14.0 mg L ^− 1 CO ₂ (pH 7.0) at 20°C, and 22.2 mg L ^− 1 CO ₂ (pH 6.7) at 30°C. Optimal CO ₂ enrichment approximately doubled RGR compared to ambient conditions at 20°C and 30°C. In contrast, NPR showed complex responses, generally peaking around pH 7.5 to 7.8 at 10°C and 20°C, coinciding with the highest calculated bicarbonate (HCO ₃ ⁻ ) molar fraction, but remaining relatively high across all CO ₂ levels at 30°C. Conclusion: The optimal CO ₂ concentration for maximizing germling cluster growth is strongly dependent on temperature. These findings provide specific targets (e.g., 14.0 mg L ^− 1 CO ₂ at 20°C, 22.2 mg L ^− 1 CO ₂ at 30°C) for optimizing CO ₂ enrichment strategies in terrestrial aquaculture systems to enhance yield, highlighting the importance of considering environmental interactions. The discrepancy between short-term photosynthesis optima and long-term growth optima suggests that factors beyond immediate carbon fixation influence overall biomass accumulation under varying CO ₂ or pH conditions.