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

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Abstract

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.

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