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 CO2 enrichment can enhance growth, its interaction with temperature is not fully quantified for the germling cluster cultivation method. The interactive effects of dissolved CO2 concentration (0.07 to 12 µatm) 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) were investigated. 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. A generalized additive model was used in the analysis. RGR peaked under different CO2 concentrations that varied with temperature, reaching a maximum value of 0.40 at 0.40 µatm for the 10 °C treatment, 0.69 at 1.41 µatm for the 20 °C treatment, and 0.67 at 1.77 µatm for the 30 °C treatment. In contrast, NPR showed complex responses, generally reaching a plateau above 5 µatm at 10 °C and 20 °C, but remaining relatively high across all CO2 levels at 30 °C. The discrepancies observed regarding the response of photosynthesis and growth to CO2 concentrations indicates that factors beyond immediate carbon fixation (e.g., the importance of time-scales) influence overall biomass accumulation under varying CO2 (i.e., pH) conditions.