<p>Terrestrial aquaculture of the economically important green seaweed <i>Ulva prolifera</i> 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 (0.249 to 121.9 matm) and temperature (10, 20, and 30 °C) on the net photosynthetic rate (NPR) and relative growth rate (RGR) of cultivated <i>U. prolifera</i> 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 0.42 day⁻¹ at 2.39 matm for the 10 °C treatment, 0.69 day⁻¹ at 9.31 matm for the 20 °C treatment, and 0.65 day⁻¹ at 26.7 matm for the 30 °C treatment. In contrast, NPR showed complex responses, generally reaching a plateau below 25 matm 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 indicate that factors beyond immediate carbon fixation (e.g., the importance of time-scales) influence overall biomass accumulation under varying CO₂ (i.e., pH) conditions.</p>