Here, we investigated the physiological responses of the economically important brown macroalgal species, Hizikia fusiformis, to solar ultra-violet (UV) radiation and increasing CO₂ concentrations. Specifically, we cultured the alga under two different CO₂ concentrations (380μL/L and 800μL/L) and two different solar radiations (PAR: photosynthetically active radiation 400-700 nm; PAB: full spectrum solar radiation, 280-700 nm) outdoors. Two radiation treatments were established in the experiment: (1) thalli that received full spectrum solar radiation (PAB treatment) in quartz tubes covered with Ultraphan 295, and (2) thalli that only received PAR (P treatment) in quartz tubes covered with Ultraphan film 395. CO₂ was supplied to the different treatments by bubbling (300 mL/min) ambient air (390μL/L) or air enriched with CO₂ (800μL/L CO₂ in the air) with a plant CO₂ incubator that automatically controlled CO₂ concentrations, with less than 5% variation. For all treatments, algae were grown at 20℃, which was regulated by a cooling unit. The growth rate, photosynthesis, dark respiration, photosynthetic pigment contents, soluble carbohydrates, proteins, and nitrate reductase activity were measured after three weeks of culture. Our results showed that elevated CO₂ significantly enhanced the relative growth rate of H. fusiformis. UV radiation appeared to have no negative effects on thalli grown under high CO₂ conditions. High CO₂ and UV culture conditions both reduced the net photosynthetic rate of H. fusiformis. However, no significant difference was found in the dark respiration of H. fusiformis among all treatments. Chlorophyll a and carotenoid contents declined when H. fusiformis was cultured under high CO₂ conditions, whereas UV had no significant effect on the content of the two pigments. There was no difference in soluble carbohydrates among all treatments; however, compared to all other treatments, soluble protein content significantly increased in thalli grown at high CO₂ and full spectrum solar radiation. Nitrate reductase activity was enhanced by high CO₂ treatment, but was inhibited in thalli grown under both UV and high CO₂ conditions. Overall, the physiological performance of H. fusiformis was most significantly influenced by CO₂ and UVR, with increasing CO₂ concentrations potentially alleviating the negative effects of UVR on thalli performance. This information is expected to help enhance the production of H. fusiformis grown in sea areas designated for future culturing.