Abstract. Narrain D, Baulroop J, Bhagooli R, Bahorun T. 2023. Differential photosynthetic, phytochemical and antioxidative responses of three macroalgae Ulva lactuca, Gracilaria salicornia and Turbinaria ornata exposed to thermal and irradiance conditions. Indo Pac J Ocean Life 7: 1-15. Worldwide climate change leads to a varied distribution of aquatic organisms due to their differences in susceptibility to environmental conditions. Being at the base of marine food webs, macroalgae are potential candidates to investigate the effects of changing environmental conditions and to study the adaptation mechanisms. This study examined the effects of in vitro thermal and irradiance conditions (Control - CLCT: 1.55±0.63 µmol quanta m-2s-1 and 28°C; Control light and high temperature - CLHT: 1.55±0.63 µmol quanta m-2s-1 and 32°C; Moderate light and control temperature – MLCT: 100±63.6 µmol quanta m-2s-1 and 28°C; Moderate light and high temperature – MLHT: 100±63.6 µmol quanta m-2s-1 and 32°C) for 1 week on the photosynthetic performance, phytochemical contents, and antioxidant potential of three macroalgae Ulva lactuca L., Gracilaria salicornia (C.Agardh) E.Y.Dawson and Turbinaria ornata (Turner) J.Agardh found in the lagoons of Mauritius Island. Our results indicate variable responses of the three test macroalgal species when exposed to combinations of temperature and light conditions. Differential responses were found to be both species- and stress-specific. Chlorophyll fluorescence measurements using a Diving Pulse-Amplitude Modulated (D-PAM) fluorometer indicated a significant increase (p<0.001) in relative maximum electron transport rate (rETRmax) of U. lactuca in all stress treatments implying higher photosynthetic activity compared to control conditions. A significant decrease (p<0.001) in rETRmax of G. salicornia under MLHT and the collapse of photosystem II (PSII) activity (Fv/Fm) in T. ornata, along with both species exhibiting visual pigment degradation, are suggestive of chronic photo-inhibition in these two macroalgal species. Antioxidant activities (FRAP and TEAC assays) correlated stronger to flavonoid contents (FRAP, r=0.909; TEAC, r=0.845) than to phenol contents (FRAP, r=0.688; TEAC, r=0.758). An increase in temperature and irradiance severely damaged the PSII of T. ornata and G. salicornia, while U. lactuca could photo-physiologically adjust to changing environmental conditions, showing its robustness. The elevated temperature significantly affected the photosynthetic performance and antioxidative activities of the tested macroalgal species (p<0.001). These findings are discussed to possible influence on defense mechanisms of these macroalgal species and their aquaculture potential in an era of climate change. Further research using field-based manipulations as well as molecular analysis is warranted to thoroughly understand the potential mechanisms involved in variable responses of these tested macroalgae.