INDO PAC J OCEAN LIFE P-ISSN: 2775-1961 Volume 7, Number 1, June 2023 E-ISSN: 2775-1953 Pages: 1-15 DOI: 10.13057/oceanlife/o070101 Differential photosynthetic, phytochemical and antioxidative responses of three macroalgae Ulva lactuca, Gracilaria salicornia and Turbinaria ornata exposed to thermal and irradiance conditions DARSHINI NARRAIN 1, 2 , JAYA BAULROOP 2 , RANJEET BHAGOOLI 2,3,4, , THEESAN BAHORUN 1,5,♥♥ 1 ANDI Centre of Excellence for Biomedical and Biomaterials Research, University of Mauritius. Réduit 80837, Republic of Mauritius 2 Department of Biosciences and Ocean Studies & Pole of Research Excellence in Sustainable Marine Biodiversity, Faculty of Science, University of Mauritius. Réduit 80837, Republic of Mauritius 3 The Biodiversity and Environment Institute. Réduit, Republic of Mauritius 4 The Society of Biology. Réduit, Republic of Mauritius 5 Mauritius Research and Innovation Council. Ebène, Republic of Mauritius. Tel.: +230-4651085, Fax.: +230-4651239,  email: r.bhagooli@uom.ac.mu, ♥♥ t.bahorun@mric.mu Manuscript received: 20 June 2022. Revision accepted: 29 September 2022. 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 -2 s -1 and 28°C; Control light and high temperature - CLHT: 1.55±0.63 μmol quanta m -2 s -1 and 32°C; Moderate light and control temperature – MLCT: 100±63.6 μmol quanta m -2 s -1 and 28°C; Moderate light and high temperature – MLHT: 100±63.6 μmol quanta m -2 s -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. Keywords: Antioxidant activity, climate change, marine macroalgae, photophysiology, phytochemistry, pulse-amplitude modulated fluorometry, thermal stress INTRODUCTION Marine macroalgae, commonly known as seaweeds, are macroscopic and multi-cellular autotrophic organisms. These aquatic organisms are ecologically, biologically, and economically important as they provide medicinal compounds (Pribadi and Kanza 2017; Kalasariya et al. 2022; Kumar et al. 2022; Naqvi et al. 2022), serve as habitats for other living organisms (Fulton et al. 2020), serve in agriculture as bio-pesticides (Asimakis et al. 2022) and bio-fertilizers, contribute in bioremediation (Voskoboinikov et al. 2021), as well as play a major role in carbon sequestration (Gao and Beardall 2022), nutrition and habitats for other living organisms. Macroalgae, the primary producers in the marine food web, macroalgae represent key components within coastal ecosystems (Setyorini et al. 2021). However, the rise in global warming- and climate change-driven environmental changes (Harley et al. 2012; Meethoo et al. 2017) are having drastic impacts on macroalgal physiology, growth, reproduction, and survival (Ji and Gao 2021). These marine autotrophs are highly affected by fluctuating environmental conditions derived from climate change and anthropogenically-driven phenomena. Elevated temperatures, intense sunlight, rapid salinity, nutrient changes, desiccation, and numerous forms of pollution are among the major stress inducers (Kakinuma et al. 2001) that make macroalgae vulnerable by influencing their performance