INDO PAC J OCEAN LIFE P-ISSN: 2775-1961 Volume 7, Number 1, June 2023 E-ISSN: 2775-1953 Pages: 79-90 DOI: 10.13057/oceanlife/o070109 Antioxidant and photo-physiological acclimatisation in tropical macroalgae at sites with distinct nutrient levels ARVIND GOPEECHUND 1,2,♥ , RANJEET BHAGOOLI 1,3,4,♥♥ , VIDUSHI SHRADHA NEERGHEEN 2 , THEESHAN BAHORUN 2,5 1 Department of Biosciences and Ocean Studies, Faculty of Science & Pole of Research Excellence in Sustainable Marine Biodiversity,University of Mauritius. Réduit 80837, Republic of Mauritius. Tel.: +230-4541041 email:  a.gopeechund@gmail.com,  r.bhagooli@uom.ac.mu 2 Biopharmaceutical Unit, Centre for Biomedical and Biomaterials Research, University of Mauritius. Réduit 80837, Republic of Mauritius 3 The Biodiversity and Environment Institute. Réduit, Republic of Mauritius 4 The Society of Biology (Mauritius). Réduit, Republic of Mauritius 5 Mauritius Research and Innovation Council. 6 th Floor, Ebene Heights, Ebène, Republic of Mauritius Manuscript received: 24 December 2022. Revision accepted: 21 February 2023. Abstract. Gopeechund A, Bhagooli R, Neergheen VS, Bahorun T. 2023. Antioxidant and photo-physiological acclimatisation in tropical macroalgae at sites with distinct nutrient levels. Indo Pac J Ocean Life 7: 79-90. The antioxidant efficacy, Total Phenolic Contents (TPC) and Total Flavonoid Contents (TFC) were investigated at Gis Gris with Lower Nutrient levels (LN site) and Bain boeuf with Higher Nutrient levels (HN site) around Mauritius Island. A field-based 7-days transplantation manipulation of Turbinaria ornata, Gracilaria salicornia and Sargassum obovatum between HN and LN sites was conducted to test the impact of different nutrient conditions on their physiology. All the species had lower antioxidant efficacies, TPC and TFC at LN compared to HN site. The glutathione peroxidase and catalase were more sensitive at reflecting the lower oxidative stress at LN site and acclimatisation to oxidative stress occurring at the HN site. ETRmax and α were significantly higher in all species at HN site. The quantum yield Fv/Fm increased in S. obovatum only, after 7 days of transplantation at HN site, while, T. ornata and G. salicornia had similar adaptability to photo-physiological stress at both sites. NPQmax decreased in S. obovatum but increased in T. ornata transplanted to HN site, indicating increased photo-protection at HN site. Lagoons with higher nutrient levels may enhance macroalgal capacity to deal with oxidative stress, thus lowering the need for further photo-protection. Keywords: Antioxidant efficacy, enzymatic antioxidant, nutrient-dependent variation, phenolic contents, photo-physiology INTRODUCTION Most shallow ocean areas receive significant amounts of human-induced chemical inputs and natural environmental conditions (Eklund and Kautsky 2003; Parida and Das 2005; Helmuth et al. 2006; Kannan and Krishnamoorthy 2006). Marine organisms' interaction with pollutants impacts their physiology (Brierley and Kingsford 2009; Dailianis 2010; Morais et al. 2012). Macroalgae may respond to altered environmental conditions in marine areas influenced by human activities by changing their levels of phenolic compounds and antioxidant activities (Orbea et al. 2002; Cunha et al. 2005; Nimptsch et al. 2005; Connan et al. 2006; Scania and Chasani 2021). Collen and Davison (1999) highlighted the exposure of Fucus spp. to desiccation or freezing stress increased Reactive Oxygen Species (ROS) production. Increased levels of the enzymatic antioxidant glutathione in the green seaweed Ulva sp. correlated with increased levels of dissolved inorganic nitrogen levels (Pereira et al. 2009). Antioxidants have thus been proposed as biomarkers of contaminant- mediated oxidative stress in various marine organisms (Cossu et al. 1997). (Cossu et al. 1997; Gopeechund et al. 2020). However, studies on macroalgae and seagrass antioxidants (Somanah et al. 2012; Ramah et al. 2014; Narrain et al. 2023) and photophysiology (Bhagooli et al. 2021a; Narrain et al. 2023) are limited in the Mauritian waters. Light plays a key role in triggering different types of stresses in marine organisms. Solar radiation, more specifically short wavelengths (UVB, 280-315 nm) can alter photo-physiological processes in plants, including protective responses (Bischof et al. 2006), DNA damage (Pakker et al. 2000; van de Poll et al. 2001) and growth (Aguilera et al. 2002). Changes in light levels are known to enhance ROS production, for e.g., the production of superoxide increased at photosystem I in the diatom Thalassia weissflogii during photosynthesis correlated with changes in the level of light (Milne et al. 2009). There is evidence of phenolic compounds helping in photo- protection. For instance, coumarins in green macroalgae like Caulerpa sp. and Dasycladus sp. help prevent radiation damage with their high UV absorption properties (Pérez- Rodríguez et al. 2003; Bischof et al. 2006). Intertidal macroalgae are speculated to be physically and physiologically adapted to cope with irradiance fluctuations and maintain optimal conditions for physiological processes such as photosynthesis (Davison and Pearson 1996). Photo-physiological adaptations also occur in response to light induced stress, whereby the photosystem activity is reduced due to strong light (Schagerl and Möstl 2011). Pulse Amplitude Modulation (PAM) fluorometry