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¹ The Key Mariculture Laboratory, Ministry of Education, College of Fisheries, Ocean University of China, Qingdao 266003, China DOI : 10.4194/TRJFAS Viewed : 43 - Downloaded : 55 This study investigates the influence of fluctuating emersion and submersion on growth and biochemical composition of Ulva australis to elucidate their mechanisms of environmental stress adaptation. Thalli of U. australis were grown under laboratory conditions with varying emersion durations i.e. 0.5 h, 1.0 h, 2.0 h, and 5.0 h every 12 h. Biochemical composition was analyzed at final stages of thalli growth focusing on total solute carbohydrate and osmolyte content. The results showed that mild emersions (0.5 h, 1.0 h, and 2.0 h every 12 h) have significantly higher growth rate and total solute carbohydrates contents, compared to thalli without emersion, while during 5.0 h emersion, growth rate was significantly lower than those without emersion. Thalli with 5 h emersion had higher osmolyte content compared to the control (P<0.05). Under mild emersion, algae show increased total solute carbohydrate content, indicating enhanced energy storage and maintenance of metabolic processes. While increased osmolyte contents act as osmoprotectant under severe emersion to prevent cellular damage from desiccation. These observed biochemical changes provide insights into the mechanisms of stress adaptation in marine macroalgae, which could have broader implications for understanding and managing coastal ecosystems under changing environmental conditions. Keywords : Macroalga Desiccation Growth Biochemical composition