The effect of temperature on the photosynthetic carbon fixation efficiency of sessile macroalgae in the mussel farming area of Gouqi Island through stable isotope.

This study aimed to investigate the carbon fixation capacity of macroalgae in the mussel farming area of Gouqi Island and to quantify further the carbon fixation effect during the cultivation process of macroalgae. Using high-abundance NaCO as a tracer combined with stable isotope technology, we quantitatively analyzed the photosynthetic carbon fixation rate, biological carbon sequestration amount, and particulate organic carbon generation rate of three macroalgae species-Ulva pertusa, Sargassum horneri, and Grateloupia turuturu-across a range of temperatures (5 °C, 10 °C, 15 °C, 20 °C, 25 °C, 30 °C). The results showed that Ulva pertusa exhibited the highest photosynthetic carbon fixation rate and carbon sequestration amount, peaking at 25 °C and 20 °C, respectively. Specifically, the photosynthetic carbon fixation rate reached 451.2 ± 21.8 μg C/(g FW·h), and the biological carbon sequestration amount was 235.5 ± 4.2 μg C/(g FW·h). Sargassum horneri followed closely, with a photosynthetic carbon fixation rate of 450.3 ± 28.1 μg C/(g FW·h) at 15 °C and a biological carbon sequestration rate of 189.0 ± 1.8 μg C/(g FW·h) at 25 °C. Grateloupia turuturu exhibited a relatively lower photosynthetic carbon fixation ability, achieving a maximum rate of 290.0 ± 20.4 μg C/(g FW·h) at 20 °C. Ulva pertusa demonstrated broad temperature adaptability and strong carbon sequestration capacity, thriving within a suitable growth temperature range of 15-25 °C. Sargassum horneri maintained a rapid growth rate at 5 °C and exhibited high photosynthetic carbon fixation efficiency at 15 °C. However, high temperatures (≥25 °C) significantly inhibited its photosynthetic activity. In contrast, Grateloupia turuturu showed relatively low photosynthetic and carbon fixation efficiency at high temperatures but had enhanced carbon fixation ability at 15 °C and 20 °C. This study provides quantitative data for evaluating the carbon fixation effects of macroalgae and demonstrates the feasibility of using stable isotope technology to rapidly and accurately determine the carbon sequestration capacity of macroalgae. These findings offer a scientific basis for the accounting of biological carbon sequestration and support the application of macroalgae in achieving carbon neutrality.