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Climate warming is among the most important threats to the health of shallow-water marine primary producers. In the present study, heat stress responses were investigated in the seagrass Thalassia hemprichii and the two macroalgae Padina boryana and Ulva intestinalis by comparing dissolved inorganic carbon (DIC) utilization, photosynthesis (as maximum quantum yield [Fv/Fm] and effective quantum yield [φPSII]), and reactive oxygen species (ROS) production after 4 h of exposure to control temperature (30 °C) or warming (40 °C). At 30 °C, DIC uptake rate of U. intestinalis was highest, whereas rates for T. hemprichii and P. boryana were comparable. Warming significantly reduced DIC use in all species. DIC use of U. intestinalis was affected to the greatest extent (from 16.38±0.88 μmol⋅g-1FW⋅h-1 in 30 °C to −1.39±0.36 μmol⋅g-1FW⋅h-1 in 40 °C). Warming significantly reduced the efficiency of photosynthesis in all species. Thalassia hemprichii showed the smallest reduction in Fv/Fm (from 0.80±0.03 in 30 °C to 0.66±0.12 in 40 °C). However, down-regulation of φPSII by warming in all species was comparable (reaching 0.15-0.18). Warming did not increase ROS accumulation in T. hemprichii (45.84±5.32 and 50.30±5.94 fluorescence units at 30 °C and 40 °C, respectively) or P. boryana (48.40±6.27 and 35.60±6.27 fluorescence units at 30 °C and 40 °C, respectively) but decreased accumulation of ROS in U. intestinalis (from 85.31±4.56 fluorescence units at 30 °C to 46.63±12.37 fluorescence units at 40 °C). Leakage of ROS from damaged algal thalli may contribute to the decrease in ROS observed in our study.
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