Light intensity regulates photosynthesis and biochemical profiles of the calcifying macroalga Corallina pilulifera under varying seawater temperatures.

Accelerated global climate warming has aggravated seawater thermal elevation and intensity of marine heatwaves. Meanwhile, light intensity constitutes another pivotal abiotic driver regulating growth of macroalgae. To disentangle the combined interactive effects of light intensity and seawater temperature on growth performance of calcifying macroalga Corallina pilulifera, a 14-day indoor controlled incubation assay was implemented with two distinct light regimes (60 and 120 μmol photons m-2 s-1) and three thermal gradients (17 °C, 21 °C, and 25 °C). Statistical analyses indicated that temperature functioned as the predominant abiotic factor shaping growth and physiological traits of C. pilulifera, with significant interactive effects detected between light and temperature variables. The treatment of optimal temperature coupled with high irradiance (21 °C, 120 μmol photons m-2 s-1) yielded maximal values for relative growth rate, net calcification rate, Fv/Fm, chlorophyll contents, extracellular carbonic anhydrase activity and tissue carbon pools. By contrast, thermal stress at 25 °C substantially retarded growth, calcification, and photosynthetic capacity, and elevated malondialdehyde and free amino acid contents. Such impacts were further potentiated under high light conditions. At the low-temperature regime of 17 °C, light intensity had no significant effect on algal growth, whereas low irradiance partially alleviated heat-driven inhibition of calcification. Light intensity acts as a modulator mediating the physiological acclimation of C. pilulifera to thermal fluctuations. These findings deliver robust theoretical underpinnings for forecasting the biogeographic distribution of Corallina species and developing refined conservation strategies for in situ algal populations.