Differences in thermal sensitivities and thermal performance curves of the three calcifying macroalgae <i>Padina boryana, Halimeda opuntia</i>, and <i>Halimeda macroloba</i>

Abstract Calcifying macroalgae play a critical role in coastal ecosystems, but rising sea temperatures pose a significant threat to their survival. This study aims to investigate the thermal sensitivity of the three marine macroalgal species Padina boryana, Halimeda opuntia , and H. macroloba . Photosynthetic performance, metabolism, pigment content, and oxidative stress–related parameters were measured at temperatures of 28°C, 32°C, 36°C, and 40°C and the thermal performance curves (TPCs) were determined for F v /F m , F v /F 0 , ϕPSII, and oxygen production to assess maximum rate (R max ), optimum temperature (T opt ), critical thermal maximum (CT max ), and thermal safety margin (TSM) of these three macroalgal species. The results showed that 40°C had the most negative effect on all three species with P. boryana demonstrating better performance compared to both Halimeda species. TPCs from photosynthetic performance revealed thermal sensitivity variations by species and P. boryana exhibited a broader thermal tolerance range compared to Halimeda . On the other hand, TPCs of oxygen production provided similar CT max values. Based on TPC projections, all three species might survive future ocean warming and marine heatwaves, though these conditions will have significant effects, with P. boryana showing greater tolerance than both Halimeda species. This study highlights the differential thermal responses and sensitivities of these macroalgae, contributing to understanding their potential resiliencies under future climate change scenarios.