Marine heatwaves (MHWs) are becoming more intense and frequent due to climate change. However, MHWs’ seasonal effects on economically relevant seaweeds have been little addressed. Therefore, we analyzed the effects of MHWs simulated for the rainy and dry seasons on the red seaweeds Gracilaria caudata and Gracilaria birdiae from Brazil. Seaweeds were cultured for two weeks under the mean sea surface temperature (Δ) of the region: 25°C during the coldest months of the rainy season and 28°C during the dry season. Subsequently, they were subjected to mean (Δ + 2°C), maximum (Δ + 4°C), and extreme (Δ + 6°C) intensity MHWs for two weeks. Finally, the temperature was decreased to Δ and the seaweeds were left to recover for two weeks. Thermally stressed seaweeds showed decreased growth and effective quantum yield, with a more pronounced decrease in seaweeds from the dry season, especially G. birdiae. Both species resisted the MHWs simulated for the rainy season, whereas G. birdiae from the dry season did not tolerate MHWs simulated for this climatic season, and G. caudata from the dry season only failed to resist the extreme MHW. Effective and maximum quantum yield, parameters of photosynthetic performance curves, and complementary energy dissipation pathways measured at the end of the recovery time demonstrated that when seaweeds resist MHWs, they can recover. Our results suggest that seaweeds can be more negatively affected by MHWs during the dry season, and G. caudata is more interesting for aquaculture in the context of climate change, as G. birdiae was less tolerant of the heat stress. |
