Effects of marine heatwaves on haploid and diploid isomorphic phases from two distant populations of Gracilaria caudata (Gracilariales, Rhodophyta).

Marine heatwaves (MHWs) are becoming more common and intense, forcing the physiological limits of marine organisms and causing major ecological impacts throughout the world. This work used a well-studied species of Rhodophyta as a model to understand how different thermal ecotypes and isomorphic life history phases respond to MHWs. The thermal ecotypic populations of Gracilaria caudata in Brazil constitute an adaptive strategy for the species to remain in a wide temperature range. In this study, tetrasporophytes and unfertilized female gametophytes of G. caudata from the Tropical Southwestern Atlantic (TSA) and the Warm Temperate Southwestern Atlantic (WTSA) biogeographic provinces were subjected to simulations of marine heatwaves based on the temperatures occurring at their collection sites. The heatwave simulations consisted of exposing G. caudata for seven days to distinct temperatures (Δ+0°C, Δ+3°C, Δ+5°C, and Δ+6°C) followed by a recovery period at control temperatures (Δ = 26°C for WTSA; Δ = 28°C for TSA). The WTSA individuals showed no differences in physiological performance among experimental treatments compared to the control groups, whereas the TSA individuals exhibited reduced growth rates and signs of thermal stress at the Δ+5°C and Δ+6°C treatments. Gametophytes from TSA showed higher growth rates than tetrasporophytes at the Δ+0°C and Δ+3°C treatments, and neither phase exhibited greater thermal resilience. Our work showed that ecotypes responded differently to MHWs, and the TSA population is more vulnerable to future events that may occur in the region. These data reinforce the importance of population analyses to determine a species' resilience to heat waves.