Consistency of Heat Stress Tolerance Across Life Stages in Macrocystis pyrifera

Abstract Kelps are vital for marine ecosystems, yet the genetic diversity underlying their capacity to adapt to climate change remains unknown. In this study, we focused on the kelp Macrocystis pyrifera (M. pyrifera), a species critical to coastal habitats. We developed a phenotyping protocol to evaluate heat stress response in 204 M. pyrifera gametophytes, subjected to heat stress treatments ranging from 21°C to 27°C. Here we show that genotypes exhibiting a heat-stress tolerant (HST) phenotype as haploid gametophytes also produced greater biomass as diploid sporophytes in a warm-water ocean farm. This correlation suggests a predictive relationship between early-stage HST and later-stage growth performance, indicating the potential for selecting resilient kelp strains under warmer ocean temperatures. However, HST kelps showed reduced genetic variation, underscoring the importance of integrating heat tolerance genes into a broader genetic pool to maintain the adaptability of kelp populations in the face of climate change.