Heat stress induces organelle alterations in <i>Macrocystis pyrifera</i> gametophytes

Kelp, brown macroalgae in the order Laminariales, provide ecosystem services vital to ocean biodiversity. However, kelp forests worldwide are declining due to abiotic stressors such as ocean warming. In this study, we present results from high-resolution confocal microscopy and in vivo imaging system imaging using protocols developed to visualize kelp gametophyte cells exposed to heat-stress treatments. Imaging revealed chloroplast mislocalization, fragmentation, and subsequent loss of chloroplasts in heat-stressed gametophyte cells. Additionally, nuclei exhibited fragmentation and a progressive loss of fluorescent signal, and the associated microbiome proliferated under various heat-stress treatments. Notably, because brown algae possess a continuous outer membrane that connects the nuclear envelope and the chloroplast envelope, these observations suggest a cellular vulnerability underlying thermal sensitivity in brown macroalgae. Finally, by comparing heat-stress tolerant and heat-stress sensitive genotypes, we found that genotypes with higher heat tolerance exhibited substantially fewer abnormalities compared to sensitive ones.