Because of the rising global temperatures, Sargassum fusiforme (Harvey) Setchell, a commercially valuable seaweed, has experienced reduced yield and quality due to high temperature from marine heatwave events. Our research showed that marine heatwave events increased oxidative damage in S. fusiforme and inhibited carbon and nitrogen absorption and assimilation, ultimately leading to negative effects on growth of algae. In the context of global warming and more frequent heatwaves, our study provides valuable insights for breeding heat-resistant strains and managing coastal aquaculture.