Impact of Carbon and Nitrogen Assimilation in <i>Sargassum fusiforme</i> (Harvey) Setchell due to Marine Heatwave Under Global Warming

Because of the rising global temperatures, Sargassum fusiforme (Harvey) Setchell, a commercially valuable seaweed, has experienced reduced yield and quality due to high temperatures from marine heatwave events. However, the mechanisms underlying the effects of heatwave stress on S. fusiforme remain unclear. In this study, the mechanisms of heatwave stress on the carbon and nitrogen assimilation processes in S. fusiforme were analyzed. These results indicated that heatwave stress, especially at 30°C for 12 days, significantly increased the levels of hydrogen peroxide (83%), malondialdehyde (84.7%), and relative conductivity (16.5%) in algae, which suggested an increase in algal damage. Morphologically, heatwave stress damaged the thylakoid structure and reduced the photosynthetic efficiency of algae and accumulated NADPH, ATP, and α-ketoglutarate significantly, resulting in decreased content of mannitol, the photosynthetic product. Additionally, physiological and transcriptomic results revealed that heatwave stress inhibited the rate of nitrate absorption rate and the activities of the most enzymes associated with nitrogen accumulation, while significantly upregulating glutamate dehydrogenase (GDH), suggesting a crucial role for GDH in S. fusiforme's adaptation to heatwave stress. In terms of amino acid composition, proline and alanine were the most sensitive to heatwave treatment. Moreover, under the natural heatwave environment simulation validation experiment, the algae showed the same physiological performance as under laboratory conditions. The results indicated that marine heatwave events increased oxidative damage in S. fusiforme and inhibited carbon and nitrogen absorption and assimilation, ultimately leading to negative effects on the growth of algae. Thus, in the context of rapid global warming exacerbating marine heatwave events, our study provides valuable insights for high-temperature-resistant breeding and ecological management in coastal aquaculture.