Temperature is a major environmental factor affecting the growth, development, and productivity of Sargassum fusiforme. We aimed to assess the metabolic processes and regulatory mechanisms in S. fusiforme during a 7-day high-temperature (27 °C and 32 °C) experiment. Changes in chlorophyll content and electrolyte leakage after high-temperature treatment were investigated. Metabolic changes in the leaves of S. fusiforme were analysed using gas chromatography–mass spectrometry. High temperatures suppressed chlorophyll content and increased electrolyte leakage. Further, a strong modulation of various metabolisms was observed: organic acids, amino acids, sugars or sugar alcohols, esters, and amines. These metabolisms were significantly enriched in ten pathways under the 27 °C treatment: aminoacyl-tRNA biosynthesis; glycine, serine, and threonine metabolism; alanine, aspartate, and glutamate metabolism; valine, leucine, and isoleucine biosynthesis; cyanoamino acid metabolism; cysteine and methionine metabolism; arginine and proline metabolism; tyrosine metabolism; citrate cycle (TCA cycle); and glucosinolate biosynthesis. The various metabolisms significantly enriched seven pathways under the 32 °C treatment, namely, alanine, aspartate, and glutamate metabolism; aminoacyl-tRNA biosynthesis; phenylalanine metabolism; tyrosine metabolism; arginine and proline metabolism; nitrogen metabolism; and isoquinoline alkaloid biosynthesis. These changes in metabolic pathways may contribute to the tolerance and adaptability of S. fusiforme to high-temperature stress.

Sargassum fusiforme is a commercially important brown seaweed that has experienced significant population reduction both from heavy exploitation and degradation of the environment. Cultivated breed strains are also in a state of population mixing. These population stressors make it necessary to investigate the population genetics to discover best practices to conserve and breed this seaweed. In this study, the genetic diversity and population structure of S. fusiforme were investigated by the genome-wide SNP data acquired from double digest restriction site-associated DNA sequencing (ddRAD-seq). We found a low genetic diversity and a slight population differentiation within and between wild and cultivated populations, and the effective population size of S. fusiforme had experienced a continuous decline. Tajima’s D analysis showed the population contraction in wild populations may be related to copper pollution which showed a consistent trend with the increase of the sea surface temperature. The potential selection signatures may change the timing or level of gene expression, and further experiments are needed to investigate the effect of the mutation on relevant pathways. These results suggest an urgent need to manage and conserve S. fusiforme resources and biodiversity considering the accelerating change of the environment.