Ocean acidification changes the interspecific competition of Sargassum fusiforme and Ulva lactuca by regulating carbon and nitrogen metabolism.

Due to different response mechanisms of cultivated Sargassum fusiforme and its main epiphyte Ulva lactuca to ocean acidification caused by increased CO2 concentrations, the effects of U. lactuca on its host may be different at elevated CO2 concentrations. In this study, the effects of U. lactuca on the photosynthetic carbon assimilation process of S. fusiforme were studied under the condition of elevated CO2 concentration. The results showed that co-culture with U. lactuca significantly inhibited the growth, photosynthesis and nitrogen metabolism of S. fusiforme, up-regulated the expression of its carbon assimilation-related genes such as antenna protein, phosphoglycerate kinase (PGK), ribulose-1,5-diphosphate carboxylase/oxygenase (RuBisCo), and down-regulated the expression of genes associated with growth and nitrogen metabolism. Under monoculture conditions, the growth and photosynthesis of S. fusiforme were significantly enhanced by elevated CO2, and the expressions of photosynthetic carbon assimilation and antenna protein-related genes were up-regulated, reflecting the promotion of S. fusiforme photosynthesis. However, the enzyme activity and gene expression of nitrogen metabolism were inhibited. Under co-culture condition, the increase of CO2 concentration also promoted the photosynthesis and growth of S. fusiforme, and up-regulated the expression of genes related to photosystem Ⅱ (PSⅡ) central proteins (including D1 and D2 proteins). The results of this study showed that U. lactuca inhibited the photosynthesis, growth and nitrogen metabolism of S. fusiforme, while ocean acidification promoted the photosynthesis of S. fusiforme by increasing activity of PSII, changing the carbon and nitrogen metabolism strategy, thus greatly alleviating the interspecific stress from its epiphyte U. lactuca. These results had important implications for exploring the ecological significance of seaweed farming and carbon sequestration efficiency in the context of future climate change.