Synthesis and release of fatty acids under the interaction of Ulva pertusa and Heterosigma akashiwo by stable isotope analysis.

Symbiosis of marine algae is inevitable in the marine environment, and species may occur interaction on the growth. In this study, the macroalgae Ulva pertusa and marine microalgae Heterosigma akashiwo were selected as target species to study the interaction mechanism between them. After the 8 days of co-cultivation, the inhibition on growth was observed for both of U. pertusa and H. akashiwo. Eight fatty acids in U. pertusa was detected, with the significant decrease in contents of polyunsaturated fatty acids (PUFAs) especially for C18:2, C18:3n-3 and C18:3n-6. Twelve fatty acids in H. akashiwo was detected, with the significant change for PUFAs. PUFA concentrations in the co-culture group were less than those in the mono-culture. Meanwhile the principal component analysis was conducted to insight into the interaction between U. pertusa and H. akashiwo by fatty acids content and carbon stable isotope ratio of fatty acids (δC). Fatty acid content could not distinguish mono and co-culture. However, δC could distinguish not only the culture time of algae, but also the living environment of algae. In addition, this study combined fatty acids content and δC to explore the release of fatty acids by algae into the seawater. The C18:3n-3 was identified as the allelochemical released by U. pertusa to inhibit the growth of H. akashiwo. The ratio of δC in seawater decreased. This study provides a theoretical basis for the symbiosis of marine algae, and a new method of compound-specific stable carbon isotopes was used to better explore the metabolism of fatty acids in algae.