The benthic dinoflagellate Amphidinium carterae can produce fatty acids and polyketide compounds, such as amphidinols. Commonly, polyketides are produced by polyketides synthase (PKS), and fatty acids are produced by fatty acids synthase (FAS). The PKS and FAS genes in dinoflagellates share a common evolutionary history. This study aimed to investigate the effect of five salinities (20, 25, 30, 35, and 40 ‰) on growth, fatty acid content, hemolytic activity, and the expression of PKS and FAS genes. The results showed that low salinity (20 ‰) induces low growth in A. carterae. Cell size was affected by salinity, with a decrease in cell size with the salinity increase. The content of fatty acids and hemolytic compounds content increased at low salinities (20 and 25 ‰). The gene expression of the PKS genes was upregulated at high salinities (35 and 40 ‰) and downregulated at low salinities (20 and 25 ‰)
FAS genes were downregulated at high (40 ‰) and low (20 ‰) salinities. The low growth rate at low salinity (20 ‰) and the high content of fatty acids and hemolytic compounds in low salinities (20 and 25 ‰) indicate that low salinities caused stress in this strain of A. carterae. The hemolytic activity at salinity of 20 ‰ probably due to a combinatory effect of high content of polyunsaturated fatty acids and amphidinols. To understand the relationship between gene expression and amphidinols biosynthesis requires the analysis of single-domain and multi-domain PKS.