Diverse steroidogenic pathways in the marine alga Aurantiochytrium

The heterotrophic alga Aurantiochytrium accumulates a high amount of squalene, which is a key precursor to the biosynthesis of sterols and steroids. However, post-squalene sterol pathways of the algae have been enigmatic. Using NMR-based chemical approaches complemented by in silico analysis of Aurantiochytrium genome, we identified 7 sterols including novel derivatives and predicted multiple sterol metabolic pathways. Based on known sterol distribution among eukaryotes, phylogeny, and our findings, we suggest that Aurantiochytrium could possess multiple sterol metabolic pathways of both photosynthetic and non-photosynthetic lineages with highly proliferated genes encoding the associated enzymes. Aurantiochytrium possibly possesses well-developed networks of sterol metabolic pathways.