The life cycle of fatty acids in oleaginous stramenopiles: from synthesis to maturation and beyond

Stramenopiles, including diatoms, eustigmatophytes and thraustochytrids, are major contributors to global primary productivity and promising platforms for lipid accumulation and very-long-chain polyunsaturated fatty acid biosynthesis. Their evolutionary history, shaped by secondary endosymbiosis, has produced highly compartmentalised cellular architectures that strongly influence fatty acid metabolism. Yet how fatty acid synthesis, trafficking, remodelling and degradation are coordinated across cellular compartments remains incompletely understood. Here, we review the life cycle of fatty acids in marine oleaginous stramenopiles, based on studies in a set of model species, from their de novo synthesis in plastids to their cellular fate. We summarise plastidial pathways producing saturated and unsaturated fatty acids and examine fatty acid export, activation into acyl-CoAs and incorporation into extraplastidial glycerolipids. Particular attention is given to elongation and desaturation pathways in the endoplasmic reticulum that generate essential very-long-chain polyunsaturated fatty acids such as eicosapentaenoic and docosahexaenoic acids. We further discuss the cytosol-to-plastid trafficking of eicosapentaenoic acid (referred to as the ‘omega pathway’, by analogy with the ‘prokaryotic’ and ‘eukaryotic pathways’ in angiosperms), the catabolic routes of fatty acid degradation, and the enzymatic as well as non-enzymatic oxidation of fatty acids to prduce oxylipins and isoprostanoids. Together, recent genetic, biochemical and lipidomic studies reveal remarkable metabolic plasticity in stramenopiles and highlight key unresolved questions relevant to cell biology and microalgal biotechnology.