Overlooked components of lipid metabolism: Odd-chain polyunsaturated fatty acids in photosynthetic microorganisms

This mini-review examines odd-carbon polyunsaturated fatty acids (odd-PUFAs), a rare but biologically significant group of fatty acids in photosynthetic microorganisms. Odd-PUFAs are straight-chain fatty acids containing an odd number of carbon atoms and two or more methylene-interrupted double bonds. They typically account for less than 1% of total fatty acids in microalgae and cyanobacteria. Current evidence indicates that their biosynthesis primarily originates from propionyl-CoA-initiated fatty acid synthesis, followed by conventional elongation and desaturation reactions. The review focuses on three major themes. First, it addresses analytical challenges in odd-PUFA identification, emphasizing the limitations of gas chromatography with a flame ionization detector (GC-FID) retention-time assignments and the need for complementary tandem mass spectrometry (MS/MS)-based approaches and derivatization techniques for reliable structural characterization. Second, it summarizes the current knowledge of odd-PUFA biosynthesis, including the metabolic origins of propionyl-CoA and its incorporation into fatty acid synthase pathways. Third, it surveys the occurrence of odd-PUFAs across photosynthetic microorganisms, ranging from trace constituents in cyanobacteria and chlorophytes to the characteristic profiles of Euglena gracilis and the very-long-chain odd-PUFAs in dinoflagellates. The primary emphasis is on photosynthetic microorganisms, although thraustochytrids are included as a comparative group because they represent the best characterized microbial system for odd-chain PUFA biosynthesis. Higher trophic organisms are discussed only briefly to illustrate the transfer of odd-PUFA algal through aquatic food webs. Advances in lipidomics and the growing interest in odd-chain fatty acids highlight the need for more research, including analytical standardization, broader taxonomic surveys, and metabolic engineering approaches for biotechnological production.