Quantification of total fatty acids in microalgae: comparison of extraction and transesterification methods
Determination of microalgaes' fatty acid content is often done with chloroform and methanol according to the Bligh and Dyer extraction, though faster methods exist. A number of comparisons between the Bligh and Dyer and faster methods have resulted in contradicting data, possibly due to differences in algae used and the different versions of the Bligh and Dyer method applied. Here, various forms of direct-transesterification (D-TE) and two-step transesterification (2-TE), including three versions developed in our lab, are compared with the original Bligh and Dyer (Can J Biochem Physiol 37: 911-917, 1959) extraction and two modifications thereof (Lee et al. J AOAC Int 79:487-492, 1996, and our own acidified version) on microalgae with different cell walls: Isochrysis galbana, Nannochloropsis oculata, and Phaeodactylum tricornutum. In total, fatty acid extracts from 11 methods were separated and quantified by gas chromatography with mass spectrometry. Results show that, for N. oculata and P. tricornutum, methods based on chloroform-methanol underestimated the fatty acid content compared with the 2-TE and D-TE methods, which gave similar results. Moreover, D-TE methods are faster than chloroform-methanol methods and use chemicals that are less toxic. Of the D-TE methods, the ones using hydrochloric acid and sulfuric acid recovered the most fatty acids, while boron trifluoride recovered slightly less. The main qualitative difference between the fatty acids recovered was that the chloroform-methanol methods recovered less saturated fatty acids in P. tricornutum.