Algaenan structure in the microalga Nannochloropsis oculata characterized from stepwise pyrolysis

Dried biomass of the eustigmatophyte Nannochloropsis oculata was subjected to stepwise pyrolysis at 50 °C intervals from 310 to 610 °C to test the potential of the technique for investigating the algaenan structure of N. oculata and for determining a broad suite of biochemical constituents. Pyrolysis at 310 °C yielded mainly free lipids such as phytol, cholesterol, C32 alkyl diols, hexadecanamide and large amounts of phytol-derived alkenes from thermal desorption. Indole generated from protein degradation first appeared at 360 °C, indicating the onset of biopolymer breakdown. The pyrolyzate of 410 °C showed a simpler composition, dominated by 2-methyl-1H-indole and indole, and accompanied by low amounts of phenol and hexadecanamide. At 460 °C, algaenan breakdown was evident from the detection of C15-C19n-alkan-2-ones with a strong predominance of C17, and of C29-C33 mid-chain ketones composed of a remarkable predominance of the symmetrical ketone C31n-alkan-16-one. Small amounts of the mono-unsaturated C31:1 mid-chain ketone and n-alkene/n-alkane doublets were also present. The chromatograms suggest that the algaenan structure of N. oculata is composed of repeating, interlinked C32n-alkan-1,17-diols, whereas the free plus bound lipids were dominated by the C32n-alkan-1,15-diol. The pyrolyzate at 510 °C was dominated by a bimodal distribution of C8-C32n-alkene/n-alkane doublets showing a maximum at n-C14:1 and n-C26, with lesser amounts of ketones. Such a wide chain length distribution implies that other aliphatic lipids, perhaps derived from the long chain n-alkenols, might also be involved in the algaenan structure. Major constituents of the 560 °C pyrolyzate were aromatic compounds, including toluene, phenol and indole, but absolute abundances were low. It is notable that high molecular weight compounds such as the n-alkene/n-alkane doublets were no longer detected at 560 °C. The results show that stepwise pyrolysis provides simpler chromatograms than a single pyrolysis of a whole sample at 610 °C and so can be a useful tool for identifying algaenan structures and/or related compounds in ancient sediments and modern microalgae.