Anaerobic biodegradation of lipids of the marine microalga Nannochloropsis salina

In order to determine the susceptibility to anaerobic biodegradation of the different lipid biomarkers present in a marine microalga containing algaenan, portions of one large batch of cultured Nannochloropsis salina (Eustigmatophyceae) were incubated in anoxic sediment slurries for various times. After 442 days, all lipids studied [mono-, di-, and tri-unsaturated hydrocarbons, long-chain unsaturated alcohols and alkyl diols, phytol, sterols, saturated and (poly)unsaturated fatty acids] showed a significant decrease in concentration, which was accompanied by a strong production of sulfide and methane. However, the studied compounds showed a wide range of reactivity and different patterns and extent of degradation. Polyunsaturated fatty acids, phytol and triunsaturated hydrocarbons were the most labile compounds and showed initially rapid degradation rates, followed by a substantial reduction in degradation rate during the later stages of incubation. Long-chain alkyl diols and unsaturated alkenols, known to constitute the building blocks of the algaenan of N. salina, showed fluctuating concentrations with time clearly indicating their release from bound fractions in parallel with their degradation. Other lipids showed a continuous concentration decrease until the end of the incubation, with alkadienes and sterols being the most resistant compounds encountered. Besides providing an extended sequence of reactivity for lipids under anoxic conditions, the results demonstrate that the presence of resistant algaenan in the outer cell wall of microalgae does not protect the other lipids of the cell from anaerobic microbial degradation.