Salinity induces unique changes in lipid classes and fatty acids of the estuarine haptophyte <i>Diacronema vlkianum</i>

Studies on the influence of salinity on microalgal lipids have focused mainly on chlorophyte species and knowledge about photosynthetic secondary endosymbionts is limited. Haptophytes exemplify this situation and represent an interesting model to advance knowledge about the high phylogenetic variability of microalgae in their response to salinity. The estuarine Pavlovophyceae <i>Diacronema vlkianum</i> underwent particular lipid class and fatty acid changes under the effect of salinity. Variability due to salinity was higher in the fatty acid profile compared with that of the lipid class profile. Triacylglycerols decreased with salinity in actively growing cells and increased only when salinity rose to 50 and cell growth ceased. Hypersalinity caused a slight, although significant, sulpholipid decrease and a 4-fold increase in phosphatidylglycerol, suggesting a key role for this phospholipid in salt tolerance. Conversely, the betaine lipid diacylglycerol hydroxymethyl-N,N,N-trimethyl-β-alanine was specifically decreased with high salinity. The fatty acids that decreased the most with salinity were 14:0, 16:1n-7, 20:4n-6, 20:5n-3 and 22:6n-3, while higher levels of 16:1n-9, 17:1, 18:2n-6 and 18:3n-3 were induced by high salinity. Despite fatty acid changes, the unsaturation index did not vary with salinity, but nutritional value of <i>D. vlkianum</i> decreased at a salinity of 50 due to a lower presence of 20-C polyunsaturated fatty acids. This pattern of lipid variation in <i>D. vlkianum</i> with salinity has not been described in any other microalgae. Although <i>D. vlkianum</i> grows within a wide salinity range, production of nutritional and functional lipids is more favourable at low salinity values.