Fatty acid composition as biomarkers of freshwater microalgae: analysis of 37 strains of microalgae in 22 genera and in seven classes

The fatty acid (FA) composition of algae is an important determinant of their food quality for consumers, and FAs can also be used as biomarkers for biochemical and energetic pathways in food webs. FA analyses of 7 freshwater algal classes and 37 strains showed clear similarity within classes and strong differences amongst classes. Class was a dominant factor (66.4 %) explaining variation in FA signatures of microalgae. The 7 algal classes comprised 4 separate groups according to their FA profiles: (1) Chlorophyceae and Trebouxiophyceae, (2) Bacillariophyceae, (3) Cryptophyceae, Chrysophyceae, and Raphidophyceae, and (4) Euglenophyceae. Each group had a characteristic FA composition, although the proportional abundance of individual FAs also differed between species and with environmental conditions. FAs found to be particularly representative for each group (i.e. diagnostic biomarkers) were as follows: 16:4[omega]3 and 16:3[omega]3 for Chlorophyceae and Trebouxiophyceae; 16:2[omega]7, 16:2[omega]4, 16:3[omega]4, 16:4[omega]1, and 18:4[omega]4 for Bacillariophyceae; 22:5[omega]6 and 18:4[omega]3 for Cryptophyceae and Chrysophyceae (Synurales), 16:3[omega]1 for Chrysophyceae (Ochromonadales), 16:2[omega]4, 16:3[omega]4, 16:3[omega]1, and 20:3[omega]3 for Raphidophyceae; and 15:4[omega]2, 20:4[omega]3, 20:2[omega]6, 20:3[omega]6, and 22:4[omega]6 for Euglenophyceae. FAs thus offer a powerful tool to track different consumer diets in a lacustrine food web. Based on the 20:5[omega]3 (eicosapentaenoic acid) and 22:6[omega]3 (docosahexaenoic acid) content among the investigated freshwater algal classes, Chlorophyceae, Trebouxiophyceae, and Chrysophyceae are of intermediate food quality for zooplankton, and Cryptophyceae, Bacillariophyceae, Euglenophyceae, and Raphidophyceae should be excellent resources for zooplankton.