Common and Species-Specific Effects of Phosphate on Marine Microalgae Fatty Acids Shape Their Function in Phytoplankton Trophic Ecology

The use of fatty acids (FA) to infer structure of phytoplankton assemblages and as indicators of microalgae nutritional value is acquiring relevance in modern phytoplankton ecology and new advances concerning factors influencing FA variability among microalgae are demanded. In this regard, the relationship between phosphorus and FA remains particularly little studied in marine phytoplankton. In the present study, we focus on phosphate effects on FA from a diversified set of marine microalgae and provide new insights into the applicability of FA in phytoplankton trophic ecology. Phosphate deprivation mainly induced monounsaturated FA production in eight out of nine microalgae and their changes were species-specific, with palmitoleic acid exhibiting extreme variation and discriminating between haptophyte classes. The important phosphate-induced and interspecific variability found for oleic acid was perceived as a concern for the current application of this FA as a trophic position indicator in grazers. Chloroplast C-16 and C-18 polyunsaturated FA were more affected by phosphate than C-20 and C-22 highly unsaturated FA (HUFA). The relative stability of stearidonic acid to phosphate in cryptophytes and haptophytes pinpointed this FA as a suited marker for both microalgae groups. Taken all species together, phosphate deprivation and taxonomy accounted for 20.8 and 50.7% of total FA variation, respectively. HUFA were minimally affected by phosphate indicating their suitability as indicators of phytoplankton trophic value. The asymptotic relationship between HUFA and phosphorus cell content suggested mineral composition (phosphorus) could be more important than HUFA content as attribute of marine microalgae nutritional value at the species level.