Concentration-Dependent Effects of Salicylic Acid and Methyl Salicylate on Chlorophylls, Carotenoids, Fatty Acids, and Phenolic Compounds in the Green Microalga Planktochlorella nurekis

Microalgae are a rich source of bioactive metabolites, including unsaturated fatty acids, chlorophylls, carotenoids, and phenolic compounds with antioxidant and nutraceutical potential. Salicylic acid (SA) and its volatile ester, methyl salicylate (MS), are well-known elicitors of secondary metabolism in higher plants, yet their mechanisms of action in green microalgae, particularly the effects of MS, remain poorly understood. Therefore, we evaluated how salicylates applied at concentrations of 0.1, 1, and 10 µM affect the growth and accumulation of bioactive compounds in a high-productivity clone of Planktochlorella nurekis, combining gas chromatography coupled mass spectrometry (GC-MS) fatty acid profiling with spectrophotometric determination of pigment levels, total phenols, and L-phenylalanine ammonia-lyase (PAL) activity. SA increased cell number and size, whereas 0.1 µM MS resulted in a six-fold gain in cell number. Accordingly, 10 µM MS raised chlorophyll b levels—nearly three-fold (from 76 to 211 µg per g D.W.)—and total carotenoids—roughly four-fold (129 to 520 µg per g D.W.)—while both elicitors significantly increased the monounsaturated fatty acid fraction (from 29% to about 40%). Notably, 10 µM SA lowered the levels of saturated fatty acids. Moreover, 10 µM MS doubled the PAL activity and increased the total phenols by 17% compared to the control. Thus, SA and MS act as effective elicitors of high-value metabolites in P. nurekis.