High-value compounds in three freshwater green microalgae using nitrogen as an abiotic stressor: A study of the antioxidant potential of ethanolic extracts

Abstract

Microalgae are a rich source of several biomolecules, some of which are recognized for their potential antioxidant activity. In this study, Chromochloris zofingiensis, Koliella longiseta, Schizomeris leibleinii were cultivated in five different media, using NaNO₃ or urea as the nitrogen source in different concentrations. BBM_STD (BBM standard), BBM (−) - NaNO₃ concentration reduced tenfold, BBM (X) - absence of nitrogen, BBM (U) - urea replacing NaNO₃ in nitrogen equivalent, and BBM (U-) - urea concentration reduced tenfold. The microalgae extracts were analysed for their production of carotenoids, chlorophylls, lipids, and phenolic compounds. In addition, antioxidant activity was assessed using three different assays (DPPH• RSA, FICA, and TAC). K. longiseta showed highest carotenoids (3.8 mg·g⁻¹) and chlorophyll (10.8 mg·g^{- 1}) content in BBM_STD (BBM standard). Indeed, the lower nitrogen supply (ten times less than BBM_STD) induced lipid accumulation, reaching a maximum of 45.1 % in C. zofingiensis (BBM (−), less NaNO₃). In contrast, urea (BBM (U), BBM using urea as nitrogen source) enhanced the phenolic contents, achieving 33.2 and 30.8 mg GAE·g⁻¹, in K. longiseta and S. leibleinii, respectively. Highest antioxidant potential was found in C. zofigiensis (43.1 % RSA, 21.7 % FICA, and 214.6 mg GAE·g⁻¹ TAC) and S. leibleinii (27.7 % RSA, 28.8 % FICA, and 253.4 mg GAE·g⁻¹ TAC). Overall, phenolic compounds seemed to have major contribution to the antioxidant activity than pigments and lipids.