Nitrogen deprivation differentially alters lipid accumulation and pyrolysis-derived volatile profiles in three microalgal species
Nitrogen deprivation (N−) is widely used to induce lipid accumulation in microalgae. However, its impact on the relationship between lipid accumulation and thermochemical behavior remains poorly understood. In this study, a comparative experimental approach was used to evaluate the effects of nitrogen availability on growth kinetics, lipid accumulation, and pyrolysis-derived volatile compounds in Chlorella vulgaris, Scenedesmus obliquus, and Nannochloropsis oculata. Under nitrogen˗replete conditions (N +), all species showed higher growth rates, whereas N− reduced proliferation but increased lipid content by 5.0-fold in C. vulgaris, 2.4-fold in S. obliquus, and 1.8-fold in N. oculata. Py-GC/MS analysis revealed that N− shifted the pyrolysis profile toward lipid˗derived compounds, particularly C16˗C18 fatty acids, long˗chain alcohols, and hydrocarbons. In C. vulgaris, oleic acid (22.3%) and alcohol derivatives dominated; in contrast, N + conditions exhibited a more heterogeneous profile, including aromatics and sterols. N. oculata maintained a predominance of lipid-derived compounds, with N− associated with a higher contribution of oxygenated compounds. In S. obliquus, the pyrolysis profile under N + was characterized primarily by esters and reactive intermediates, while under N− it was dominated by hydrocarbons (13.6%) and alcohols. Nitrogen deprivation increased lipid accumulation in all three microalgal species, but the volatile compounds generated during pyrolysis remained species-dependent. Despite the higher lipid content under nitrogen deprivation, the corresponding pyrolysis profiles did not show a consistent relationship with lipid accumulation. These results indicate that lipid-rich biomass does not necessarily exhibit similar thermochemical behavior across microalgal species and suggest that evaluating feedstocks based solely on total lipid content may overlook species-specific differences relevant to bioenergy production.