The interaction between nitrogen source and light intensity affects the biomass and phenotypic plasticity of Scenedesmus obliquus.

As critical environmental factors, nitrogen and light not only regulate phytoplankton growth but also influence their phenotypic plasticity. Scenedesmus obliquus, an alga which is famous for its remarkable phenotypic plasticity, was studied to understand its response to varying combinations of nitrogen source and light intensity. It was cultured in media containing different nitrogen sources (NaNO₃, NH₄Cl, CO(NH₂)₂) under a range of light intensities (25, 50, 75, 100, 150 µmol photons m^-2 s^-1). Results showed that growth rates increased with higher light intensities across all nitrogen sources. Photosynthetic efficiency (F_v/F_m and Φ_PSII) remained stable in NaNO₃ treatments, but declined with rising light intensity in NH₄Cl and CO(NH₂)₂ treatments. The highest proportions of multicellular colonies were observed at 150 µmol photons m^-2 s^-1 for NH₄Cl and NaNO₃ treatments, while colonies in CO(NH₂)₂ treatments peaked at 100 µmol photons m^-2 s^-1, with colony size stabilized at approximately 2.1, 4.0, and 1.0 cells per particle under NaNO₃, NH₄Cl, and CO(NH₂)₂ treatments, respectively. Nitrogen removal efficiency improved with increasing light intensity across all treatments, though S. obliquus exhibited varying capacities to remove nitrogen depending on the sources. These findings demonstrated how S. obliquus adapts to varying nitrogen sources and light intensities in its growth, photosynthesis, and morphology, providing new evidence for our insights into its ecological versatility. This study established a theoretical foundation for optimizing culture conditions in applications such as wastewater treatment and bioenergy production.