Melatonin buffers antioxidant defense but only partially sustains photosynthesis under combined abiotic stresses in Chlamydomonas reinhardtii

Melatonin (MLT) is a key regulator of stress responses across photosynthetic systems, yet its role remains uncertain under combined abiotic stresses. Here, we examined MLT-mediated antioxidants, photosynthetic, and transcriptional responses in algae (Chlamydomonas reinhardtii) under single and combined abiotic stress (cadmium, heat, salinity, and red light). MLT significantly enhanced catalase and peroxidase activities, specifically under dual, triple, and quadruple stress combinations. However, superoxide dismutase displayed limited changes. This enzymatic response was supported by the upregulation of antioxidant genes (CAT, POD3, SOD-Mn1/3, GPX3, and GSTs). In contrast, photosynthetic performance was impaired under stress, as chlorophyll a/b contents declined across most treatments, with only condition-specific improvement under cadmium + MLT, despite moderate induction of photosynthesis-related genes (Chla/b, and PSI-psaA/B). Transcriptional exploration further highlighted the selective induction of transcription factors (NAC1 and MYB2, positively correlated with antioxidant genes), while MLT biosynthesis-related genes showed minimal changes. This suggests that exogenous MLT acts independently of the endogenous pathway in algae. In short, these findings demonstrate that MLT functions as a "stress-buffering modulator", favorably enhancing antioxidant defense while providing limited protection to photosynthesis under multifactorial stress combinations. This differential regulation highlights the dynamic role of MLT to promote algal tolerance and adaptation to multi-stress conditions.