Effects of Spirulina platensis and Sargassum polycystum extracts on morpho-physiological and metabolic responses of Pak Choi under cadmium stress

Cadmium (Cd) contamination in agricultural systems presents a significant risk to plant growth and development, food safety, and human health owing to its high toxicity and environmental mobility. It is crucial to develop eco-friendly strategies to alleviate Cd stress in crops, and algal extracts have demonstrated potential in improving plant tolerance to heavy metal stress. This study investigated the potential of Spirulina (Limnospira) platensis, Sargassum polycystum, and their combination in ameliorating cadmium (Cd) phytotoxicity and enhancing the remediation efficiency of hydroponic Pak Choi. Seedlings were exposed to 100 mg L-1 Cd alongside 0, 25, 50, and 100 mL L-1 (v/v) of algal extracts. Cd exposure alone severely impacted plant growth, reducing shoot and root length, shoot and root fresh weight (FW), shoot and root dry weight (DW), and leaf area by 61.45%, 66.03%, 88.67%, 63.55%, 79.33%, 67.80%, and 59.24%, respectively. Algal extracts significantly mitigated these negative effects. Chlorophyll a, chlorophyll b, and carotenoids decreased under Cd stress by 39.60%, 62.62%, and 41.74%, respectively, but T10 treatment (100 mg L-1 Cd and 100 mL L-1 S. platensis + S. polycystum) increased them by 55.61%, 157.30%, and 70.05%. Photosynthetic efficiency (Fv/Fm) dropped to 0.74 under Cd stress, while T10 treatment improved it to 0.80. Cd exposure reduced photosynthetic rate, stomatal conductance, transpiration rate, and internal CO2 by 55.91%, 50.00%, 50.24%, and 17.61%, respectively, but T7 treatment (100 mg L-1 Cd and 100 mL L-1 S. polycystum) improved photosynthetic efficiency significantly. Oxidative stress markers (MDA, H2O2) increased by 127.05% and 142.67% under Cd stress, but T10 treatment reduced them by 48.45% and 51.64%. Algal extracts treatments significantly reduced Cd accumulation in root cells. GC-MS analysis identified 52 metabolites, with algal extracts restoring or maintaining their levels. Principal Component Analysis (PCA), Hierarchical Cluster Analysis (HCA), and Partial Least Squares Discriminant Analysis (PLS-DA) of GC-MS data revealed distinct clusters within treatment groups, showing a positive correlation between metabolite changes and algal treatments. The PLS-DA model suggests algal extracts ameliorate Cd toxicity by regulating metabolite profiles. This study concludes that algal extracts, particularly at 100 mL L-1 (v/v), demonstrated the most pronounced mitigation of Cd toxicity. It also highlights the potential mechanisms behind their protective effects, which include enhanced photosynthetic performance, reduced oxidative stress, and metabolite regulation in Cd-stressed Pak Choi.