Cadmium (Cd) is one of the highly toxic, non-essential heavy metals that inhibit plant growth and development by prompting chlorophyll loss and affecting photosynthetic activities. This study investigated the efficacy of Spirulina platensis
and Sargassum polycystum
extracts in alleviating Cd stress in Pak Choi at morpho-biochemical, anatomical, and molecular levels. Different concentrations (0, 25, 50, and 100 mL/L) of S. polycyctum
(SAR), S. platensis
(SPI), and a mixture of both extracts (SS) were exposed to 100 mg/kg Cd-contaminated Pak Choi seedlings. Non-Cd-treated Pak Choi and Cd-contaminated Pak Choi without algal extracts were assigned as positive and negative controls, respectively. The results showed that the application of algal extracts increased the plant height, fresh weight (FW), and dry weight (DW) as the extract level increased. This was greatest in 100SS with 37.51% (shoot length), 68.91% (root length), 110.8% (shoot DW), and 216.13% (root DW), while an increase of 176.7% (shoot FW) and 256.9% (root FW) was seen in the 100SPI treatment. Chlorophyll a
carotenoid, and chlorophyll fluorescence increased significantly after the treatment with 100SS. Antioxidant enzymes CAT, APX, POD, and protein were significantly increased in 100 mL/L extracts by 28.13% (100SS), 36.40% (100SAR), 46.92% (100SS), and 153.48% (100SS), respectively. The same treatment was also identified to dominate the development of root structures such as total length, surface area, projected area, diameter, volume, tips, and fork number. The highest reduction of Cd content in the root and shoot of Pak Choi was observed in 100SS with a 53.8% and 39.88% decrease, respectively. Increasing algal extract concentration also improved the leaf histological characteristics substantially, such as stomatal size and opening, mesophyll tissues, and vascular bundles. In addition, the fold change ratio of the Rubisco gene at 100SS treatment was noticeably greater than other algal extract treatments, with a 0.99 fold change when compared with the untreated sample. This present study illustrated that Sargassum polycyctum
and Spirulina platensis
extracts have the potential to effectively alleviate Cd stress in Pak Choi plants, especially with the application of 100 mL/L of an algal extract mixture. These findings contribute to the development of sustainable strategies for mitigating Cd toxicity in crops.