Optimization of extracellular polysaccharides (EPS) production by Stenotrophomonas rhizophila JC1 and its protective effect on alfalfa under Pb stress.

Stenotrophomonas rhizophila JC1 and its extracellular polysaccharides (EPS) have been shown to effectively adsorb heavy metals in previous studies. The fermentation conditions of EPS by S. rhizophila JC1 were optimized using the Box-Behnken design (BBD). The composition, structural characteristics, and heavy metal adsorption capacity of EPS were systematically evaluated. The alleviation mechanism of Pb stress on alfalfa was investigated through EPS inoculation. The maximum EPS yield reached 0.313 %. EPS consisted of glucose, glucosamine, galactose, and mannose in a molar ratio of 12.20:1:22.29:1.68. EPS also contained four distinct polymers with molecular weights of 623,683.71 Da, 144,072.27 Da, 105,892.21 Da, and 51,094.79 Da. The adsorption processes conformed to the pseudo-second-order model and Langmuir isotherm model. High Pb concentrations significantly reduced germination percentage, germinative force, root length, fresh weight, and soluble protein, inhibited photosynthesis, exacerbated oxidative stress, and caused damage to the antioxidant system, thereby inhibiting seedling growth. EPS at low concentrations can promote alfalfa seed germination and mitigate Pb stress by reducing the aforementioned damage. This study highlights the potential of EPS in soil remediation and enhancing plant resistance to heavy metal stress.