Structural characterization of novel extracellular polymer substances from an Alternaria sp. 17463 and their impact on soil metabolism.

Dark septate endophytes (DSE) are widely used in ecological restoration. In this study, we first discovered that the metabolic products of the DSE strain Alternaria sp. 17463 contain extracellular polymeric substances (EPS). This study aimed to optimize the culture conditions, characterize the structural composition of EPS, and evaluate its effects on soil improvement. The ethanol precipitation method was used to extract EPS from the metabolites of Alternaria sp. 17463, increasing the yield 4.48-fold to 6.96 g L⁻¹ through optimized cultivation conditions. The molecular mass of EPS was 533.754 kDa, with an exopolysaccharide content of up to 77.8% and a protein content of 8.4%. The EPS were mainly composed of glucan units T)-Glc p-(1 → and → 4)-Glc p-(1 → , with minor fractions of mannose and galactose. By acting as a cementing agent, EPS strengthened the linkages between soil particles. At an optimal concentration, it significantly enhanced the activity of extracellular enzymes involved in carbon, nitrogen, and phosphorus metabolism, thereby improving soil nutrient transformation and cycling. This also promoted changes in the metabolism of small molecules in the soil. This research reveals why this strain can accelerate ecological restoration and lays the foundation for further exploration of its metabolic products and potential applications in this field, particularly in open-pit dump reclamation and agricultural soil improvement.