Co-fermentation of dark septate endophytes and Trichoderma viride can effectively degrade cellulose in different Chinese medicinal herbal residues

Chinese medicinal herbal residues (CMHRs) are currently a widely overlooked renewable biomass resource, and developing controllable microbial fermentation technology for CMHRs will help achieve the resource utilization of CMHRs. In this study, the activity of various cellulases, reducing sugar content and cellulose degradation rate were combined to evaluate the solid state fermentation (SSF) potential of dark septate endophytes (DSE) and Trichoderma viride in separate fermentation and co-fermentation of different CMHRs (single Astragalus mongolicus residue and compound residues of Panax notoginseng and Salvia miltiorrhiza). The results showed that the cellulase activity of some DSE strains was higher than that of T. viride. T. viride fermentation alone could increase the content of substrate reducing sugars, while DSE fermentation could reduce the content of substrate reducing sugars. DSE strains such as Macrophomina pseudophaseoline, Paraboeremia selaginellae, Paraphoma chlamydocopiosa and Paraphoma radicina had the potential to co-ferment with T. viride. Optimization of the fermentation process by inoculation of T. viride after 3 d or 6 d of DSE effectively increased the cellulase activity of co-fermented CMHRs. After optimization, the activities of filter paper cellulase, carboxymethyl cellulase, and β-glucosidase in different fermentation combinations reached 2.51–5.61, 2.86–4.72 and 12.85–32.35 U/g, respectively. Further, the co-fermentation of DSE and T. viride effectively degraded cellulose in different CMHRs with a degradation rate of 54.43%–67.38%. Scanning electron microscope confirmed that the co-fermentation of DSE and T. viride could increase the external surface area and porosity of CMHRs, and destroy the structure of CMHRs. The results of this study both provided a basis for the microbial degradation of single and compound CMHR, which is of great significance for solving the treatment of waste such as Chinese herbal residue and further resource utilization.