Transforming Tilapia into Indoleacetic Acid-Containing Biostimulants: Synergistic Effect of Enzymolysis and Multi-Strain Fermentation

Following new trends in green development, many studies have focused on the high-value utilization of fish resources through green biological processes. This study innovatively introduced a one-step process of mixed strain–enzyme synergy (MES) with which to prepare tilapia hydrolysates and explored the synergistic effects of strains and enzymes on both the protein hydrolysis process and its products’ characteristics via comparative experiments. Further, soybean was used as a model crop to verify the agronomic effects of the hydrolysates. The addition of exogenous papain increased hydrolysis by 31.94% compared to the fermentation-only group. Peptides and amino acids contents in the mixed strains were higher than those in the single fermentation process (p < 0.05), while 8.46 mg/L of indoleacetic acid was produced through fermentation. Hydrolysates promoted the growth of lateral roots in soybean seedlings (p < 0.05) via the use of a 2500-fold dilution of the biostimulant, increasing the root area and stem length and reducing the sugar content of soybean seedlings by 1.59-, 1.44- and 1.69-fold compared to those in Hoagland’s nutrient solution. These results lay a foundation for the biological preparation of biostimulants for hydroponic vegetables through the utilization of fish waste resources, aligning with green development goals.