Closing the loop: Agricultural applications of hydrolyzed polyhydroxyalkanoates production waste

The increasing global demand for sustainable materials and circular economy solutions has driven research into innovative applications of polyhydroxyalkanoates (PHA) and their by-products. While PHAs are well-recognized biodegradable bioplastics, the potential reuse of waste streams generated during their production remains largely unexplored. This study investigates the feasibility of utilizing hydrolysates derived from PHA post-fermentation biomass as biostimulants for rapeseed (Brassica napus L.) growth. Waste biomass from two bacterial fermentation processes - Zobellella denitrificans and Pseudomonas putida CA-3 - was subjected to acid hydrolysis to generate liquid formulations suitable for soil application. Their effects on plant growth, photosynthetic efficiency, phytohormone profiles, soil chemical composition, and microbiome dynamics were evaluated through microbiotests and pot experiments. Results indicated that hydrolysate application enhanced biomass accumulation and improved soil nutrient content, while also modulating key physiological responses, including chlorophyll fluorescence dynamics and hormone levels, suggesting a role in stress resilience. Additionally, hydrolysates induced a shift in the rhizosphere microbiome, decreasing bacterial dominance while enhancing diversity, particularly through the promotion of plant growth-associated microbial taxa. These findings support the integration of PHA production residues into agriculture, aligning with circular bioeconomy principles by converting industrial by-products into value-added biostimulants. Future research should focus on optimizing hydrolysate formulations, assessing their long-term agronomic impact, and evaluating their commercial feasibility for sustainable crop production.