Upcycling of PHA-producing bacteria for biostimulants production and polyhydroxyalkanoates recovery

The use of petroleum-based plastic has led to its accumulation in the environment, with negative impacts on the ecosystem and the biota. Polyhydroxyalkanoates (PHAs), biobased and biodegradable plastics produced by microbes, have many commercial applications, however their high production cost means they cannot yet compete with traditional plastics. At the same time, the problem of the growing human population implies that improved crop production is needed to avoid malnutrition. Biostimulants enhance plant growth and have the potential to improve agricultural yields; they can be obtained from biological feedstock, including microbes. Therefore, there is scope for coupling the production of PHAs with that of biostimulants, making the process more cost-efficient and minimizing by-products generation. In this work, low-value agro-zoological residues were processed to obtain PHA-storing bacteria via acidogenic fermentation; PHAs destined for the bioplastic market were extracted, and the protein-rich by-products were turned into protein hydrolysates using different treatment methods, assessing their biostimulant effects in growth trials with tomato and cucumber plants. The results indicate that the best hydrolysis treatment, realizing the highest amount of organic nitrogen (6.8 gN-org/L) while achieving the best PHA recovery (63.2 % gPHA/gTS), is obtained with strong acids. All the protein hydrolysates were effective in improving either roots or leaf development, with various results, depending on the species and the growth method. The acid hydrolysate was the most effective treatment to enhance the development of shoots (21 % increase compared to the control) and roots (16 % increase for the dry weight and 17 % for main root length) of hydroponically-grown cucumber plants, while pot-grown tomatoes, biostimulated via foliar spray, developed bigger shoots (up to 41 %) with the hydrolysate obtained from the alkaline treatment. These preliminary results indicate that simultaneous production of PHAs and biostimulants is feasible, and that commercialization could be achievable given the expected reduction in production costs.