Exploiting starch and cellulose biodegradation for microbial polyhydroxyalkanoate (PHA) production

In Colombia, annual plastic consumption reaches nearly 1.2 million tons, generating a significant impact on the environment and public health, as most of this waste ends up in landfills that pollute mangroves, rivers, and seas. To address the problems associated with non-biodegradable and non-renewable plastics, alternative materials such as biodegradable and non-toxic polymers have been promoted, among which polyhydroxyalkanoates (PHA) stand out. These microbially derived biopolymers are biocompatible, can be produced from various renewable carbon sources, and are suitable for both single-use plastics and biomedical applications, making them consistent with circular economy principles.

This study aimed to evaluate PHA production by Bacillus thuringiensis (C01) using starch and carboxymethylcellulose (CMC) as carbon sources, and to optimize the process through a central composite design with response surface methodology. Initially, fermentation growth variables were assessed using glucose as a control, followed by evaluation of the experimental substrates under optimized conditions. With CMC, a PHA production of 0.09 g/L was obtained, with a yield of 49.5% (w/w). In contrast, optimization with starch (14.168 g/L starch, 3.616 g/L ammonium sulfate, yeast extract in a 1:1 ratio, and an inoculum of approximately 6×10⁸ CFU/mL) resulted in 3.71 g/L biomass and 2.78 g/L PHA, corresponding to a yield of 74.9% (w/w). These findings demonstrate the ability of Bacillus thuringiensis to degrade starch and channel it into PHA synthesis.

FTIR, TGA, DSC, and MALDI-TOF analyses confirmed that the biopolymer obtained was poly-3-hydroxybutyrate (P(3HB)). In conclusion, the amylolytic activity of Bacillus thuringiensis highlights its potential for using starch-rich agroindustrial residues as substrates for sustainable PHA production.