Hints from nature for a PHA circular economy: Carbon synthesis and sharing by Pseudomonas solani GK13

Polyhydroxyalkanoates (PHAs) are a well-known group of biodegradable and biocompatible bioplastics that are synthesised and stored by microorganisms as carbon and energy reservoirs. Extracellular PHA depolymerases (ePhaZs), secreted by a limited range of microorganisms, are the main hydrolytic enzymes responsible for their environmental degradation. Pseudomonas sp. GK13, initially identified as P. fluorescens GK13, produces PHA and a prototypic ePhaZ that specifically degrades mcl-PHA. In this study, a comprehensive characterization of strain GK13 was performed. The whole genomic sequence of GK13 was consolidated into one complete chromosome, leading to its reclassification as P. solani GK13. We conducted a detailed in silico examination of the bacteria genomic sequence, specifically targeting PHA metabolic functions. From the different growth conditions explored, PHA accumulation occurred only under carbon/nitrogen (C/N) imbalance, whereas ePhaZ production was induced even at balanced C/N ratios in mineral media. We extend our study to other bacteria belonging to the Pseudomonas genus revealing that the ePhaZ production capacity is closely associated with mcl-PHA synthesis capacity, as also suggested by metagenomic samples. This finding suggests that these types of microorganisms could contribute to the carbon economy of the microbial community, by storing PHA in carbon-rich times, and sharing it with the rest of the population during times of carbon scarcity through PHA hydrolysis. The conclusion pointed that carbon cycle metabolism performed by P. solani GK13 may contribute to the environmental circular economy at a microscopic scale.