Characterizing the growth of PHA-producing microorganisms on short-chain carboxylic acids

Short-chain carboxylic acids are important chemical intermediates in anaerobic treatment or thermal hydrolysis of biowaste. They are a crucial precursor for the production of Polyhydroxyalkanoate (PHA) by various microorganisms. However, there is limited data on the growth behavior of PHA-producing microorganisms on carboxylic acids and their tolerance range. Therefore, this study aims to systematically determine the growth behavior of four commonly used PHA-producing microorganisms (Cupriacidus necator, Pseudomonas putida, Azohydromonas australica, and Haloferax mediterranei) on different carboxylic acids (formic, acetic, propionic, valeric, and levulinic acid). Batch experiments were conducted in a microbioreactor to determine the maximum specific growth rates (µ_max) on varying acid concentrations. Additionally, the data was analyzed using the Han-Levenspiel growth model to determine the inhibitory effect of the acids on the microorganisms. Our experiments showed that P. putida had the highest µ_max on propionic acid (0.165 h^{- 1}) and levulinic acid (0.16 h^{- 1}). Likewise, H. mediterranei also showed the highest µ_max on propionic acid (0.174 h^{- 1}). For C. necator, the highest µ_max was determined in acetic (0.102 h^{- 1}) and levulinic acid (0.109 h^{- 1}). Among all the tested carboxylic acids, acetic acid was found to be the least toxic acid and was also the only substrate that enabled meaningful growth of A. australica. Furthermore, formic acid seems to be the least suitable substrate for the growth of these microorganisms except H. mediterranei as only the growth of this microorganism was observed. Based on these results, we concluded that carboxylic acids are not a suitable carbon source for the growth of A. australica. While C. necator and P. putida are more versatile in utilizing various acids, except formic acid. They showed the largest optimum growth or tolerance range in acetic and levulinic acid, making these acids the most preferred substrate for growth. Lastly, H. mediterranei could grow in all carboxylic acids, however, the requirement of a highly saline medium would be challenging.