Isolation and identification of a psychrotolerant dimethyl phthalate-degrading bacterium from selected frozen soil of high-latitude areas in China and optimization of its fermentation conditions using response surface methodology

Large quantities of phthalic acid esters (PAEs) are released and migrate into the environment continuously and become ubiquitous in soil, surface water, groundwater and other areas, eventually leading to serious environmental pollution. Therefore, the methods of PAEs mineralization have become a hot research topic. In this study, a highly efficient dimethyl phthalate (DMP)-degrading bacterium capable of utilizing DMP as the sole source of carbon and energy was isolated from frozen soil collected from a refuse disposal station. The DMP-degrading bacterium was identified as Pseudomonas sp. QDF12 according to the phylogeny of 16S rDNA sequence. The degradation process well fitted a first-order kinetic model, with a half-life of 9.8 h and the highest degradation rate of 20.19 mg/L•h at 15.0 °C. The fermentation conditions that enhanced the fermentation of biomass were optimized via Plackett-Burman design and response surface methodology. Finally, the carbon-nitrogen ratio, CaCO3, temperature and shaking speed were identified as key factors. Verification experiments indicated that the maximal biomass was 1.85 × 109 cfu/mL under the optimized conditions, which was 134.18% and 66.67% higher than those obtained under the Luria broth medium and the basic fermentation conditions, respectively. The time and temperature of fermentation were reduced to 12 h and 12.0 °C respectively. In conclusion, the strain QDF12 is more suitable for application in low-temperature areas owing to its fast, efficient fermentation and low-temperature resistance. Because of the shorter fermentation period and lower fermentation temperature, the large-scale commercial application of strain QDF12 will reduce the production costs and accelerate the fermentation speed.