Biodégradation de deux types d’emballages plastiques en microcosme aquatique par les bactéries Pseudomonas aeruginosa et Staphylococcus aureus

Abstract

In Cameroon, like in several countries around the world, the environmental pollution problem by plastic waste is acute. The microbial biodegradation of these plastics would vary according to the enzymes involved and could also be influenced by the abiotic factors of the environment. The present study aims at evaluating in aquatic microcosm as a fu nction of time, pH and different temperature ranges, the biodegradation of Low Density Polyethylene (LDPE) and Polypropylene (PP) by the bacteria Pseudomonas aeruginosa and Staphylococcus aureus. Sterilized films of 0.065g LDPE and 0.1g PP were immersed in mineral solutions of pH 5, 7 and 9 UC, in glass vials containing various cellular abundances of P. aeruginosa and S. aureus. Bacteria were incubated at 7°C, 23°C, 37°C and 44°C for 10 days, 20 days, 30 days, 40 days, 50 days and 60 days. Several parameters were studied to assess the degradation of both types of polymers, including mass loss, cell abundance, pH and electrical conductivity of the solutions. In the presence of P. aeruginosa cells, a decrease in the weights of LDPE and PP films was observed an increase in incubation time. The highest mass losses after 60 days of incubation were 6.1% for LDPE recorded at 23°C and initial pH 5, and 5.5% for PP observed at 23 and 37°C and at initial pH 5. The values of the electrical conductivity initially between 3000-3500 μS/cm increased with the incubation period and temperatures for the 2 types of polymers. The highest values after 60 days of incubation were 4835 μS/cm and 5900 μS/cm respectively for PP recorded below 44°C at initial pH 9, and for LDPE recorded below 44°C at initial pH 7. On the overall, the pH of the solutions werelowered to acidity during the incubation period. Cell abundances varied throughout the incubation period depending on the temperatures and initial pH values of the medium. The apparent rate of change in cell abundance was positive under 7°C, 23°C and 37°C, reflecting relative cell growth, and negative under 44°C, reflecting relative cell inhibition. The highest apparent relative cell growth rates were 985 and 662 CFU/10 days below 23°C at baseline pH 9, respectively, for LDPE and PP. In the presence of S. aureus, the masses of the films of the 2 polymers also decreased during incubation. The highest mass losses after 60 days were 7.6% for LDPE recorded below 23°C and at initial pH 5, and 7% for PP observed below 44°C and at initial pH 9. Electrical conductivity increased gradually with the incubation period and pH values decreased overall. The conductivity values initially between 3000-3500 μS/cm, reached after 60 days of incubation, 6610 μS/cm for PP below 23°C and at initial pH 5, and 5330 μS /cm for LDPE recorded below 37°C and at initial pH 5. Cell abundances varied from one incubation

temperature to another and according to initial pH values. The apparent rates of change in cell abundances overall decreased with the increase in incubation temperature at initia l pH 9. However, they increased at initial pH 5 with the increase in the incubation temperature from 7°C to 37°C. The highest levels for LDPE (73 CFU/10 days observed below 23°C), and for PP (196 CFU/10 days observed below 7°C), were all observed at baseline pH 7. The variation in cell abundance of each bacterial species was significant between the different incubation temperatures for the 2 types of polymers. Variations in pH, electrical conductivity of solutions, masses of films LDPE and PP, and cell abundances were significant (P<0.05) between different incubation times and different initial pH values of the solutions. Significant differences were observed in the degradation parameters of the 2 types of polymers for each bacterium on the one hand, and between the degradation parameters of the 2 bacterial species for the same polymer on the other hand. The degradation of the 2 types of polymers seems to be more favorable at pH 5 and 9. LDPE degradation was high at temperatures 7 and 23°C, and PP degradation at temperatures 23, 37 and 44°C. Of the 2 bacteria tested, S. aureus showed the best degradation rates for the 2 polymers. LDPE showed the highest rate of degradation than PP in the presence of the 2 bacteria. The incubation period seems to have played a major role in the degradation of the 2 types of polymers, followed by the temperature, then the initial bacterial abundance and finally the initial pH of the medium. The 2 bacteria P. aeruginosa and S. aureus thus seem important for the restoration of the aquatic environment polluted by plastic wastes. It would be necessary to explore the impact of other environmental conditions in order to optimize the degradation of these polymers.