Nitratireductor and Oricola bacteria as moderately thermophilic polypropylene degraders in marine environments

Two bacterial strains degrading liquefied polypropylene (PP) at a moderately high temperature of 50°C were obtained after enrichment on liquefied PP in surface seawater supplemented with nitrogen, phosphorus, and iron nutrients at 50°C. The strains, designated R1 and R4-2, belonged to Nitratireductor and Oricola, respectively. These genera are phylogenetically close. Nitratireductor R1 or its close relatives were detected as the main bacteria (12% of the total bacteria) and Oricola R4-2 or its close relatives were also significantly detected (5%) in the liquefied PP-enriched seawater, suggesting that these bacteria utilized liquefied PP as a carbon source. Nitratireductor R1 showed liquefied PP-degrading activity at 50°C but not at 28-42°C, while Oricola R4-2 showed the activity at 42-50°C but not at 28°C. At 50°C, these two strains degraded liquefied PP of all lengths detected, isoprenoid-derived branched alkane (pristane), and n-alkane (n-hexadecane) to a similar extent, and also degraded aromatic compounds. Pristane and n-hexadecane appeared not to induce the PP-degrading activity. These results indicate that Nitratireductor and Oricola bacteria could be used in industrial applications for degrading liquefied PP and polyethylene and aromatic structures of liquefied aromatic-containing plastics at 50°C.