| dc.description.abstract | In this work, two isolates, Achromobacter mucicolens (A2) and Bacillus paramycoides (A17), isolated from naturally weathered plastic enrichment cultures, were studied genetically, morphologically, biochemically and for plastic biodegradation capability. Characterization studies revealed a Gram-variable, rod-shaped A2 strain, with growth ranges of pH 5-9, 30-37°C, salt tolerance of 1-8%, and produced circular, crateriform, smooth, mucoid, and transparent punctiform colonies with entire margins. Strain A2 showed the ability to utilize few carbon substrates, could grow in the presence of multiple antibiotics and chemical inhibitors, and showed evidence of swimming motility in 0.03% semisolid agar down to 4°C, as well as the ability to form biofilms on polystyrene. The Bacillus species was revealed to be Gram-positive and rod-shaped, with growth ranges of pH 4-9, 22.5-48°C, salt tolerance of 1-8%, and produced circular, crateriform, smooth, wrinkled, and pigmented milky colonies with entire margins. Strain A17 showed the ability to utilize many carbohydrate substrates and showed less resistance to chemical sensitivity assays than A2.
Genetic enzymatic potential for the ability to complete the plastic biodegradation pathways of polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) was assessed for both isolates. Both strains showed enzymatic potential to complete the degradation process for PE, PP and PVC from polymer to polyhydroxyalkanoate (PHA). A 45-day biodegradation experiment of PE, PP, and PVC with pure cultures and a co-culture treatment showed successful biodegradation of all three plastics, with the best degradation being of PE, followed by PP, and the least degradation of PVC. As the molecular structure of the polymers contain additional components (PE being the simplest with just a hydrocarbon backbone, then PP with an additional methyl group, and PVC having the most components with an additional chlorine group) all treatments showed a decrease in degradation efficiency. This data supported the hypothesis that with additional components to the polymer structure the biodegradation will be more difficult.
The plastic biodegradation study showed more plastic weight loss by strain A2 than strain A17, and the co-culture treatments consistently showed more weight loss than strain A17 as well as more weight loss of PE than both pure cultures. Additionally, A17 showed a decrease in weight loss that was significantly larger than the decrease of the A2 as the plastic polymers contained additional components. FTIR analysis showed significant differences in functional group changes of PP and PVC plastics, while PE had nearly identical FTIR peak patterns between the treatments and the control. This could be due to the strains surpassing the oxidation step, which is the most energetically taxing step of the biodegradation pathway, or due to functional group changes not being evident in every plastic particle. The GC chromatogram data provided insight to which strain was the largest contributor to the plastic biodegradation in the co-culture treatments. The co-culture PE GC chromatogram was the most similar to A17’s PE GC chromatogram, indicating A17 was most likely the larger contributor to the degradation in the co-culture. However, the co-culture GC chromatograms of PP and PVC were both more similar to strain A2’s GC chromatograms, indicating A2 was most likely the larger contributor to the degradation of PP and PVC in the co-culture treatments. This followed the trend of the weight loss, as A17 showed a large decrease in weight loss of PP and PVC.
Overall, A2 showed a better potential for plastic biodegradation application than A17, with the ability to form biofilms on a plastic surface, a limited ability for carbon substrate utilization, chemical resistance to many chemical stressors, and a higher rate of plastic biodegradation regardless of plastic polymer structures of PE, PP and PVC. | en_US |
