Effects of polyethylene microplastics and addition of nitrogen sources on bacterial communities in agricultural soil and biodegradation efficiency by isolated bacterial consortia

Abstract

Polyethylene microplastics (PE MPs) are emerging pollutants in agricultural soil, and biodegradation is a potential solution where microbial enzymes attack the polymer chain to use it as a nutrient (the 1st pathway) with a recent study indicated nitric oxide to have a role in PE biodegradation via the pathway of nitrogen metabolism (the 2nd pathway). The objective of this study is to investigate the response of soil bacterial community to PE MPs (degradation efficiency, change in community, and enzyme activity) with and without nitrogen sources and to isolate the enriched consortia to confirm the possession of the 2nd pathway. Urea and nitrate were used as nitrogen sources and added to the microcosm of soil and 5% PE MPs at C/N of 10. The microcosm with nitrogen source enhances the biodegradation of PE MPs with a percent degradation of 28.262% (with nitrate) and 12.748% (with urea) compared to 10.125% (no nitrogen) after 60 days of incubation. In addition, the activity of total enzymes related to the 1st pathway by FDA hydrolysis increased in the condition with both nitrogen sources and PE MPs while the bacterial community is greatly changed, enriching Actinobacteria with Nocardia, Mycobacterium, Nocardioides, and Actinomadura as the key players. After that, enrichment in liquid media with soil samples from microcosms was performed, and the consortium with nitrate exhibited the highest PE MPs’ weight loss (10.334%), whereas the consortium with urea exhibited the least weight loss (6.682%) after 30 days of incubation. Consortia with nitrogen sources also showed nitric oxide production, which is the marker of the 2nd pathway. Community analysis showed that 2 key degraders were lost during enrichment, leaving only Nocardia and Mycobacterium but new key degraders of Proteobacteria phylum were also obtained which are Sphingopyxis, Dokdonella, and Pseudoxanthomonas. This study signifies that different conditions of PE MPs degradation yield different key degraders and supplementation of nitrogen sources can cause biodegradation of PE via both pathways, improving the degradation efficiency, and changing the community, while nitrate was found to be the best nitrogen source.