SYNERGISTIC PLANT-MICROBE INTERACTION IN MODULATING MICRO/NANO PLASTIC DEGRADATION FOR SUSTAINABLE ECOSYSTEM

Abstract

Contaminated soil is one of today's most difficult environmental issues, posing serious hazards to human health and the environment. Contaminants, particularly micro-nano plastics, have become more prevalent around the world, eventually ending up in the soil. Numerous studies have been conducted to investigate the interactions of micro-nano plastics in plants and agroecosystems. However, viable remediation of micro-nano plastics in soil remains limited. A significant amount of leftover plastic from the extensive usage of plastic film mulch and effluents from surface runoff and industrial activities has accumulated and ultimately formed microplastics (MPs) in agricultural soils. However, it is uncertain how crops would be impacted by microplastics from plastic mulch film. In order to observe the effects of plastic fragments especially microplastics in plant and soil, the growth, physio-biochemical characteristics, and morphology of Brassica juncea (mustard plants) exposed to two types of HDPE microplastics – HDPE_MPs and HDPE_beads, were studied. Upon interaction with MPs and beads, the height, biomass, chlorophyll content, phenolic content and proline content of Brassica juncea plant were drastically lowered. This work emphasizes that MPs may have higher detrimental impacts for terrestrial ecosystems, which warrants additional investigation in future studies, and offers a fresh insight into the possible effects of MPs with varying biodegradability’s on soil-plant systems. Secondly, to observe the impacts of microplastics on wild plants, a simulated dump yard model was prepared studying impact of two different types of microplastics: high-density polyethylene (HDPE) and nylon-6,6, on tropical wild plants: Cynodon dactylon (L.) and Portulaca grandiflora. The effects of microplastics on the two plants were evaluated using confocal laser scanning microscopy for morphological inspection, antioxidant activity, chlorophyll content analysis, and biometrical parameters (root and shoot height, biomass v output). The uptake of microplastics by plant parts could be observed through the symplastic and apoplastic pathways. The morphological studies could confirm the mechanism of uptake within plant parts. The accumulation of microplastics within the root and aerial parts of leaves could provide a phytoremediation strategy by phytoextraction of microplastics. Mechanisms showing the uptake of MNPs by plants is demonstrated by explaining the apoplastic and symplastic pathways. The major accumulation occurs in the root hairs and aerial parts of leaves thereby showing a phytoextraction strategy. Finally, synergistic plant-microbe interaction was studied to determine the capability of soil microbes in degrading microplastics and also harnessing the plant nutrition and growth. For this, the isolation of soil microorganisms was carried out using metagenomics sequencing to identify the bacterial strain that showed the most degradation efficiency. Also, two other microplastics, PP and PVC, were taken for the research study to ascertain the importance of bacterial isolate for microplastic degradation. To confirm microplastic degradation by the isolated bacterial strain, Acinetobacter baumannii, both the microplastics were subjected to FTIR analysis, thermogravimetric study, weight loss % for a span of 50 days and morphological characterization to observe the changes in the structure of microplastics post bacterial inoculation. The results confirmed degradation efficiency in both the microplastics stating the efficacy of microbes for microplastics elimination for sustainable ecosystem. These results conclude the effectiveness of isolated bacteria in microplastic degradation and potentially leading to the development of more effective and sustainable solutions for managing plastic waste. This thesis is summarized in five chapters: • Chapter 1 discusses a brief introduction about agricultural pollution of microplastics and its remediation technologies. It also talks about the sources and impacts of microplastics on human health and surrounding ecosystem. • Chapter 2 outlines the materials and methods involved in carrying out the objectives of the research study. • Chapter 3 focuses on results and discussion for the objectives designed for the research. vi • Chapter 4 focuses on the summary, conclusion and future scope of the research study. • Chapter 5 discusses the references that were used in the research study. To better comprehend the finding of this research, future insights on live imaging of microplastics within plant parts could provide substantial information on phytoaccumulation of microplastics. The buildup of microplastics in soil is the last point that needs more attention. The remediation potential of soil could be determined by analyzing the amount of microplastics left over after accumulation in plants. Overall, it can help in the sustainable remediation of soil containing microplastics in nearby groundwater system for cleaner environment.