Bioremediation Techniques for Microplastics Removal

Plastics’ unique physical and chemical properties made them indispensable parts of our everyday life and technology. Due to the mismanagement of plastic wastes, 10% of global plastic production annually entering the ocean accounts for 60–80% of marine debris. With the current plastic production rate, more plastics will exist in the oceans than fish by 2050. Plastic waste does not decompose in nature, or its decomposition takes a long time. Among plastic contaminants, microplastics, which are plastic pieces less than 5 mm in size, have attracted much attention because of their potential risks to organisms’ lives. This chapter discusses plastic polymers, their types, and their features that affect plastics’ degradation. Here, we present the interaction between organisms and microplastics and their hazardous effects on living organisms. Bioremediation and biodegradation are explained. Also, new approaches in biodegradation, such as enzyme engineering, are introduced. Plastic polymers’ chemical and physical features such as molecular weight, molecular backbone’s atoms, chemical bonds, crystallinity, hydrophobicity, and additives presence are important factors in vulnerability to decomposing agents. Aging and weathering by abiotic factors including sunlight, heat, moisture, and oxygen decrease the microplastics’ surface hydrophobicity and facilitate microorganism attachments and biofilm formation. Microplastics, because of releasing toxic additives, metallic and organic toxic compounds’ adsorption on their surfaces, threaten organisms’ lives. Microplastics’ harmful effects on marine organisms, especially the primary producers’ food chains such as microalgae, can directly or indirectly influence food web consumers such as fish, aquatic birds, and even humans. Antibiotic adsorption on microplastics and, therefore, enrichment of potentially pathogenic and antibiotic-resistant bacteria and antibiotic-resistance genes through horizontal gene transfer are other microplastics-related concerns. Following the biofilm formation, microorganisms’ activity and their secreted enzymes and agents deteriorate the microplastics and lead to molecular fragmentation and depolymerization. Assimilation and mineralization of the fragmented molecules are the last biodegradation steps that give rise to CO2, H2O, CH4, and biomass production. Some genus and species of fungi and bacteria and their powerful enzymes such as oxidoreductases and hydrolases are key players in bioremediation by microorganisms. Electron microscopy, spectroscopy techniques, weight loss measurements, mechanical properties, molar mass changes, CO2 evolution/O2 consumption, radiolabeling, clear-zone formation, enzymatic degradation, and controlled composting test are employed for biodegradation evaluation. Since more than 99% of prokaryotes and some eukaryotic microbes are unculturable, hence, to select plastic-decomposing microorganisms, culture-independent methods, i.e., metagenomic analysis, are utilized. The metagenome analysis and in silico mining lead to a deeper investigation of the explored and unexplored nature to find efficient enzymes and microorganisms for microplastics’ bioremediation. Using microbial consortia and engineered microorganisms and their enzymes are other promising approaches for plastics bioremediation.