Plastic pollution has become a pervasive environmental issue, with microplastics (MPs) and nanoplastics (NPs) emerging as significant contaminants in aquatic ecosystems. Defined as plastic particles less than five millimeters in diameter, MPs originate from both primary sources, such as microbeads in personal care products, and secondary sources, resulting from the fragmentation of larger plastic debris. The River Thames, one of Europe's most prominent urban waterways, serves as a critical case study for exploring the dynamics of MP pollution and its ecological ramifications.
The proliferation of MPs in surface waters has garnered attention due to their potential to act as vectors for pathogenic bacteria and toxic compounds. This vectoring capability raises concerns about the exacerbation of waterborne diseases, the spread of antimicrobial resistance, and the bioaccumulation of hazardous substances within aquatic organisms. The intricate interactions between MPs, microbial communities, and environmental pollutants necessitate a comprehensive understanding to mitigate the adverse impacts on both ecosystem health and human well-being.
This dissertation delves into the multifaceted role of MPs and NPs as carriers of pathogenic microorganisms and toxic substances in the River Thames. By examining the formation and composition of microbial biofilms on plastic particles, the study aims to elucidate the mechanisms by which MPs facilitate the transport and persistence of harmful agents in aquatic environments.
Microplastics have been identified as ubiquitous pollutants across various aquatic habitats, including marine, freshwater, and estuarine environments. Their small size and lightweight nature enable MPs to be easily transported by water currents, leading to widespread distribution and accumulation in sediments and biota. Studies have reported high concentrations of MPs in urban rivers like the Thames, which act as conduits for plastic debris from inland sources to coastal zones.
MPs provide unique substrates for microbial colonization, facilitating the formation of biofilms that consist of diverse bacterial communities. These biofilms can harbor both benign and pathogenic bacteria, creating microenvironments that favor microbial survival and proliferation. The surface properties of MPs, including their hydrophobicity and surface texture, influence the extent and composition of microbial colonization.
MPs possess the ability to adsorb various environmental pollutants, including heavy metals, persistent organic pollutants (POPs), and endocrine-disrupting chemicals (EDCs). The high surface area-to-volume ratio of MPs enhances their capacity to concentrate these toxic substances, which can then be transported across different compartments of aquatic ecosystems. This accumulation not only increases the toxicity of MPs themselves but also facilitates the bioavailability of pollutants to aquatic organisms that ingest or come into contact with plastic particles.
Comprehensive sampling was conducted along various points of the River Thames, including the Royal Docks, to capture spatial variations in MP distribution. Both water and sediment samples were collected systematically over a 12-month period to account for seasonal fluctuations. Neuston nets with a mesh size of 300μm were employed for surface water collection, while sediment samples were obtained using Van Veen grab samplers to ensure representative sampling of the benthic environment.
The isolation of MPs from environmental samples involved density separation using zinc chloride solution (density 1.6 g/cm³) followed by filtration through graduated sieves. Extracted particles were subjected to Fourier-Transform Infrared Spectroscopy (FTIR) for polymer identification and Scanning Electron Microscopy (SEM) for detailed surface morphology analysis. Microscopic examination further facilitated the assessment of particle size distribution and physical characteristics.
Biofilms on MPs and NPs were analyzed using membrane filtration techniques for bacterial isolation, followed by culture-based methods for pathogen identification. Advanced molecular techniques, including 16S rRNA gene sequencing, were utilized to characterize the diversity and composition of microbial communities. Antibiotic susceptibility testing was conducted to evaluate the presence of antimicrobial-resistant strains within the biofilms.
The assessment of toxic compound accumulation on MP/NP surfaces employed Gas Chromatography-Mass Spectrometry (GC-MS) for organic pollutants and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for heavy metal detection. Persistent Organic Pollutants (POPs) and other hazardous substances were quantified to determine their concentration gradients and potential environmental risks.
The analysis revealed a predominance of polyethylene (PE) and polypropylene (PP) particles, consistent with their widespread use in consumer products. Particle sizes ranged from 0.3 to 5mm, with significant weathering and surface degradation observed under SEM. These altered surfaces provided enhanced sites for microbial attachment, promoting biofilm formation.
Biofilm analysis uncovered a diverse array of bacterial species, including potentially pathogenic taxa such as Vibrio spp. and Escherichia coli. The presence of antibiotic-resistant strains was notably higher in biofilms compared to planktonic bacterial populations. The microbial communities exhibited metabolic capabilities for hydrocarbon degradation, indicating potential roles in MP biodegradation processes.
MPs and NPs demonstrated significant accumulation of heavy metals, including lead, cadmium, and mercury, as well as organic pollutants like polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). The concentration of these toxic compounds on plastic surfaces often exceeded that found in the surrounding water and sediment matrices, highlighting MPs' role as concentration agents for environmental contaminants.
The structured surfaces of weathered MPs provide ideal habitats for microbial colonization, promoting the formation of complex biofilms. These biofilms not only enhance the persistence of bacteria in aquatic environments but also facilitate horizontal gene transfer, potentially accelerating the spread of antibiotic resistance. The ability of MPs to support diverse microbial communities underscores their role as keystone structures influencing aquatic microbiomes.
MPs serve as effective vectors for pathogenic bacteria, enabling their transport across extensive distances within river systems. The association of pathogens with MPs increases their resilience to environmental stressors, such as UV radiation and predation, thereby enhancing their survival and dissemination. This vectoring capacity poses significant risks for the transmission of waterborne diseases and the proliferation of antimicrobial-resistant pathogens in aquatic ecosystems.
The high surface area of MPs facilitates the adsorption of toxic compounds, effectively concentrating pollutants and increasing their bioavailability. Aquatic organisms ingesting MPs are exposed to elevated levels of contaminants, which can lead to bioaccumulation and biomagnification of hazardous substances within food webs. The synergistic effects of MPs, microbial pathogens, and adsorbed toxins exacerbate the overall toxicity and ecological impact of plastic pollution.
Understanding the interactions between MPs, microbial communities, and toxic compounds is essential for developing effective bioremediation strategies. Harnessing the metabolic capabilities of specific bacterial strains capable of degrading MPs offers potential avenues for mitigating plastic pollution. Additionally, addressing the sources of toxic compound emissions and enhancing wastewater treatment processes are critical components of comprehensive pollution management frameworks.
The research underscores the pivotal role of microplastics and nanoplastics as vectors for pathogenic bacteria and toxic compounds in the River Thames ecosystem. The intricate interplay between MPs, microbial communities, and environmental pollutants facilitates the transportation and persistence of harmful agents, amplifying their ecological and public health risks. Mitigating the adverse impacts of MP pollution requires an integrated approach that encompasses source reduction, advanced wastewater treatment, and the development of innovative bioremediation technologies. Future studies should focus on elucidating the long-term consequences of MP-mediated pollutant transport and exploring sustainable solutions to address this multifaceted environmental challenge.
This dissertation provides an in-depth analysis of the role of microplastics and nanoplastics as vectors for pathogenic bacteria and toxic compounds in the River Thames. By integrating multidisciplinary research approaches, the study offers valuable insights into the complex interactions between plastic pollution and aquatic ecosystems, highlighting the urgent need for effective mitigation strategies to safeguard environmental and public health.