Ecotoxicological impacts of micro and nanoplastics on bivalve molluscs.
Supervisor: Matthew Cole (Plymouth Marine Laboratory, UK)
Microplastic (MP) and nanoplastic (NP) pollution in the ocean is an ecological issue of growing worldwide concern. Over the last fifteen years ecologists have studied the ecotoxicological impacts of MP/NPs throughout different organisms (e.g. fish, marine mammals, and invertebrates). Field investigation is insufficient to understand the effects of MP/NPs, as in nature they interact with other chemicals resulting in more complicated health impacts, thus experimental studies are conducted under controlled conditions to avoid any chemical interaction with MP/NPs. Two approaches were used in this study. Initially my project focused on investigating histological and histopathological impacts of two different shapes of MPs (particles and fibers) on adult mussels (Mytilus spp.); I was trained to use the required equipment and instruments (e.g. cryotome, microscopes and FT-IR). Owing to the Covid-19 pandemic, the UK government imposed a lockdown in March 2020, banning all direct contact and shutting schools, universities and research institutes including Plymouth Marine Laboratory (PML). Therefore, experimental work was cancelled, and a dry project was sought by data-mining the most up-to-date literature on the ecotoxicological impacts of MP/NPs on bivalve species (Mussels, Clams and Oysters) to perform a systematic review. Data was extracted from 30 studies. 90 biological endpoints were identified and put together in 8 categories: genotoxicity, neurotoxicity, reproduction, antioxidation system, digestive system functions, fitness, immune response, cellular toxicity. Among the identified endpoints, 80% were significantly affected during MP/NPs exposure. Although genotoxicity category has been investigated only in 7 publications, it revealed the highest significant impacts (95%), that was mainly expressed by DNA tail%. On the contrary, neurotoxicity category did not show any negative effects in all treatments. To differentiate between possible effects of MP/NPs, dataset was divided into classes, based on particles’ characteristics: size, shape and concentration. Chi-square test was applied for inter classes comparison, using categorical data, that based on significance of impacts (p < 0.01 or 0.05) which were estimated in the reviewed studies. Data analysis revealed significant differences (p < 0.01) among size classes, as well as all concentration classes; No significant differences were found between the two shape classes (p > 0.01). Heatmapswere created to facilitate visualization of ecotoxicological differences among MP/NPs classes and to confirm results of Chi-square test. NPs showed the highest impacts among all size classes and 106-108 particles.L-1 have triggered the highest levels of damage among concentration classes. Although in general MP/NPs shape did not reveal statistical significance, microbeads showed more toxic effects than microfibers corresponding to most of the biological endpoints. This result might be attributed to the lack of publications investigated microfibers. Based on this review, suggestions for future research directions were proposed, to cover all MP/NPs types, shapes, sizes and concentrations, aiming at better understanding of ecotoxicological impacts on bivalve species.