Macro- and micro-plastic ingestion and their impacts on marine biota

Abstract

Plastic production has rapidly increased in recent decades, exceeding 400 million tonnes annually, driven by factors such as population growth, urbanization, and industrialization, while the COVID-19 pandemic has further intensified reliance on single-use plastics. This high demand for plastic, coupled with its extended durability in the marine environment, has made it the most common type of litter found in the oceans. Plastics persist in the marine environment for extended periods, breaking down into smaller pieces undergoing mostly photodegradation and mechanical fragmentation. Plastics are often categorized by size in: macroplastics (> 25 mm), mesoplastics (5-25 mm) and microplastics (< 5 mm). Microplastics (MPs) are characterized by their ubiquity in the marine ecosystems, being detected in all environmental compartments: in the water column, the seafloor sediments, the beaches, or ingested by a plethora of marine species. Plastics, independently of their size, have emerged as a significant and growing threat to marine ecosystems and their resident wildlife.Marine organisms interact with plastic through both entanglement and ingestion, with a particular focus on the latter in the present study. The ingestion of plastics, particularly larger ones, can have serious consequences for marine life, including injury, malnutrition, and death. Plastics, including MPs, can also be dangerous due to chemical toxic substances carried on their surfaces either when manufactured or adsorbed while in the marine environment. These chemically loaded MPs can be consumed by smaller marine organisms like fish, sea urchins or mussels and potentially induce adverse effects related to the oxidative stress, biotransformation, genotoxicity, neurotoxicity etc. Recent studies suggest that changes in biomarker levels may be indicative of elevated MP ingestion by marine biota. Although information on plastic ingestion increases, their interaction with marine species in Greek waters is scarce and so is the assessment of the impacts of plastic ingestion on biota. Investigating the susceptibility of species with high ecological or commercial significance to plastics is crucial, in order to improve our understanding on the potential effects deriving from this interaction. To that extend, this thesis studies the alarming issue of plastic litter and its escalating threat to the marine environment and wildlife, with particular emphasis on the levels of macro and MP ingestion and their potential effects on various species in Greek waters. The first contribution of this dissertation, presented in the second chapter, focuses on the macro and MP ingestion in the loggerhead sea turtle Caretta caretta. This species is globally considered as a vulnerable species, with a decreasing population trend worldwide. Due to their wide distribution and their propensity to plastic ingestion, the loggerhead sea turtles have been recently proposed as suitable indicator species for monitoring the presence and impact of plastics at a basin scale in the Mediterranean Sea. In the Mediterranean basin, loggerhead interaction with marine litter has been reported, however, no information on plastics ingestion by sea turtles was available for Greek waters. To address this, stranded dead loggerhead turtles (n=36) were collected along the Greek coastline from a plethora of beaches in both the Ionian and Aegean Seas, revealing that 72% of these turtles had ingested plastic debris. On average, each turtle had consumed 7.94 ± 3.85 (SE) plastic items. A total of 286 plastic items were extracted from the turtles' gastrointestinal tracts and were categorised by size, shape, color, and polymer type. The dominant types of polymers were identified as polypropylene and polyethylene. Furthermore, variations in plastic ingestion among different life stages of the loggerhead specimens were explored. Expanding the dataset, in the study presented in Annex I, in 56 stranded dead sea turtles, including both loggerhead turtles (C. caretta) and green turtles (Chelonia mydas) from the North Aegean, South Aegean, Levantine, and Ionian regions of the Greek Seas, a total of 497 plastic items were detected, with over half of them being plastic sheets, primarily associated with single-use products. Notably, the highest levels of ingested plastic sheets were detected in turtles from the Levantine region, aligning with model simulations of floating plastic bags distribution. Polypropylene and polyethylene remained the dominant polymer types of plastic in all regions studied. This research provides essential evidence of plastic ingestion by sea turtles in Greek waters, shedding light on possible sources and high-risk areas for these vulnerable species.The third chapter of the present thesis centres on the MPs and their potential impact on benthic organisms, with a specific focus on sea urchins (Paracentrotus lividus), an herbivorous species, with wide distribution in the Mediterranean Sea. In many areas, wild populations of P. lividus are declining exponentially due to intensive harvesting for human consumption, as this species is the most important echinoderm consumed and its gonads are considered a delicacy. Sea urchins and seabed sediments samples were collected from three coastal areas of Zakynthos Island in the Ionian Sea, each exposed to different levels of anthropogenic pressures: the port of Zakynthos, the tourist area on the northeast coast and the Marine Protected Area (MPA). A consistent pattern in MP abundance, shape, and color in both sea urchins and seafloor sediment samples was revealed. While there were no significant effects of MP ingestion observed in terms of oxidative stress, neurotoxicity, and genotoxicity assessed by biomarkers in the sea urchins, a positive correlation between glutathione S-transferase (GST) activity and ingested MPs suggested a possible activation of the detoxification system in response to MP ingestion. The last contribution of this dissertation, presented in chapter four, investigates the ingestion of MPs by mussels (Mytilus galloprovincialis) from various marine environments and assesses the associated effects using biomarkers indicative of oxidative stress, biotransformation, genotoxicity and neurotoxicity. Mussels, being sessile organisms, are considered suitable as bioindicators for monitoring MPs pollution at local scale in near-surface waters along the coastline. The mussels' ecological importance coupled with their role in fisheries and aquaculture underscores the urgency of understanding the effects of MPs ingestion on these species. As filter-feeding organisms, they are vulnerable to MPs, but although their susceptibility to MPs ingestion has been widely studied in the Mediterranean Sea and globally, information on potential effects driven by the MPs ingestion in mussels from the field is scarce. To address this issue, mussels were collected from sites exhibiting distinct anthropogenic pressures, including the MPA at Zakynthos Island (Ionian Sea), a mussel farm in Nea Peramos and a fish farm in Sounion (Aegean Sea). The results revealed higher MPs ingestion in mussels from the fish farm, likely due to the use of plastic equipment. Biomarkers reflecting oxidative stress and biotransformation, as well as the integrated biomarker response (IBR) index, revealed significant physiological changes in mussels in relation to MPs ingestion levels. To verify whether biomarker responses in the field were driven by the MPs ingestion or other confounding environmental factors, an experimental setup was designed: mussels were exposed to known MPs concentrations excluding all other factors that could potentially influence biomarker responses. Additionally, exposing mussels to increasing concentrations of MPs would help clarify at which MPs load biomarker responses are induced, shedding light on the extent to which mussels are affected by MP loads. The findings of this dissertation highlight the pervasive issue of plastic pollution in Greek waters, emphasizing their adverse effects on marine organisms. From the alarming rates of plastic ingestion observed in loggerhead sea turtles to the significant biomarker responses in mussels from the site where MP ingestion levels were the highest, this research underscores the urgent need for comprehensive investigations into plastics ingestion across various marine species in Greek waters. By shedding light on the impact of plastics ingestion, this study not only contributes to the global understanding of plastic pollution but also serves as a first step towards establishing threshold limits for plastic ingestion to protect marine life. Moreover, the up-to-date methodologies employed, including field samplings as well as, laboratory experiments, provide valuable insights into the complexities of the interaction between plastics and biota in real-world scenarios. Overall, this research offers a significant contribution to the ongoing global effort to mitigate the adverse effects of plastic pollution on marine ecosystems.

Η διατριβή αυτή αποτελεί την πρώτη συνολική έρευνα σε σχέση με τα επίπεδα κατάποσης πλαστικού από θαλάσσιες χελώνες, αχινούς και μύδια, καθώς και τις επιπτώσεις της κατάποσης στα ελληνικά ύδατα. Τα ευρήματα αυτής της έρευνας δείχνουν ότι τα επίπεδα κατάποσης πλαστικών μεταξύ των τριών μελετημένων ειδών είναι συγκρίσιμα με αυτά που αναφέρονται στην παγκόσμια βιβλιογραφία και δείχνουν να επηρεάζονται από την εκάστοτε περιοχή και τις ιδιαίτερες συνθήκες που τη χαρακτηρίζουν. Στην παρούσα εργασία υπογραμμίζεται η σημασία της κατάταξης των πλαστικών σε κατηγορίες ορισμένων βασικών χαρακτηριστικών, όπως το χρώμα, το σχήμα, το μέγεθος, και ιδίως ο τύπος πολυμερούς, καθώς προσφέρουν ενδείξεις για πιθανές πηγές προέλευσης και συσχέτισης μεταξύ των πλαστικών που βρίσκονται στους οργανισμούς με αυτά που υπάρχουν στο θαλάσσιο περιβάλλον. Παρόλο που οι ενδείξεις των βιοδεικτών που χρησιμοποιήθηκαν δείχνουν διαφοροποιήσεις στις περιοχές με τα περισσότερα πλαστικά στους οργανισμούς, οι διαφορές στις τιμές των βιοδεικτών δε μπορούν να συσχετιστούν άμεσα με την κατάποση πλαστικού ούτε στη περίπτωση των αχινών ούτε σε αυτή των μυδιών. Περαιτέρω έρευνα σε οργανισμούς που εκτίθενται σε υψηλά επίπεδα ρύπανσης από πλαστικά και με τη χρήση επιπλέον βιοδεικτών, είναι απαραίτητη για να αποκτηθεί μια πιο σαφής κατανόηση των επιπτώσεων της κατάποσης πλαστικού στους οργανισμούς. Τα αποτελέσματα της διατριβής συμβάλλουν στη βαθύτερη κατανόηση της ρύπανσης από πλαστικά και των επιπτώσεων της στο θαλάσσιο περιβάλλον, στη θέσπιση ανώτερων ορίων για την κατάποση πλαστικών από τους θαλάσσιους οργανισμούς και κατ’ επέκταση στον μετριασμό των επιπτώσεών τους.