Τύχη των πλαστικών και μικροπλαστικών στο θαλάσσιο περιβάλλον

The ease of use and versatile properties of plastics have turned them from the novelty product they were 80 years ago to one of the most non-replaceable families of materials of our time, with predictions for increased demand in the following years despite recent bans. In 2019 alone, 640 Mt of plastics were used. Various types of plastics have been developed, from simple polyolefin, such as polyethylene (PE) and polypropylene (PP) to more complex, such as polystyrene (PS) and polyethylene terephthalate (PET). Traditional fossil-based plastics account for 8% of the global petroleum production. The demand for single use plastic materials, mostly used for packaging, understandably contributes to the production of vast amounts of plastic waste. In 2020, 29.5 Mt of plastic waste was generated in the EU alone. Worldwide, 22% of the plastic waste is mismanaged, ending up in landfills or being discarded, and eventually ending up in the environment, especially the seas. Mismanaged plastic land originating waste corresponds to 80% of the plastic waste entering the marine environment. There exist disagreements between the estimated amounts of plastic in the seas. Model outputs are significantly lower than calculations based on waste production and management statistics. That could be attributed either to a combination of model underestimations and plastic waste input overestimations, or to the fact that during their residence in the marine environments interact with it in ways that result in “obscuring” it. For instance, owing to their inherent density, or due to the attachment of biological factors (biofouling), plastics can move vertically within the water column and float or settle to the benthos. The effect of the UV fraction of solar radiation, heat and microorganisms can lead to photodegradation, thermodegradation or biodegradation, while mechanical stress can result in the breakdown of plastics to smaller pieces (fragmentation). While plastic particles of smaller sizes enter the ocean directly (primary partilces), fragmentation results in the formation of secondary microplastics (<5mm) and nanoplastics (<1 μm). All plastics have negative impacts on the environment and the health of marine life, but microplastics and nanoplastics can enter more organisms and in more than one way and have more pronounced impacts on the health of the receiving biota. For that reason, it is important to understand the fate of plastics and microplastics in the marine environment, what are the processes that affect them and their interactions. The aim of this dissertation is to attempt to study these processes as they occur in the marine environment, so that their role and contribution to the fate of plastics can be better understood. That was achieved in a series of microcosm, mesocosm and field experiments. These allowed the study of the plastic polymers themselves, as well as their interactions with radiation and marine organisms, the establishment of the baseline of the south-eastern Mediterranean plastic colonizing community and the examination of their effect on the polymers. The multiple scale of the experiments revealed whether it is possible to simulate the marine environment in the sea and extrapolate the results of smaller scale experiments in the actual environment. In the microcosm experiment, secondary low-density polyethylene (LDPE) and high-density polyethylene (HDPE) microplastics were incubated with two marine communities. One of the communities (Souda) was wild and the other (Agios) had previously been acclimatized to utilize plastics as a carbon source. Over the 120 days of the experiment, biofilm was formed on the surface of the microplastics of both treatments, revealing that both communities had the potential to survive with weathered plastics as the sole carbon source. Examination of the plastic particles with FTIR (Fourier-transform infrared spectroscopy) revealed chemical changes concurrent with biodegradation. Dynamic light scattering (DLS) allowed the observation of plastic particles with sizes between 56 nm and 4.5 μm and confirmed that biodeterioration and biofragmentation, prerequisite processes for biodegradation, occurred in the microcosms. Having established that prior acclimatization is not required for the utilization of plastics by microorganisms, polypropylene (PP) pellets with and without prior artificial weathering were incubated in mesocosms non-acclimated marine communities under semi-realistic environmental conditions for 180 days. Bacterial attachment and biofilm formation were observed in this experiment, as well. Surface changes, observed with ATR-FTIR (attenuation total reflectance Fourier-transform infrared spectroscopy) revealed that also in that setup the polymer was altered as a result of solar radiation and the activity of microorganisms. No microscopic particles could be detected during dynamic light scattering (DLS) examination. Measurements of zeta-potential and colloid particle hydrodynamic diameter contained in the seawater implied that any microplastic or nanoplastic particles produced were trapped in marine aggregates. Intriguingly, a phase shift could be observed between the mesocosm containing the virgin and artificially weathered pellets, in terms of number of viable cells, in accordance with the chemical alterations of the surface of the pellets. It was thus shown that while weathered polymer substrates are readily available for biodegradation, leading to the removal of the affected layer to reveal fresh virgin polymer, virgin polymers must first undergo weathering through abiotic processes to be able to act as carbon source for marine communities. The vertical movement of plastic particles in the water column was examined over a 300-day incubation period in the bay of Souda. 5 types of plastic films (PS, PET – denser / LDPE, HDPE, PP lighter than seawater), and 3 types of plastic pellets (LDPE, HDPE, PP), before and after weathering, were examined. The accumulation of biofouling on the surface of the samples was studied. Simultaneously, their sinking velocity was determined, using a novel semi-theoretical methodology. The sinking velocity of the same plastics, after the biofouling had been removed allowed the establishment of the determining effect of biofouling on the sinking characteristics of the plastic samples. The removal of biofouling by two severe storm events allowed the examination of removal and reattachment of the biofouling agents and the better understanding of its effect on sinking and allowed the correlation of sinking velocity with the biofouling quantity. Density and sample form were found to play a role in the sinking behavior of the plastics, while weathering did not significantly affect the sinking fate of the pellets. Thus, semi-empirical mathematical expressions of sigmoid nature were proposed for the description of both the fouling development rate, as well as the changes in sinking velocity as it progressed.The same plastics used for the examination of their sinking characteristics were examined at four timepoints (35, 152, 202 and 242 days) for the fungal and bacterial community composition and succession via next generation sequencing, along with the chemical changes of their surface to investigate biodegradation. Bacterial communities were found to be more diverse and variable, dominated by Proteobacteria and Bacteroidetes. Fungal Ascomycota, which have been found to dominate epiplastic communities in the literature and potentially biodegrade plastics, dominated communities were more stable. Biodegradation was not actively confirmed in this work. The effect of one of the storm events which affected the samples for sinking behavior examination also affected the samples used for this chapter, allowing the determination of the effect of stochasticity on the plastisphere communities. Plastic type did not play a significant role towards the determination of the plastisphere community, in contrast with the stage of biofilm development along with the stochastic effect of the storm-induced biofouling removal. In conclusion, it was determined that microcosms and mesocosms are effective tools of studying plastic biodegradation, since their small volume and high concentrations allow the examination of parameters which the infinite dilution of the sea would deem impossible. The cyclical pattern of weathering and de-weathering by microorganisms was confirmed in all three scales employed here. The unreliable nature of weight measurements as a biodegradation metric was demonstrated, unless thorough sample pre-treatment has preceded. The determining effect of biofouling on the sinking behavior of samples of various sizes and densities was shown, which until now, had only been speculated. The south-easter Mediterranean plastisphere, and much more so the fungal side of it, was studied for the first time, implying that further and deeper study is needed, since plastic biodegrading taxa were observed. However, since biodegradation could not be proven in the field scale experiment, it is natural to assume that plastic biodegradation in the marine environment is minimal. Finally, while the results from scale to scale are transferrable, the process of translating lab or mesocosm derived results must be realized with extreme caution.Η πληθώρα χρήσεων και η ευκολία στη χρήση που προσφέρουν τα πλαστικά τα ανέδειξαν από τα καινοτόμα προϊόντα που ήταν πριν 80 χρόνια σε μια από τις αναντικατάστατες ομάδες υλικών της εποχής μας, με προβλέψεις για αύξηση της ζήτησής τους, παρά τις πρόσφατες απαγορεύσεις. Το 2019 μόνο, χρησιμοποιήθηκαν 640 Mt πλαστικών. Διάφοροι τύποι πλαστικών έχουν εμφανιστεί, από τις απλές πολυολεφίνες, όπως το πολυαιθυλένιο (PE) και το πολυπροπυλένιο (ΡΡ) ως πιο πολύπλοκοι, όπως το πολυστυρένιο (PS) και το τερεφθαλικό πολυαιθυλένιο (ΡΕΤ). Τα παραδοσιακά πλαστικά που αποτελούν παράγωγα του πετρελαίου αποτελούν το 8% της παγκόσμιας παραγωγής πετρελαίου. Η απαίτηση για πλαστικά μιας χρήσης, που χρησιμοποιούνται κυρίως ως υλικά συσκευασίας, συντελούν στην παραγωγή τεράστιων ποσοτήτων πλαστικών αποβλήτων. Το 2020 παράχθηκαν 29,5 Mt πλαστικών αποβλήτων στην ΕΕ μόνο. Παγκοσμίως, το 22% των πλαστικών αποβλήτων δεν χαίρει ορθής διαχείρισης και καταλήξει σε χωματερές ή απλώς απορρίπτεται και καταλήγει στο περιβάλλον, ειδικά στη θάλασσα. Κακώς διαχειρισμένα πλαστικά απορρίμματα με προέλευση από τη στεριά αντιστοιχεί στο 80% των πλαστικών αποβλήτων που καταλήγουν στο θαλάσσιο περιβάλλον. Διαφωνίες υπάρχουν για τις αναμενόμενες ποσότητες πλαστικών στις θάλασσες. Τα μοντέλα δίνουν σημαντικά χαμηλότερες τιμές από αυτές που υπολογίζονται στατιστικά, λαμβάνοντας υπόψιν την παραγωγή αποβλήτων και τη διαχείρισή τους. Αυτό θα ήταν δυνατό να αποδοθεί είτε σε ένα συνδυασμό υποεκτίμησης από πλευράς μοντέλων και υπερεκτίμησης των εισροών πλαστικών, είτε στο γεγονός ότι κατά την παραμονή τους στο περιβάλλον τα πλαστικά αλληλεπιδρούν με αυτό με τρόπους που καταλήγουν στο να «κρύβεται» μέρος τους. Για παράδειγμα, λόγω της πυκνότητάς τους, ή της επικάθισης βιολογικών παραγόντων, τα πλαστικά μπορούν να κινηθούν κατακόρυφα εντός της υδάτινης στήλης και να καταλήγουν να επιπλέουν ή να βυθίζονται. Η επίδραση του υπέρυθρου (UV) κλάσματος της ηλιακής ακτινοβολίας, της θερμότητας και των μικροοργανισμών μπορούν μα οδηγήσουν σε φωτο-, θερμο- ή βιοαποδόμηση, ενώ η μηχανική καταπόνηση μπορεί να καταλήξει στο θρυμματισμό σε μικρότερα κομμάτια. Μικρού μεγέθους πλαστικά σωματίδια μπορούν να εισέλθουν στους ωκεανούς ως έχουν παραχθεί (πρωτογενή), ωστόσο, ο θρυμματισμός συντελεί στο σχηματισμό δευτερογενών μικροπλαστικών, με μέγεθος μικρότερο των 5 mm, και νανοπλαστικών (