Physical, chemical and biological processes and fates of petroleum-based plastic and bioplastic pollutants in aquatic environments
Creators: name: Gundry, Taylor
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 310703
subject: Microbial ecology
classificationCode: 4104
subject: Environmental management
classificationCode: 410404
subject: Environmental management

description: By the mid to late 20th Century the use of petroleum-based plastics had become widespread. Much of this plastic has been and continues to be littered, leading to plastic pollution becoming ubiquitous in marine environments. Plastic pollutants can cause physical harm to marine organisms, via entanglement or ingestion. Persistent organic pollutants (POPs) such as some brominated flame retardants (BFRs) are known to accumulate onto plastic pollutant surfaces in marine environments, and may provide a novel pathway for exposure to these chemicals to organisms. Additionally, plastic surfaces containing microbial biofilms have been suggested as a vector for the transport of harmful algae and pathogens beyond their natural ranges. In recent decades, bioplastics (plastics derived from biological based materials) have been developed and utilised as an alternative to petroleum-based plastics. However, the environmental fates of bioplastic pollutants, the processes of BFR accumulation and biofilm development onto bioplastics remains undetermined.<br><br> This thesis sought to advance current knowledge of the fate of pollutant petroleum-based plastics and bioplastics within aquatic ecosystems. This was addressed via an experimental approach in which polypropylene (PP) as a model petroleum-based plastic, polylactic acid (PLA) as a model bioplastic and glass slides as non-plastic control substrate were deployed in an exposure experiment at five sites along a freshwater-marine continuum of the Yarra River into Port Phillip Bay, Melbourne, Australia. The three specific objectives were to; compare variation in the structural properties of PP and PLA, via analysis of surface hydrophobicity, tensile strength, crystallinity and chemical structure (Chapter 3); determine the potential for BFRs to accumulate onto PP and PLA with comparison to glass substrates (Chapter 4); and compare spatial-, temporal- and substrate-specific (PP, PLA and glass) variation in the structure and composition of microbial (prokaryotic and eukaryotic) biofilm communities forming on polymer and glass surfaces and with comparison to those in the surrounding water (Chapter 5).<br><br>Research in Chapter 3 revealed that between Day 1 and Month 12 of the exposure experiment there were no significant changes in either the water contact angle (WCA) (a proxy used to assess surface hydrophobicity), or Young's Modulus (a measure related to tensile strength) for either PP and PLA substrates. There was no overall trend of an increase of the Max load at break (Max Load) (a measure related to tensile strength) of the PLA substrates between the other sampling dates. However, there was a significant increase (P &lt; 0.05) of the Max Load for the PLA substrates but not the PP substrates. Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) (crystallinity) analysis of the plastics were undertaken to compare polymers between Day 1 and Month 6. There was an increase in the crystallinity of the PLA substrates but not for the PP substrates. Neither the PP nor the PLA substrates exhibited any change in FTIR spectra between Day 1 and Month 6 and indicated no change in the chemical structure of either plastic type.<br><br>Research in Chapter 4 investigated accumulation of two groups of BFRs onto the PP, PLA and glass substrates. The targeted BFRs were polybrominated diphenyl ethers (PBDEs) and novel brominated flame retardants (NBFRs). The selected BFR analytes were extracted from the PP and glass substrates using a selective pressurised liquid extraction (S-PLE) method. A novel dual vortex and sonication method was successfully developed and implemented for the extraction of BFR analytes from the PLA substrates. Analysis of the selected BFRs was undertaken using an Agilent 7000C gas chromatograph coupled to a triple quadrupole mass spectrometer (GC-MS/MS). Differences in BFR concentrations between the three substrate types was not able to be assessed due to sample loss from sample frames over the course of the experiment. At least one PBDE congener and one NBFR compound were detected in all samples, although the mean ∑PBDE and ∑NBFR concentrations on the substrates were low, 12.3 ng g-1 ± 7.4 ng g-1 and 23 ng g-1 ± 23 ng g-1, respectively.<br><br>Research in Chapter 5 explored structural and compositional changes in the prokaryotic and eukaryotic microbial biofilm communities as well as water communities via high-throughput DNA amplicon sequencing of the 16S and 18S rRNA genes, respectively. The structure of the microbial biofilm communities on substrates were distinct from those in the surrounding water environment and differed principally with sample site, and then with sampling date. There was no significant (P &gt; 0.05) difference in the composition of microbial biofilm communities between any of the three substrate types. The prokaryotic biofilms communities were dominated by Proteobacteria (alpha-, beta-, and gamma- classes) and Bacteroidetes, and the eukaryotic biofilm communities were dominated by diatoms and ciliates. Both the prokaryotic and eukaryotic water microbial community types had higher mean numbers of observed operational taxonomic units (OTUs) and Shannon diversity when compared to the coupon-biofilm communities. The relative abundance of three key functional guilds of potential plastic degraders, pathogenic bacteria and harmful algae were assessed. None of these three functional guilds had relative abundances greater than 1 % of the overall community; although three fish pathogens Pseudomonas anguilliseptica, Acinetobacter johnsonii and A. lwoffii, were frequently identified, being detected in over two thirds of the biofilm communities.<br><br>This research has shown that PLA is as physically- and chemically- stable as PP over a 12- month period in aquatic environments. It was hypothesised that substantial degradation of the plastics did not occur because the plastics were quickly biofouled within days from deployment, and that biofouling in water would have reduced the rate of photo-oxidative degradation, one of the main degradation processes to occur to plastics in natural environments. PLA, PP and glass substrates were all found to have the capability to accumulate PBDEs and NBFRs. The plastic biofilm communities were shown to be diverse and distinct from those in the surrounding water communities, and the plastic biofilm (both PP and PLA) communities were highly similar to those forming on glass, demonstrating that plastic biofilm communities consists of predominantly generalist surface colonisers. Three fish pathogens (P. anguilliseptica, A. johnsonii and A. lwoffii) were frequently identified within the substrate biofilm communities, indicating that aquatic plastic debris may be a long- term novel exposure pathway for pathogen exposure in fish due to the high number of plastic fragments in aquatic environments, and the ability of plastics to passively travel vast distances. The lack of plastic degrading organisms identified on the plastics raises doubts that PLA, will be biodegraded to any significant extent in aquatic environments. Therefore, bioplastics should be held in the same regard as petroleum-based plastics by government, policy makers and industry leaders as they work towards solutions that reduce the impacts of both petroleum-based plastics and bioplastics within aquatic ecosystems.