The effects of plastic leachate on marine Prochlorococcus
Marine plastic pollution is a worldwide, growing problem and has the potential to affect marine organisms in different ways. In addition to physical hazards related to plastic ingestion and entanglement, marine plastics can leach a variety of substances which adversely impact surrounding organisms, including microorganisms.
Photosynthetic marine bacteria play a crucial role in carbon cycling and other global biogeochemical processes. Despite this, there is currently virtually nothing known regarding how plastic pollution might impact these important microorganisms. Thus, this study aimed to investigate how leachate from common plastic items affects Prochlorococcus, a key marine photosynthetic bacterium which is highly abundant in many marine regions and contributes significantly to global oxygen production.
Firstly, I investigated how leachates from common plastic products affects in vitro growth and primary productivity of two strains of Prochlorococcus, NATL2A and MIT9312, representing model low light and high light adapted ecotypes respectively (Chapter 2). In this study, each strain was exposed to leachates from new high-density polyethylene (HDPE) plastic bags and polyvinyl chloride (PVC) plastic matting at a range of concentrations. Both leachates negatively affected growth and primary productivity in a dose-dependent manner, with the strains showing some differences in the extent and timing of their response to each plastic leachate.
Further investigations were performed to determine whether environmental weathering of plastic items altered leachate composition and toxicity, using the same two strains of marine Prochlorococcus (Chapter 3). Prochlorococcus NATL2A and MIT9312 were exposed to leachates derived from HDPE and PVC materials that had been weathered in estuarine water for 17 and 112-days, in parallel with unweathered leachates. This showed that the weathered HDPE and PVC continued to leach substances which negatively affect growth rates, photophysiology and membrane integrity but to a lesser extent than for equivalent leachates derived from unweathered material. Strain specific differences were again observed, with MIT9312 responding more strongly but more slowly than NATL2A, in terms of declines in photosynthetic efficiency and membrane integrity.
Zinc (Zn), a commonly used additive in plastic manufacturing, was among the elements most strongly enriched in leachates from unweathered and weathered HDPE and PVC. In order to investigate the role that this specific leachate component may have in leachate toxicity, I looked at the response of Prochlorococcus NATL2A and MIT9312 to Zn exposure, using a range of concentrations, and also tested closely related marine Synechococcus strains CC9311 and WH8102 (Chapter 4). The results from this study suggest that zinc likely contributes to the toxicity of plastic leachates observed in the earlier chapters. Together, my thesis shows that plastic leachates have the potential to negatively affect important marine picocyanobacteria and provides insight into substances likely to contribute to this toxicity.
