Microbial colonization, biodegradation, and photodegradation of marine plastics in (sub)tropical waters
Authors:   , Goudriaan Maaike 
Published: August 2024

Plastic pollution in the world’s oceans is a critical issue with wide-reaching effects on marine life, human health, and economies dependent on coastal tourism and fisheries. The problem extends beyond visible impacts to disrupt marine microbial life and biogeochemical processes. Plastics, primarily made from petrochemicals, are integral to modern life, but their increased use has led to significant plastic waste, particularly in the form of commodity polymers like polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), and polyamides (PA, e.g., nylon). These materials make up a large portion of marine debris, originating from both land and sea-based activities, such as fishing and maritime transport.

Plastic waste accumulates globally due to inadequate waste management, leading to the widespread presence of potentially toxic plastic products and their derivatives in terrestrial and aquatic environments. While most research has focused on plastics floating on the ocean’s surface, it is suspected that significant amounts also exist in the deep sea, either suspended in the water column or on the seafloor. Processes like biofouling, where microbes colonize plastic surfaces, play a role in moving plastics from the surface to deeper waters. However, the specific microbial communities that colonize different types of plastics and their role in plastic degradation remain poorly understood.

Plastics persist in the ocean for hundreds of years, presenting long-term challenges even if plastic emissions were to stop. Marine plastic debris undergoes both abiotic and biotic breakdown processes, which cause fragmentation. Abiotic degradation is driven by factors like UV light and mechanical stress, while biodegradation involves microorganisms that remove plastic-derived carbon from the environment. The stages of microbial plastic degradation include biodeterioration, biofragmentation, molecule uptake and assimilation, and energy gain through mineralization. However, current analytical methods struggle to differentiate between these processes or to accurately measure them.

Research outlined in this thesis aimed to better understand the fate of marine plastics in (sub)tropical coastal waters, focusing on bacterial colonization, biotic and abiotic degradation, and improving methods to study biodegradation. Fieldwork was conducted in the Caribbean and Mediterranean Seas, areas severely affected by plastic pollution. The studies examined the effects of polymer type, environment, and time on plastic-associated bacterial communities using techniques like 16S rRNA gene amplicon sequencing. It was found that bacterial communities on plastics differ by polymer type initially but become more similar over time, with bacteria capable of degrading hydrocarbons playing a significant role in shaping these communities.

A key finding was that habitat and location have a greater influence on microbial community development on plastics than polymer type. The use of isotopically labeled plastics in experiments helped trace plastic-derived carbon into microbial biomass, providing insights into microbial assimilation and degradation of plastics. Additionally, the research highlighted the significant role of UV-induced photodegradation in the removal of marine plastics, though the ecological impacts of the degradation products remain uncertain.

Overall, the research underscores the complex interactions between plastics and marine microbial communities, the importance of environmental factors over polymer type, and the need for improved methodologies to study plastic degradation. The findings contribute to a better understanding of the long-term persistence and ecological impacts of plastics in the ocean.