Buoyant microplastics in the marine environment

Despite growing plastic discharge into the environment, researchers have struggled to detect expected increases of marine plastic debris in sea surfaces, especially at sizes <5 mm. These “missing plastics” not only sparked discussions about final sinks for such debris, but also about our lack of adequate methods to find and quantify smaller size fractions (<1 mm), which all could be contributing to the observed gaps in the plastic budget. Deep-sea sediments have been suggested as the final sink for microplastics (generally particles < 5 mm). The meta-analysis I present in this thesis (Chapter 2) highlights that in open oceans, microplastic polymer types segregated in the water column according to their density. Lower density polymers, such as polypropylene and polyethylene, dominated sea surface samples but became less abundant through the water column, whereas denser polymers (i.e. polyesters and acrylics) were enriched with depth. The need for methods better suited to quantify small microplastics in environmental samples has been flagged. Chapter 3 of this thesis details the optimisation and implementation of a protocol that allows high throughput detection and automated quantification of small microplastic particles (20–1000 μm) using the dye Nile red, fluorescence microscopy and image analysis software. The preliminary application of this protocol showed a power-law increase of small microplastics (i.e. <1 mm) with decreasing particle size in coastal sea surface water. This finding suggests that part of the “missing” plastic fraction may have been missed due to the inefficiency of traditional methods to quantify the smaller fraction of microplastics. On sea surfaces, plastic debris is rapidly colonized by a diverse community of microorganisms, and speculation arose about microbes using such plastics as a carbon source. In Chapter 4, I show that weathered polyethylene became enriched by distinct genera within the biofilms, but only during early stages of colonization (i.e. after 2 days) in coastal marine water. Given the lack of persistent enrichment over time, common non-hydrolysable polymers might not serve as an important source of carbon for mature colonizing communities and these mainly persist by consuming labile photosynthate generated by primary producers. Overall, this thesis shows that buoyant plastics appear to be more prevalent on sea surfaces than earlier research had suggested, and that plastic biodegradation is likely limited to a minor process that occurs within the biofilm, but can be sped up when combined with abiotic weathering.