Microplastics are often presented in media as a single type of contaminant. However, researchers have been working for years to understand their diversity in polymer structure, chemistry, size, shape and weathering extent – to name a few. Truly, the most unifying aspect of microplastics may be their ubiquity and persistence in the environment! As such, an abundance of recent research has addressed the toxicity of different types of microplastics to biota, most measuring mortality and, to a lesser extent, reproductive output. The potential impacts of microplastics to systems or ecosystems are more difficult to elucidate, both due to the diverse contaminant nature and the importance of sub-lethal effects.

As a PhD student, I recently investigated the response of salt marsh sediment microbial ecosystems to microplastic pollution. These sediment microbiomes are critical in driving carbon utilization and nutrient turnover in coastal zones, where microplastic pollution can be introduced from coastal runoff, storm drains and wastewater effluent. Working with microbial ecologist Bongkeun Song, as well as my advisor and environmental chemist, Robert C. Hale, microbial community structure and functional implications for nitrogen cycling were probed using a microcosm incubation. Individual microcosms were amended with four diverse microplastics: polyethylene (PE), polyurethane foam (PUF), polyvinyl chloride (PVC) and polylactic acid (PLA), a ‘biopolymer’. To elucidate the ecosystem response, we both monitored the sediment microbial community structure (taxa and their abundance) and key aspects of the nitrogen cycle, nitrification and denitrification. In combination, nitrification and denitrification act as an important nitrogen removal processes for coastal ecosystems.

This research reports significant changes in microbiome structure between microplastic treatments. The most significantly distinct community was found to be in the PVC-amended sediments. This treatment also showed significantly reduced nitrification and denitrification. Alternatively, the PLA- and PUF-amended sediments significantly enhanced nitrification and denitrification, compared to our control. This confirmed our hypothesis that when microplastics are abundant in coastal sediments, they not only change the sediment microbial community and associated functionality, but also that different types of microplastics have different effects. As such, this work suggests that sedimentary microplastic pollution could be affecting the ability of sediment microbial communities to provide key ecosystem services.

As in any study, however, there are limitations to what this research alone can tell us. For example, plastic treatments presented here do not exactly mimic known sedimentary pollution (i.e. no mix of polymers or pre-biofouled microplastics). At the same time, this work is foundational in nature, and points out a potentially very impactful aspect of microplastic pollution that warrants further research. As we work collaboratively to understand microplastic complexity, we too should address the potential for ecological shifts, systems changes, and what drives them (be they microplastic shape, polymer chemistry, additive content, or otherwise). This line of inquiry is both exciting, and necessary in order to shape policy that can effectively mitigate the deleterious effects of ever-increasing plastic pollution.

Referenced work: M.E. Seeley, B. Song, R. Passie, & R.C. Hale. 2020. Microplastics affect sedimentary microbial communities and nitrogen cycling. Nature Communications. https://www.nature.com/articles/s41467-020-16235-3

Published September 10, 2020.