Plastic: A Potential Carbon Source for Sedimentary Biogeochemical Cycling

Sediment is the ultimate sink for microplastics. Given the current trends in plastic production and waste treatment, plastic deposition to the sediment is expected to continue. Some plastics, such as polyethylene (PE) and polystyrene (PS) exhibit biodegradability, albeit at very slow rates and not determined in conditions found in sediment. Plastic degradation is aided by the presence of light and oxygen; however, after these plastics settle to sediment it remains unclear how dark, low oxygen conditions typical of sediment influence biofilm development, degradation or conservation of plastic. To explore the biogeochemistry of plastic under these conditions, wood, PE, and PS were placed in the water column and sediment of Svanemøllen Harbor, a Danish marina in central Copenhagen. Throughout a year of exposure, these substrates were periodically subsampled to analyze for biofilm development and potential plastic degradation. Concurrently, sediment cores were analyzed for sulfide, phosphate, dissolved inorganic carbon, methane, and ammonium to track redox zonation over time. The plastic-associated biofilm was characterized using DNA sequencing, biofilm assay, and He-ion microscopy and compared to the ambient microbial community in the water column and sediment. He-ion microscopy was also used to visually inspect the surface of the plastic for evidence of degradation and to describe the microbial community. Several species of diatoms were present on the plastics and potentially rotifers, known diatom predators. Preliminary results show that, during the spring bloom, the sulfate reduction zone corresponded with the most biofilm development on PE in the sediment. After six months, the biofilm present on PE and PS increased, but the biofilm on PE was more stable and constant at all core depths.