Biodegradable plastics can alter carbon and nitrogen cycles to a greater extent than conventional plastics in marine sediment

The seabed constitutes a global sink for plastic debris, where they can remain for centuries. Biodegradable plastics offer the advantage of having less persistence in the environment than conventional ones. The seabed is responsible for key ecosystem functions related to the cycling of elements by decomposing the labile fraction of organic matter and fueling primary production, while storing the most recalcitrant part of this organic matter and limiting CO emissions. Although plastics are expected to affect these processes, knowledge on this matter is scarce. In controlled microcosms, we show that biodegradable plastics can stimulate the decomposition of marine-buried carbon and reduce the release of inorganic nitrogen. We found that conventional and biodegradable plastics promoted anaerobic sediment metabolic pathways. Biodegradable plastics produced a two-fold CO release to the water column, which suggests the decomposition of not only plastics, but also of buried organic carbon. The stimulation of sediment metabolism could be due to excessive carbon consumption by bacteria that derives from a rise in the carbon:nitrogen ratio. Accordingly, the NH flux to the water column lowered. As NOx fluxes also lowered, biodegradable plastics might promote nitrification-denitrification coupling. If biodegradable plastics become a major component of marine pollution, then sediment biogeochemical cycles might be strongly influenced, which could affect the carbon sequestration of coastal ecosystems and compromise their mitigation capacity against climate change.