Biodegradable microplastics aggravate greenhouse gas emissions from urban lake sediments more severely than conventional microplastics

Abstract

Freshwater ecosystems, such as urban lake sediments, have been identified as important sources of greenhouse gases (GHGs) to the atmosphere, as well as persistent sinks for ubiquitous microplastics due to the high population density and frequent anthropogenic activity. The potential impacts of microplastics on GHG production, however, remain underexplored. In this study, four types of common biodegradable microplastics (BMPs) versus four conventional non-biodegradable microplastics (NBMPs) were artificially exposed to urban lake sediments to investigate the responses of nitrous oxide (N₂O) and methane (CH₄) production, and make a comparison regarding how the biodegradability of microplastics affected GHG emissions. Importantly, results suggested that BMPs aggravated N₂O and CH₄ production in urban lake sediments more severely than conventional NBMPs. The production rates of N₂O and CH₄ increased by 48.78–71.88 % and 30.87–69.12 %, respectively, in BMPs groups, while those increased by only 0–25.69 % and 6.46–10.46 % with NBMPs exposure. Moreover, BMPs insignificantly affected nitrification but facilitated denitrification, while NBMPs inhibited both processes. BMPs not only created more oxygen-limited microenvironment, greatly promoting N₂O production via nitrifier denitrification pathway, but also provided dissolved organic carbon favoring heterotrophic denitrification, which was primarily supported by the enriched denitrifiers and functional genes. In contrast, NBMPs slightly upregulated nitrifier denitrification pathway to generate N₂O, and showed a toxic inhibition on both nitrifiers and denitrifiers. In addition, both BMPs and NBMPs promoted hydrogen-dependent methanogenic pathway but suppressed acetate-dependent pathway. The greater enhancement of CH₄ production with BMPs exposure was attributed to the additional organic carbon substrates derived from BMPs and the stimulated microbial methane metabolism activities.