Temperature Dominates Microplastic Effects on Microbial and CO2/CH4 Responses in Seagrass Sediments

Xue Li; Fan Sheng; Dong Li; Junchang Jin; Mingjiao Li; Jiwei Luo

Abstract

Microplastics and warming may jointly affect sediment microbial processes and carbon cycling in coastal ecosystems, yet their relative importance and interactions remain poorly understood. Here, we conducted a 28-day sediment–water microcosm experiment using seagrass-bed sediments to compare the effects of polyethylene (PE) and polypropylene (PP) under four temperature regimes (21, 28, 35, and fluctuating 28/35 °C). Temperature was the dominant driver of bacterial community structure and carbon-substrate utilization, whereas the overall effect of microplastic type was not significant based on PERMANOVA. Biolog profiles likewise showed stronger temperature-associated than polymer-associated variation. In contrast, PE and PP were associated with distinct and temporally variable responses in selected biogeochemical variables and CO₂ and CH₄ emissions. NH₄⁺ declined across treatments, with a greater early decline under PE, whereas PP was associated with higher DOC under fluctuating temperature on day 7. CO₂ and CH₄ emissions also varied with temperature and polymer type, with CH₄ generally highest at 35 °C. However, these gas responses cannot be directly attributed to specific microbial pathways because archaeal communities, methanogenic functional genes, methane oxidation, and relevant process rates were not characterized. Overall, temperature exerted stronger and more consistent effects on community-level microbial responses than microplastics, while PE and PP produced comparatively weaker, polymer-specific, and context-dependent changes in sediment biogeochemistry and CO₂ and CH₄ dynamics. These findings highlight temperature as a key control on short-term microbial responses to microplastic exposure in seagrass sediments.

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