Microplastics in Cyanobacterial Harmful Algal Blooms: Facilitators of CO₂ and CH₄ Emission Hotspots

Algal blooms, carbon emission hotspots in aquatic ecosystems, typically are accompanied by high microplastics (MPs) levels. However, the role of MPs in algae-driven carbon cycling remains elusive. Here, Microcystis aeruginosa(M. aeruginosa) was used to investigate the responses of CO₂ and CH₄ emissions to MPs during cyanobacterial harmful algal blooms (cHABs, a predominant algal bloom type), integrating physicochemical analysis and high-resolution chemical and microbial approaches to elucidate the role of MPs in carbon emissions throughout growth-decline phases of algal blooms. The results exhibited three phases. In phase I, MPs facilitated M. aeruginosa growth, thereby stimulating aerobic CH₄ production through photosynthesis-coupled reactive oxygen species mechanisms. As aggregate settling dynamics varied with heteroaggregation, the survival patterns of M. aeruginosa changed, reducing CO₂ and CH₄ production through decreasing carbon substrates in phase II. However, M. aeruginosa decay, accelerated by MPs, not only enhanced anaerobic organic matter utilization in water, but also elevated the nominal oxidation state of carbon in sediment, promoting hydrogenation and altering the balance between methanogenesis and methanotrophy, subsequently increasing CO₂ and CH₄ production in phase III. Ultimately, MPs exacerbated cumulative carbon emissions of cHABs, suggesting MPs as facilitators behind algal blooms being foci of carbon emissions.