Enhancement of lake carbon emission by cyanobacteria-derived organic carbon input: Stimulatory effect of co-metabolism on sediment.

Lake carbon emissions have increased markedly under climate warming and eutrophication, yet the underlying mechanisms remain poorly understood. Here, we reveal a clear spatiotemporal expansion of cyanobacterial blooms in Lake Taihu from 2000 to 2023 based on the floating algae index (FAI) analysis, indicating a year-on-year increase in bloom frequency and intensity. Furthermore, this study examined how cyanobacterial-derived organic matter inputs influence carbon emissions and sediment carbon pool structures across three representative zones: a cyanobacterial-dominated region, a macrophyte-dominated region, and the lake center. Field observations showed that CH4 and CO2 fluxes in the cyanobacterial-dominated region were significantly higher than in the other regions, reaching 133.9 and 2361.2 μg/m2·min, respectively. The total organic carbon (TOC) content in the surface sediment was highest at 58.5 g/kg, while microbial biomass carbon (MBC) reached 554.1 mg/kg; both declined sharply with depth. In contrast, the macrophyte-dominated region displayed the highest TOC in bottom sediment (49.5 g/kg), indicating strong carbon burial. Microcosm experiments confirmed that cyanobacterial-derived organic carbon input induced co-metabolic processes in sediments, and significantly increased CH4 and CO2 fluxes-especially in regions with high recalcitrant organic carbon (ROC) content. The relative increase in sediment carbon emissions was positively correlated with ROC and negatively with labile organic carbon (LOC). Moreover, the proportion of ROC in sediments significantly decreased before and after incubation, indicating that co-metabolism promotes ROC degradation and enhances the role of lakes as carbon sources. However, based on microcosm experiments, where cyanobacterial input enhanced carbon emissions from sediments while TOC concentration still increased, lakes remain net carbon sinks overall. These findings improve our understanding of the role of co-metabolism in regulating lake carbon source-sink dynamics and provide critical insights for regional and global carbon assessments.