Seasonal bloom alternation drives periodic shifts in carbon sink function via differential dissolved organic matter processing in a plateau lake.

Abstract

Frequent algal blooms alter dissolved organic matter (DOM) dynamics and carbon sink functions in eutrophic lakes, yet how bloom type governs DOM molecular transformation and microbial carbon pump (MCP) direction remains unresolved. Integrating Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), metagenomics, Biolog EcoPlate, and incubation experiments, we investigated DOM composition, microbial functions, and refractory dissolved organic carbon (RDOC) formation during cyanobacterial (Pseudanabaena sp.) and dinoflagellate (Peridinium sp.) blooms in Lake Erhai. Cyanobacterial blooms released CHON-enriched, high-molecular-weight (HMW) DOM with elevated carboxyl-rich alicyclic molecules (CRAMs, 39.53 %) accumulation, exhibiting expanded synthesis-dominated meta-metabolome networks and intracellular carbon storage modules (GT35, GH13) with progressively broadening substrate utilization. Conversely, dinoflagellate blooms produced low-molecular-weight (LMW), oxidized, and sulfur-rich DOM with elevated polycyclic aromatic hydrocarbons (PAHs, 14 %), characterized by removal-dominated networks and extracellular degradation modules including polysaccharide lyases and peptidoglycan-degrading enzymes (GH24, CBM50) with specialized catabolic activity. Summer warming promoted cyanobacterial biomass and DOM accumulation, yet enhanced microbial activity functioned as a carbon turnover engine that suppressed net RDOC accumulation. In contrast, lower temperatures in autumn and winter suppressed dinoflagellate biomass, but substrate-specific enzymatic catalysis sustained efficient RDOC formation via activated carbohydrate-active enzymes (CAZymes). These findings suggest that seasonal alternation between cyanobacterial and dinoflagellate blooms modulates MCP direction, driving periodic shifts in Lake Erhai's carbon sink function and indicating that plateau lake carbon management should integrate seasonal temperature variations and algal community composition.