Thermal regimes during overwintering recovery shape microbial network and dissolved organic matter complexity in Microcystis-dominated systems.

The overwintering recovery of Microcystis aeruginosa represents a critical but underexplored phase in the seasonal development of cyanobacterial blooms. Although the role of temperature in driving bloom onset is recognized, its effects on microbial assembly and the molecular transformation of dissolved organic matter during reactivation remain insufficiently characterized. In this study, 16S rRNA gene sequencing, excitation-emission matrix fluorescence spectroscopy coupled with parallel factor analysis, Fourier transform ion cyclotron resonance mass spectrometry, and metabolomics were applied to examine how three thermal recovery regimes-constant temperature, gradual warming, and cold-dark preconditioning-shape microbial succession and dissolved organic matter dynamics. Constant temperature accelerated the dispersal limitation of bacterial communities and promoted rapid dissolved organic matter (DOM) turnover, whereas gradual warming and cold-dark preconditioning induced more undominated community structures, and the accumulation of nitrogen- and sulfur-rich DOM compounds. Cold-dark pretreatment notably enhanced the formation of structurally complex, recalcitrant DOM, and delayed microbial reactivation. The network of relationships between microorganisms and dissolved organic matter revealed distinct coupling patterns across treatments, with enhanced microbial processing of aromatic and humic-like molecules occurring under thermal fluctuation or stress. Metabolomic profiling further indicated different physiological adaptation strategies, with stress-linked metabolites enriched under variable-temperature conditions. These findings highlight the mechanistic links between temperature-driven microbial recovery and dissolved organic matter transformation, providing new insights into how winter conditions influence cyanobacterial bloom trajectories in freshwater ecosystems.