Abstract: Algae-derived dissolved organic matter (ADOM) originates from algal exudates or lysed algal cells in eutrophic water. ADOM due to promoting the formation of harmful disinfection by-products (DBPs) in water plants poses a potential threat to the safety of drinking water. In order to investigate the microbial-driven release and transformation of ADOM during the algal disappearance period, this study collected decaying cyanobacterial scums from Lake Taihu and carried out indoor incubation experiments using lake water only and lake water with algae under dark conditions. Our study comprehensively characterized DOM transformation at both the macroscopic and microscopic levels using spectroscopy and high-resolution mass spectrometry. The results showed that: (1) At the short term of decay, a large amount of cyanobacterial organic matter was released, and the concentrations of dissolved organic carbon (DOC), dissolved oxygen nitrogen (DON) and dissolved organic phosphorus (DOP) increased significantly, which could promote free-living bacterial growth. As the degradation time increased, 45.23% of DOC, 82.07% of DON and 81.40% of DOP were mineralized in the cyanobacterial organic matter together with the microbial community (include bacteria, actinomycetes, and viruses). (2) Ultraviolet-visible (UV-Vis) analysis showed that the content and molecular weight of Chromophoric dissolved organic matter (CDOM) decreased after microbial degradation. Parallel factor analysis and excitation-emission matrix spectroscopy showed that most algal-derived protein-like substances were degraded into recalcitrant humic-like substances by microorganisms. (3) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that the composition of ADOM and lake DOM after microbial treatment tended to be similar, and the identified removed molecular formulas are primarily composed of algae- derived saturated lipids during microbial action. The generated molecular formulas showed a high degree of unsaturation. (4) In chlorine disinfection of algae-laden waters, ADOM is considered to be the source of precursors for the formation of toxic DBPs. Some suspected molecular formulas of disinfection by-product precursor (i.e., C₁₈H₃₁O₉N₃-compounds and C₂₀H₃₁O₁₁N-compounds) could still be detected in water after long-term microbial degradation. The study showed that microbial treatment had a significant impact on the concentration and occurrence of ADOM in water. In the earlier decay of cyanobacterial blooms, microbial treatment can promote ADOM release. While in the later decay of cyanobacterial blooms, microbial treatment can reduce the impact of ADOM on water quality. However, there are still small amount of disinfection byproduct precursor DOM molecules present in water. Therefore, during the algal-disappearance period, it is still necessary to pay attention to the safety of water quality and the treatment of disinfection byproduct precursor DOM.