Algal community composition drives lake greenhouse gas emissions via dissolved organic matter transformation and microbial processing.

Lakes are important sources of greenhouse gases, yet bloom-driven emissions are often assessed from total algal biomass, ignoring algal functional composition. This study examined how cyanobacteria (Microcystis aeruginosa), green algae (Chlorella vulgaris), diatoms (Cyclotella meneghiniana), and dominance-based mixtures regulate DOM transformation and CO2/N2O production under eutrophic conditions. It integrated to pure-culture experiments, water-sediment microcosms, sterilization controls, DOM fluorescence spectroscopy, gas monitoring, and metagenomics to resolve an algae-DOM-microbe-gas cascade. Cyanobacteria produced protein-like DOM and stimulated carbon mineralization, with CO2 exceeding 20 mmol L-1 by day 36; cyanobacteria-dominant mixtures followed a similar high-CO2 trajectory. Green algae generated tyrosine-like DOM and caused the strongest NO2- accumulation, reaching 5.21 mg L-1 by day 21, corresponding to the highest N2O production; this pattern also occurred in green-algae-dominant mixtures. Diatom-only and diatom-dominant treatments favored humic-like DOM, organic carbon retention, and the weakest short-term CO2/N2O accumulation. Sterilization reduced inorganic carbon and greenhouse gas production, supporting microbial control. Background summer metagenomics provided functional context, showing algal-DOM turnover potential through carbon metabolism, glycolysis/gluconeogenesis, pyruvate metabolism, and the TCA cycle, while nirK and other nitrogen genes indicated capacity for substrate-driven incomplete nitrogen reduction. Functional differentiation among Candidatus_Planktophila, Limnohabitans, Rhodoferax, and Cyanobium linked DOM processing with potential gas-production pathways. These results show algal community composition, rather than biomass alone, regulates greenhouse gas production by shaping DOM quality, nutrient intermediates, and microbial C-N pathways. Incorporating algae composition into greenhouse gas assessment, this novel algae-DOM-microbe-gas framework provides mechanistic support for improving eutrophication management and lake-emission mitigation.