Contrasting fate and GHG emissions of algal vs. manure DOM under light and microbes in lakes.

The escalating dissolved organic matter (DOM) inputs from algal (ADOM) and livestock manure (MDOM) sources are altering lake ecosystems, yet their fate under light and microbes in steppe lakes is unclear. Using microcosms, we found that photodegradation suppressed ADOM bioavailability by 4.8% via substrate competition and reactive oxygen species (ROS) inhibition (especially singlet oxygen), while light enhanced MDOM bioavailability by 5.4%. ADOM mineralization released 1.6× more CO2 than MDOM. Multi-group structural equation modeling (SEM) indicated that efficient ADOM microbial transformation drove higher CO2 emissions, whereas light-induced ROS shifted microbial communities and transformation pathways. ADOM and MDOM inputs predominantly enhanced CO2 emissions and water browning, respectively. These processes are interconnected-exogenous MDOM can promote algal blooms and subsequent greenhouse gas release. For eutrophic steppe lakes, integrated management combining exogenous load reduction and endogenous bloom suppression is critical to mitigate ecological risks and greenhouse effects.