Cold-Temperate Mountainous Freshwater Produces Methane by Algal Metabolism

We reported important environmental drivers of dissolved CH4 concentrations (d-CH4) in nutrient-limited mountainous freshwater in a cold-temperate region and explored the potential for multiple known oxic CH4 production pathways. Field investigation revealed consistent supersaturated d-CH4 in surface water (relative to the theoretical value of d-CH4 at atmospheric equilibrium), with significant seasonal variations. Statistical analysis highlighted the direct impact of algal dynamics and the indirect effect of temperature and nutrients on d-CH4. Further lab-scale incubation demonstrated that CH4 production decreased by 55.25 to 93.65% with algae removal, while it increased 4 to 10 times with methylphosphonate (MPn) amendment. These findings argued that CH4 produced from algal metabolism related to MPn had a high potential for supersaturated d-CH4. It also verified the pivotal role of cyanobacteria in this mechanism, with temperature and light acting as regulatory factors. Through highlighting the role of algae for CH4 characteristics in cold-temperate mountainous freshwater and proposing the potential of oxic CH4 production through MPn metabolism in nutrient-limited lakes, this study enriches comprehension of aquatic CH4 cycle and warns about the importance of preserving environmental balance in freshwater with minimal human disturbance.