Photosynthesis-driven methane production in oxic lake water as an important contributor to methane emission

Recent discovery of methane (CH₄) production in oxic waters challenges the conventional understanding of strict anoxic requirement for biological CH₄ production. High-resolution field measurements in Lake Stechlin, as well as incubation experiments, suggested that oxic-water CH₄ production occurred throughout much of the water column and was associated with phytoplankton especially diatoms, cyanobacteria, green algae, and cryptophytes. In situ concentrations and δ13C values of CH₄ in oxic water were negatively correlated with soluble reactive phosphorus concentrations. Using 13C-labeling techniques, we showed that bicarbonate was converted to CH₄, and the production exceeded oxidation at day, but was comparable at night. These experimental data, along with complementary field observations, indicate a clear link between photosynthesis and the CH₄ production-consumption balance in phosphorus-limited epilimnic waters. Comparison between surface CH₄ emission data and experimental CH₄ production rates suggested that the oxic CH₄ source significantly contributed to surface emission in Lake Stechlin. These findings call for re-examination of the aquatic CH₄ cycle and climate predictions.