The ‘CH4 oversaturation paradox’ has been observed in oxygen-rich marine and lake waters, and viewed to significantly contribute to biosphere cycling of methane, a potent greenhouse gas. Our study focused on the intriguing well-defined pelagic methane enriched zone (PMEZ) in freshwater lakes. Spiking Yellowstone Lake PMEZ samples with 13C-labeled potential methanogenesis substrates found only 13C-methylphosphonate (MPn) resulted in 13CH4 generation. In 16S rRNA gene Illumina libraries, fourPseudomonassp. operational taxonomic units surprisingly accounted for ∼11% abundance in the PMEZ community.Pseudomonassp. isolates were also obtained from MPn enrichments with PMEZ water; they were most aggressive in MPn metabolism and their 16S rRNA gene sequences matched 35% of the Illumina PMEZPseudomonasreads. Further, two key genes encoding C-P lyase (phnJL, an important enzyme for dealkylation of MPn), were only amplifiable from PMEZ DNA and all PCR generatedphnJLclones matched those of thePseudomonassp. isolates. Notably, methanogen 16S rRNA signatures were absent in all Illumina libraries andmcrAwas not detected via PCR. Collectively, these observations are consistent with the conclusion that MPn metabolism contributes significantly to CH4 oversaturation in Yellowstone Lake and likely other oxic freshwater lake environments, and thatPseudomonassp. populations are critical participants.