Crenothrix are major methane consumers in stratified lakes

Methane-oxidizing bacteria represent a major biological sink for methaneand are thus Earth’s natural protection against this potent greenhouse gas. Here we show that in two stratified freshwater lakes a substantial part of upward-diffusing methanewas oxidized by filamentous gamma- proteobacteriarelated to Crenothrix polyspora. These filamentous bacteria have been known as contaminants of drinking water supplies since 1870, but their role in the environmental methaneremoval has remained unclear. While oxidizing methane, these organisms were assigned an ‘unusual’ methane monooxygenase(MMO), which was only distantly related to ‘classical’ MMO of gamma-proteobacterial methanotrophs. We now correct this assignment and show that Crenothrix encode a typical gamma-proteobacterial PmoA. Stable isotope labelingin combination swith single-cell imaging mass spectrometry revealed methane-dependent growth of the lacustrine Crenothrix with oxygen as well as under oxygen-deficient conditions. Crenothrix genomes encoded pathways for the respiration of oxygen as well as for the reduction of nitrate to N2O. The observed abundance and planktonic growth of Crenothrix suggest that these methanotrophscan act as a relevant biological sink for methanein stratified lakes and should be considered in the context of environmental removal of methane.