Lipid Biomarkers Respond to Seasonal Blooms of Methane Oxidizing Bacteria in a Eutrophic Lake

Lakes are key components of global carbon cycling, accounting for 6-16 % of natural methane (CH₄) emissions. Methane release from lakes is largely regulated by aerobic methane oxidizing bacteria (MOB), which oxidize up to 75% of the methane produced in lakes to CO₂. Over the past two centuries, anthropogenic environmental change and climate forcing have led to rapid changes in lake systems including eutrophication and stronger stratification. The implications of these ongoing processes for MOB communities and the key methane sink they represent remains unclear. A deeper understanding of how MOB communities in lakes responded to systemic change in the past is essential for discerning their future dynamics. This highlights the need for a reliable biomarker that can track changes in MOB communities in lakes across timescales. The hopanoid bacterial membrane lipids, bacteriohopanepolyols (BHPs), exhibit specificity to MOB types and genera, therefore holding potential as biomarkers allowing us to track community assemblages. The peri-alpine lake Rotsee, located in central Switzerland, is a prime example of a monomictic eutrophic lake, making it an ideal study site to further understand and develop BHPs as biomarkers for MOB communities. Here we present initial results of a seasonal study of Rotsee where we used BHPs coupled with eDNA to investigate the MOB assemblages in the lake’s present-day water column, allowing us to ascertain how rapidly seasonally changing conditions affect MOB communities and the lipid biomarker assemblages they produce. We carried out intact polar lipid (IPL) extraction on suspended particulate matter (SPM) filtered from Rotsee’s 16-meter-deep water column at three-meter intervals and at the oxycline, from August 2025 to January 2026, and at two depths (surface and 15 meters) from May to December 2019. IPLs were measured on an Ultra High Precision Liquid Chromatography-Quadrupole-Orbitrap High-Resolution Mass Spectrometer (UHPLC-Orbitrap-HRMS). Genetic material was extracted from the SPM samples and sequenced targeting bacterial 16S rRNA. Preliminary results show that MOB-specific BHPs: aminotriol, aminotetrol, and aminopentol, are present in the Rotsee water column. The relative abundances of BHPs in the epilimnion remains low and steady during spring and summer but spike during lake overturn in November, whereupon most of the methane is released and oxidized after accumulation in the hypolimnion. 16S rRNA data indicates that the MOB communities are entirely made up of Gammaproteobacteria and match BHP seasonal trends with MOB-specific sequences being more abundant during overturn in the surface water. Additionally, surface waters in November are characterized by a higher abundance of aminopentol, which is scarcely found during spring and summer. Interestingly surface water 16S rRNA data also show that the MOB community compositions change considerably in November and December, shifting from Methylomonas to Methylobacter-dominated. Therefore, preliminary results show that BHP abundances respond to seasonal MOB blooms and show promise towards tracking seasonal community dynamics in eutrophic stratified lakes.