Eric Bollinger

• Climate change and its drivers are the most important concern for human and environmental wellbeing. Although focus is largely shifted towards anthropogenic emissions of carbon dioxide (CO2), methane (CH4) from natural inland waters contributes extraordinarily much to global warming. Despite natural emissions being on the rise, its drivers remain poorly understood with chemical stressors of anthropogenic origin (such as antibiotics) - although omnipresent – being completely ignored. To address this, we assessed their toxic action on structure (i.e., phenology and gene expression) and functioning (i.e., methanogenesis) of natural freshwater sediment prokaryotic communities in anaerobic incubations. Antibiotics mostly increased the production dynamics of CH4 driven by changes in community structure rather than by shifts in utilized carbon sources (while acetate is the most important). These effects were most prominent at higher temperatures and shifted into the opposite direction at lower temperatures. Disparate impacts of antibiotics were also present in communities with different exposure history. In sediment from a natural reserve, antibiotics increased methanogenesis while decreasing it in a community downstream of a wastewater treatment plant. An experimentally invoked adaptation phase did not significantly impact the sensitivity of those communities towards antibiotics. Across all studies, relative abundances of Methanomicrobia closely followed the pattern in CH4 production. This finding is a key mechanistic link that is presumed to be of mostly indirect nature by affecting bacterial substrate providers. These results are the first to show the impacts of antibiotics on methanogenesis in natural communities. Further, the enhancing nature of these compounds is most relevant in a warming world and even seemingly adapted communities are vulnerable. This prompts towards a contribution of anthropogenic chemical stressors to increasing natural CH4 emissions, while emergence and generalization of effects constitute future challenges.