Methane fueled lake pelagic food webs in a Cretaceous greenhouse world.

Methane (CH) is a potent greenhouse gas but also an important carbon and energy substrate for some lake food webs. Understanding how CH incorporates into food webs is, therefore, crucial for unraveling CH cycling and its impacts on climate and ecosystems. However, CH-fueled lake food webs from pre-Holocene intervals, particularly during greenhouse climates in Earth history, have received relatively little attention. Here, we present a long-term record of CH-fueled pelagic food webs across the Cretaceous Oceanic Anoxic Event 1a (~120 Mya) that serves as a geological analog to future warming. We show an exceptionally strong expansion of both methanogens and CH-oxidizing bacteria (up to 87% of hopanoid-producing bacteria) during this Event. Grazing on CH-oxidizing bacteria by zooplankton (up to 47% of ciliate diets) within the chemocline transferred substantial CH-derived carbon to the higher trophic levels, representing an important CH sink in the water column. Our findings suggest that as Earth warms, microbial CH cycling could restructure food webs and fundamentally alter carbon and energy flows and trophic pathways in lake ecosystems.