Fish acting as sinks of methane-derived carbon in Neotropical floodplains

Floodplains function as global hotspots for the natural production of methane. Some of this methane can be oxidized by methanotrophic bacteria and assimilated into their biomass before reaching the atmosphere. Consequently, aquatic invertebrates that feed on methanotrophic bacteria may transfer methane-derived carbon to higher trophic levels in the aquatic food chain. Our objective was to investigate the proportion of methane-derived carbon in the biomass of apex fish across 34 lakes from four major Neotropical floodplains (Amazon, Pantanal, Araguaia, and Paraná) using stable isotopes of carbon (δ¹³C). We found that methane-derived carbon contributed between 5 % and 16 % to the biomass of 37 apex fish species, providing, for the first time, evidence of the fish's role in the methane cycle in the Neotropics. Consumers in the Amazon and Pantanal floodplains, the largest and most significant regions for methane production, exhibited higher levels of methane-derived carbon in their biomass (11.06 ± 2.87 % and 9.84 ± 3.08 %, respectively). These results underscore the role of aquatic consumers in mitigating methane emissions in floodplains, as methane oxidation and assimilation are linked to reduced emissions. Therefore, conserving fish assemblages in floodplains through strategies that maintain the natural dynamics of these ecosystems is essential for controlling natural methane emissions.