Epixylic microbial communities as key regulators of methane emissions from submerged wood in a tropical hydroelectric reservoir

Abstract Tropical hydroelectric reservoirs are recognized hotspots for greenhouse gas emissions, yet the role of submerged forests in these emissions remains poorly understood. In this study, we investigated the contribution of standing dead trees and their associated epixylic biofilms to methane (CH 4 ) and carbon dioxide (CO 2 ) emissions in the permanently stratified Petit Saut reservoir, French Guiana. We conducted in situ measurements of greenhouse gas fluxes from submerged trunks at multiple depths using static chambers. To unravel the microbial mechanisms underlying CH 4 dynamics, we characterized the active microbial communities and quantified methane production and oxidation rates in both wood and biofilm samples collected above and below the oxycline, using 16S rRNA sequencing and laboratory incubations. Our findings revealed that submerged wood serves as an unrecognized source of in situ CH 4 production, primarily via acetoclastic methanogenesis dominated by Methanosaeta . In contrast, epixylic biofilms acted as effective CH 4 sinks, with methane oxidation largely driven by Methylocystis . Microbial composition and activity showed strong vertical stratification, closely linked to oxygen availability. In situ fluxes mirrored these microbial patterns, with net CH 4 and CO 2 efflux observed below the oxycline and net influx above it. This study highlights the dual role of submerged wood and its surface biofilms in CH 4 cycling, emphasizing the importance of microbial interactions in regulating greenhouse gas emissions from flooded tropical forests. These findings underscore the need to include wood‐mediated processes in greenhouse gas budgets of tropical reservoirs, especially as dam construction continues to expand in forested tropical regions.