Trees microbiome from tropical reservoir

Reservoirs emit significant amounts of C but the understanding of the C sources driving these emissions is still limited. In particular, the contribution of flooded forests to these emissions remains unknown while it may constitute the largest C pool in the reservoirs, notably the tropical ones. In this study, we examined the fate of standing dead trunks of the flooded forest of a tropical reservoir (Petit Saut, French Guiana) and the underlying microbial communities and activities including the resulted greenhouse gases (GHG) production. We sampled wood and associated biofilms and we measured in situ GHG fluxes at the trunks surface along the standing dead trees at three depth corresponding to aerial, oxic and anoxic conditions. Microbial assemblages and associated CH4 production and consumption were examined in wood and biofilm samples. Heterotrophic bacteria and methanogens dominated microbes colonizing wood. In contrast, microbial assemblages from the wood-associated biofilms were dominated by phototrophs mainly cyanobacteria at the oxic depth and anaerobic phototrophs (e.g., Chlorobia) at the anoxic one. Compared to wood, biofilm samples exhibited lower abundance in methanogens but higher methanotrophs resulting in lower net methanogenic activities than those measured in wood samples and higher CH4 consumption rates. Standing dead trees constitutes quite stable habitats colonized by a surface biofilms shaped by bacteria using C derived from wood as energy source and acting as either C sinks or C sources according prevailing light and oxygen conditions.