Shifts in bacterial and archaeal community structures during the batch biomethanation of Ulva biomass under mesophilic conditions

•Mesophilic biomethanation of Ulva biomass was demonstrated with a high CH4 yield.•Changes in microbial community structure during the biomethanation were monitored.•Bacterial community had dynamic structural shifts while archaeal community did not.•Hydrogenotrophic Methanolinea-like population was likely the dominant methanogen.

Mesophilic biomethanation of Ulva biomass was performed in a batch bioreactor, and a high organic removal of 77% was obtained on the basis of chemical oxygen demand (COD) after a month of operation. The estimated methane yield was 0.43 ± 0.02 L CH4/g CODremoved which is close to the theoretical methane potential. Transitions of bacterial and archaeal community structures, associated with process performance data, were investigated using a combination of molecular fingerprinting and biostatistical tools. During the operation, archaeal community structure had no significant changes while bacterial community structure shifted continuously and dynamically. The reactor completely stabilized volatile fatty acids (primarily acetate and propionate) accumulated from the acidogenesis phase, with Methanosaeta- and Methanolinea-related microbes respectively being the main aceticlastic and hydrogenotrophic methanogens. Methanolinea- and Syntrophobacter-related populations were likely the key members to form a syntrophic propionate-degrading consortium. A Methanolinea-related population was likely the dominant methane producer in the experimental reactor.