No abstract

Mesophilic and thermophilic anaerobic digesters (MD and TD, respectively) utilizing Gracilaria and marine sediment as the substrate and inoculum, respectively, were compared by analyzing their performances and microbial community changes. During three successive transfers, the average cumulative methane yields in the MD and TD were 222.6 ± 17.3 mL CH4/g volatile solids (VS) and 246.1 ± 11 mL CH4/g VS, respectively. The higher hydrolysis rate and acidogenesis in the TD resulted in a several fold greater accumulation of volatile fatty acids (acetate, propionate, and butyrate) followed by a larger pH drop with a prolonged recovery than in the MD. However, the operational stability between both digesters remained comparable. Pyrosequencing analyses revealed that the MD had more complex microbial diversity indices and microbial community changes than the TD. Interestingly, Methanomassiliicoccales, the seventh methanogen order was the predominant archaeal order in the MD along with bacterial orders of Clostridiales, Bacteriodales, and Synergistales. Meanwhile, Coprothermobacter and Methanobacteriales dominated the bacterial and archaeal community in the TD, respectively. Although the methane yield is comparable, both MD and TD show a different profile of pH, VFA and the microbial communities.

Anaerobic microbial consortia in AD or fermentation oxidise or reduce a target organics (including nutrients) in order to share (i.e. extract and utilise) thermodynamic enthalpy energy (i.e. ATP with thermal energy). Herein, the oxidation and reduction (i.e. electrochemical reaction) is a type of biochemical reaction that involves a transfer of electrons or hydrogens between two species and or taxa. The review discusses an electrochemical communication in the bacterial communal society leading to a 'bacterial cartel' which can be a type of struggling for life (to obtain the biochemical energy constantly). Interestingly, syntrophic bacteria (mostly acetogenic bacteria) bind or flocculate the AD bacterial consortia and build two-layer biofilms or bioflocs to obtain the energy while producing a peculiar profile of fatty acids. The hydrolytic fermentative bacteria also dissociate with acidogenic bacteria for an association with the syntrophic bacteria when ∆ψ approaches at between-200 and-250 mV. Three examples (singlemethanogenesis, long-chain fatty acid (LCFA) degradation and acid-fermentation process) explain in the electrochemical origin. This concept remains quite controversial, but if true, may have major implications in broad areas of environmental and biological processes.