Illustration of the microbial community selected by optimized process and nutritional parameters resulting in enhanced biomethanation of rice straw without thermo-chemical pretreatment.

Effects of different process and nutritional parameters on microbial community structure and function were investigated to enhance the biomethanation of rice straw without any thermochemical pre-treatment. The study was performed in a mesophilic anaerobic digester with cattle dung slurry as inoculum. The highest methane yield of 274 ml g-1 volatile solids was obtained from particulate rice straw (1 mm size, 7.5% solids loading rate) at 37 °C, pH-7, when supplemented with urea (carbon: nitrogen ratio, 25:1) and zinc as trace element (100 µM) at 21 days hydraulic retention time. The optimization of conditions selected Clostridium, Bacteroides, and Ruminococcus as dominant hydrolytic bacteria and Methanosarcina as the methanogen. Analysis of metagenome and metatranscriptome revealed wide array of bacterial lignocellulolytic enzymes that efficiently hydrolyzed the rice straw. The methane yield was >80% of the theoretical yield, making this green process a sustainable choice for efficient extraction of energy from rice straw.