Effect of operating temperature on anaerobic digestion of the Brazilian waterweed Egeria densa and its microbial community

•For effective treatment of E. densa, temperature of anaerobic digestion was examined.•The microbial diversity decreased and Firmicutes was dominant at 55 °C and 65 °C.•Acetate oxidation and hydrogenotrophic methanogenesis may have been enhanced at 65 °C.•55 °C is a suitable, however, care must be taken to prevent the ammonia inhibition.

To develop an effective treatment for the globally invasive Brazilian waterweed Egeria densa, anaerobic digestion was observed at 37 °C, 55 °C, and 65 °C. The average methane production rate at 55 °C was 220 mL L−1 day−1, which was two-fold that at 37 °C and 65 °C. Volatile fatty acid accumulation was detected under thermophilic conditions; however, although there was methane production, the system did not shutdown. The microbial communities differed between mesophilic (37 °C) and thermophilic (55 °C and 65 °C) conditions. A bacterial community consisting of the phyla Bacteroidetes (43%), Firmicutes (37%), Proteobacteria (9%), Synergistetes (5%), Spirochaetes (1%), and unclassified bacteria (5%) were detected under mesophilic condition. In contrast, the phylum Firmicutes was dominant under thermophilic conditions. In the archaeal community, Methanosaeta concilii (40%), Methanolinea sp. (17%), and unclassified euryarchaeota (43%) were detected under mesophilic condition. Methanosarcina thermophila (87% at 55 °C, 54% at 65 °C) and Methanothermobacter thermautotrophicus (13% at 55 °C, 46% at 65 °C) were detected under thermophilic conditions. At both 37 °C and 55 °C, acetoclastic methanogenesis likely occurred because of the lower abundance of hydrogenotrophic methanogens. At 65 °C, the growth of the acetoclastic methanogen Methanosarcina thermophila was limited by the high temperature, therefore, acetate oxidation and hydrogenotrophic methanogenesis may have occurred.