Determination of methanogenesis by nutrient availability via regulating the relative fitness of methanogens in anaerobic digestion

Response of microbial community to nutrient availability in anaerobic digestion (AD) remains elusive. Prokaryotic communities in AD batch cultures with 0, 1, 3, 5, 7, 11, 15, 20, and 25 g/L peptone were monitored using massive parallel sequencing and quantitative PCR over a 34-day experimental period. Methane production displayed a hump-shaped response to the nutrient gradient (peaking at 15 g/L peptone). Moreover, total and acetoclastic methanogens showed hump-shaped responses (both peaking at 11 g/L peptone). However, prokaryotic population increased with nutrient concentration (linear regression, R2 = 0.86) while diversity decreased (R2 = 0.94), and ordination analysis showed a gradual succession of community structure along the first axis. Network analysis revealed that extent of interspecific interactions (e.g., edge number and clustering coefficient) exhibited a hump-shaped response. The combined results indicate that abundant species became more dominated with increasing nutrient, which can result in a gain or loss of interspecific interaction within the community. Network module analysis showed that one module dominated the network at each nutrient level (comprising 41%–65% of the nodes), indicating that AD community formed a core microbial guild. The most abundant phylotypes, Macellibacteroides and Butyricicoccaceae, were consistently negative with acetoclastic methanogens in the dominant modules. Their predominance at ≥15 g/L peptone can explain the hump-shaped responses of methanogenesis and methanogens. Collectively, methanogenesis and microbial network exhibited hump-shaped responses, although microbial community exhibited monotonic responses. Therefore, nutrient availability can determine the methanogenesis through regulating the relative fitness of methanogens within the community.