Microbial community response to ammonia levels in hydrogen as-sisted biogas production and upgrading process

Biological conversion of carbon dioxide in biogas into methane using hydrogen derived from surplus renewable energy (wind power) as reducing power is a novel technology for biogas upgrading. High ammonia concentrations were recently found toxic to this biogas upgrading process, however the mechanisms behind the inhibition as well as the microbial stress response in such unique upgrading system have never been reported. Thus in the present study, the effect of high ammonia concentrations on microbial community during hydrogen induced biogas upgrading process was evaluated. The results showed that a change from aceticlastic pathway to hydrogenotrophic pathway occurred when ammonia concentration increased from 1 to 7 g NH4+-N L-1. In addition, the bacteria, potentially syntrophic associate with hydrogenotrophic methanogens, were enriched under high ammonia concentrations. Moreover, the growth of some bacteria (e.g., Halanaerobiaceaeen and Leucobacter) which were vulnerable to ammonia toxicity were restored upon hydrogen injection. On the other hand, the tolerance both archaea (e.g., Methanothermobacter thermautotrophicus and Methanobrevibacter) and bacteria (e.g., Thermoanaerobacteraceae, Ruminococcaceae and Clostridium) to ammonia inhibition were further improved by adding hydrogen into reactors. Therefore, besides syntrophic bacteria and hydrogenotrophic methanogens, hydrogen injection under high ammonia concentration could also promote the growth of some hydrolytic and fermentative bacteria.