Anaerobic Digestion $($AD$)$ is a process in which an organic substrate such as food and kitchen waste, animal manure, and agricultural and forestry residues is converted into biogas using a consortium of different microorganisms comprising various bacteria and archaea in the absence of oxygen. The produced biogas contains on average $55-70\%$ of CH$4$ and $30-45\%$ of CO$2.$ To inject the produced biogas from anaerobic digestors into the natural gas infrastructures the CO$2$ must be removed. The chemical and physical methods of CO$2$ removal are expensive and not sustainable. An alternative method is injecting hydrogen $($H$2)$ gas into the digestor to convert the CO$2$ to CH$4$ but the process of H$2$ production is energy intensive. Syngas is a cheap source of H$2,$ but it also contains CO which has been reported as inhibitory to microbial communities. A curious researcher is investigating the possibility of using syngas injection in biogas reactors for CO$2$ conversion to CH$4.$ Some methanogenic archaea in the biogas reactors can convert H$2$ and CO$2/$CO to CH$4.$ Three continuously stirred tank reactors are started in the lab using a microbial mixed culture derived from a commercial biogas facility. The reactors are fed daily with food waste and after $90$ days the biogas composition of the reactors performing AD of food waste is in the range of $64-68\%$ CH$4$ and $32-36\%$ CO$2$ and is stable. The researcher starts the syngas injection into the reactors with a very low flow rate of $0\mathrm{.}05$ ml$/$min on day $91$ of the process. On day $105,$ the CH$4$ concentration increased to $98\%.$ The researcher further increased the syngas flow rate to $1$ ml$/$min on day $107$ of the process and the gas composition remained the same. $1.$ a$)$The supervisor is interested in the microbial community composition in the reactors. What are the steps for the researcher to investigate the microbial communities during the experiment? Discuss in detail the methods of your choice for the analysis, the experimental planning, and the principals involved. $2.$ b$)$To increase the gas productivity of the reactor on day $112$ they further increased the syngas loading rate to $2$ ml$/$min$.$ The CH$4$ content was reduced to $0\%$ and the AD process failed. The researcher speculated that CO in the syngas might have inhibited the microbial communities in the reactors. The researcher is curious to find the mechanism of syngas inhibition in this case, what kind of studies would they need to perform? What is the experimental procedure? justify your answers in detail.