Figure 2. Factors that limit methanogen growth.

In a closed system such as in sealed anaerobic glass culture tubes, the metabolic productivity of any organism can be estimated by the Gibbs' free energy (ΔG°′) of the rate-limiting biochemical transformations occurring. For most methanogens, this is C and/or H₂ metabolism. Other factors, such as physical stress (pH, temperature, and water activity) and net metabolite fluxes, also affect population growth by increasing entropy of the cell systems, thus exerting a negative vector on ΔG°′ and resulting in increased BEQ. Finally, informational entropy in the form of spatial organization, gene content, and gene regulation also affects whether cells optimally convert chemical energy into biomass. At the extremum are non-growing diffusion-controlled cell systems and at the other are compact solid-state cells in which metabolism is flux-controlled. Red, net entropy (chemical, informational); blue, specific growth rate.