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Gas fermentation normally involves gases such as hydrogen and methane which have very poor solubility in aqueous
media. This will limit the extent to which biomass can grow and hence risk the commercial viability of a process. The
graph below shows autotrophic growth of Cupriavidus necatar Re2133 at 1 bar with growth stopping at an optical
density of just over 0.2.
Improvements in reactor, agitation and sparger design can only improve gas transfer by a limited amount and any
major improvements need a more radical solution. The blue line on the graph shows the effect of a stepwise pressure
increase up to ~ 4 bar in an identical experimental set up. The experiments were conducted in a high pressure parallel
bioreactor system, BioXplorer (system rated to 10 bar). Elevated pressure led to an optical density of ~ 1.2 before the
gas availability became limiting (results courtesy of Prof. Stephane Guillouet, LISBP, INSA, France).
The graph also shows the specific growth rate of the strain and whilst this is seen to drop at elevated pressure, it still
remains significantly high. Gas fermentation at higher pressures has the potential to make this technology
commercially viable in the near future, especially with continued advances in bacterial strain development and
innovations in elevated pressure bioreactors.