In most autotrophic fermentations with R. eutropha reported in the literature, the initial gas mixture (typically 8:1:1 H2:CO2:O2) is within the explosive range for the H2 and O2 gas concentrations. The low aqueous solubility of both H2 and O2 presents challenges in making these gases bioavailable to R. eutropha cells [14] . As with many aerobic microbial biotransformations, the rate of gas mass transfer (dissolu­tion) from the gas to the liquid phase represents another potential rate-limiting step.

One strategy to reduce the explosion risk during autotrophic growth of R. eutro — pha is to keep the H2 and O2 gas streams physically separated. This can potentially be performed by the hollow fiber reactor setup discussed in Sect. 4.2 (Fig. 7). However, for initial screening, microfermenters (bioreactors with 1 mL or less working volume, discussed in [87-90]) can be used to optimize growth and produc­tion conditions prior to culture scale-up. Risk of explosion still exists in such sys­tems, but the small scale of the reactors would control the potential damage.