The facultatively chemolithoautotrophic hydrogen bacterium was able to utilize organic and inorganic substrates concomitantly, i.e. to grow mixotrophically. The mixotrophic capabilities were investigated under succinate-limited growth of with molecular hydrogen in a gas atmosphere devoid of carbon dioxide. At a dilution rate () of 0.2 h, the mixotrophic cellular yield was increased by 135% over the heterotrophic yield with succinate alone. Total carbon analysis revealed that under these conditions 95% of the succinate carbon was converted to cell carbon. The mixotrophic yield decreased only slightly at dilution rates lower than 0.2 h but significantly at higher dilution rates and was only 18% above the heterotrophic yield at = 0.32 h. Unlike other facultative chemoautotrophs, mixotrophic growth of required both H oxidation (Hox) and autotrophic CO fixation (Cfx), as evident from mutants defective in either H oxidation (Hox) or autotrophic metabolism (Cfx), as well as from incorporation studies of radioactive substrates. The cellular yield of a Cfx mutant, HF17, increased only slightly (by 14%) upon the addition of H, indicating that the ability of to change the metabolism of a heterotrophic substrate was limited. Hox mutants did not increase their cellular yield under identical growth conditions.


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