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Abstract
The facultatively chemolithoautotrophic hydrogen bacterium Alcaligenes eutrophus was able to utilize organic and inorganic substrates concomitantly, i.e. to grow mixotrophically. The mixotrophic capabilities were investigated under succinate-limited growth of A. eutrophus with molecular hydrogen in a gas atmosphere devoid of carbon dioxide. At a dilution rate (D) of 00·2 h−1, 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 00·2 h−1 but significantly at higher dilution rates and was only 18% above the heterotrophic yield at D = 00·32 h−1. Unlike other facultative chemoautotrophs, mixotrophic growth of A. eutrophus required both H2 oxidation (Hox) and autotrophic CO2 fixation (Cfx), as evident from mutants defective in either H2 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 H2, indicating that the ability of A. eutrophus 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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