Mixotrophic capabilities of Alcaligenes eutrophus

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 0.2 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 0.2 h-1 but significantly at higher dilution rates and was only 18% above the heterotrophic yield at D = 0.32 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.