Heterotrophic Carbon Metabolism by Beggiatoa alba

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Abstract
The assimilation and of CO2 and acetate by B. alba strain B18LD was investigated. Although B. alba required CO2 for growth, the addition of excess CO2 (as NaHCO3) to the medium in a closed system did not stimulate growth. Approximately 24-31% of the methyl-labeled acetate and 38-46% of the carboxyl-labeled acetate were oxidized to 14CO2 by B. alba. The apparent Vmax values for combined assimilation and oxidation of [2-14C]acetate by B. alba were 126-202 nmol min-1 mg protein -1 under differing growth conditions. The Vmax values for CO2 assimilation by heterotrophic and mixotrophic cells were 106 and 131 pmol min-1 mg protein -1, respectively. The low Vmax values for CO2 assimilation, coupled with the high Vmax values for acetate oxidation, suggested that the required CO2 was endogenously produced from acetate. Exogenously supplied acetate was required by B. alba for the fixation of CO2. From 61-73% of the [14C]acetate assimilated by washed trichomes was incorporated into lipid. Of the assimilated [2-14C]acetate, 55% was incorporated into poly-.beta.-hydroxybutyric acid. This was consistent with chemical data showing that 56% of the heterotropic cell dry weight was poly-.beta.-hydroxybutyric acids. Succinate and CO2 were incorporated into cell wall material, proteins, lipids, nucleic acids and amino and organic acids, but not into poly-.beta.-hydroxybutyric acid. Glutamate and succinate were the major stable products after short-term [1-14C]acetate assimilation. Glutamate and aspartate were the first stable 14CO2 fixation products, whereas glutamate, a phosphorylated compound, succinate and aspartate were the major stable 14CO2 fixation products over a 30-min period. The CO2 fixation enzymes isocitrate dehydrogenase (NADP; reversed) and malate dehydrogenase (NADP decarboxylating) were found in cell-free extracts of both mixotrophically grown and heterotrophically grown cells. The typical autotrophic CO2 fixation mechanisms are absent from B. alba B18LD and the CO2 and acetate metabolism pathways are probably linked.