Heterotrophic Carbon Metabolism by Beggiatoa alba

Abstract

The assimilation and metabolism of CO 2 and acetate by Beggiatoa alba strain B18LD was investigated. Although B. alba was shown to require CO 2 for growth, the addition of excess CO 2 (as NaHCO 3 ) to the medium in a closed system did not stimulate growth. Approximately 24 to 31% of the methyl-labeled acetate and 38 to 46% of the carboxyl-labeled acetate were oxidized to 14 CO 2 by B. alba . The apparent V max values for combined assimilation and oxidation of [2- 14 C]acetate by B. alba were 126 to 202 nmol min −1 mg of protein −1 under differing growth conditions. The V max values for CO 2 assimilation by heterotrophic and mixotrophic cells were 106 and 131 pmol min −1 mg of protein −1 , respectively. The low V max values for CO 2 assimilation, coupled with the high V max values for acetate oxidation, suggested that the required CO 2 was endogenously produced from acetate. Moreover, exogenously supplied acetate was required by B. alba for the fixation of CO 2 . From 61 to 73% of the [ 14 C]acetate assimilated by washed trichomes was incorporated into lipid. Fifty-five percent of the assimilated [2- 14 C]acetate was incorporated into poly-β-hydroxybutyric acid. This was consistent with chemical data showing that 56% of the heterotrophic cell dry weight was poly-β-hydroxybutyric acid. Succinate and CO 2 were incorporated into cell wall material, proteins, lipids, nucleic acids, and amino and organic acids, but not into poly-β-hydroxybutyric acid. Glutamate and succinate were the major stable products after short-term [1- 14 C]acetate assimilation. Glutamate and aspartate were the first stable 14 CO 2 fixation products, whereas glutamate, a phosphorylated compound, succinate, and aspartate were the major stable 14 CO 2 fixation products over a 30-min period. The CO 2 fixation enzymes isocitrate dehydrogenase (nicotinamide adenine dinucleotide phosphate; reversed) and malate dehydrogenase (nicotinamide adenine dinucleotide phosphate; decarboxylating) were found in cell-free extracts of both mixotrophically grown and heterotrophically grown cells. The data indicate that the typical autotrophic CO 2 fixation mechanisms are absent from B. alba B18LD and that the CO 2 and acetate metabolism pathways are probably linked.