Lithoautotrophic growth of the freshwater strain Beggiatoa D-402 and energy conservation in a homogeneous culture under microoxic conditions

The freshwater filamentous bacterium Beggiatoa D-402 was shown to grow lithoautotrophically in a homogeneous culture under microoxic conditions only, the growth yield being the highest at 0.1 mg O(2) l(-1). High activities of the Calvin cycle key enzymes and of the dissimilatory path thiosulfate oxidation enzymes were found in the bacterial cells. The rate of CO(2) fixation above 112 nmol min(-1) (mg protein)(-1), an about 90% increase in the protein carbon at the expense of CO(2) carbon and an increase in the molar yield up to 12 mg dry weight (mmol oxidized thiosulfate)(-1) indicate the bacterial growth was autotrophic. Thiosulfate was oxidized by the strain almost completely into sulfate. The metabolically useful energy was conserved by oxidative phosphorylation that was coupled to oxidation of sulfur compounds. The bacterial membranes were found to contain CO-binding cytochromes b and two cytochromes c with M(r) 23 and 26 kDa, the terminal part of the respiratory chain containing presumably a cbb(3)-type oxidase. A cytochrome c with M(r) 12 kDa was detected in the soluble fraction.     

Regulation of Metabolic and Electron Transport Pathways in the Freshwater Bacterium Beggiatoa leptomitiformis D-402

The biomass yield of freshwater filamentous sulfur bacteria of the genus Beggiatoa, when grown lithoheterotrophically or mixotrophically, has been shown to increase 2 to 2.5 times under microaerobic conditions (0.12 mg/l oxygen) as compared to aerobic conditions (9 mg/l oxygen). The activity of the glyoxylate cycle key enzymes have been found to increase two to three times under microaerobic conditions (at an O₂ concentration of 2 mg/l), and the activities of the sulfur metabolism enzymes increased three to five times (at an O₂ concentration of 0.1–0.5 mg/l). It has also been found that, under microaerobic conditions, thiosulfate was almost completely oxidized to sulfate by the bacteria, without accumulation of intermediate metabolites. At the same time, a 2- to 15-fold decrease in the activities of the tricarboxylic acid cycle enzymes involved in the reduction of NAD and FAD was observed. Reorganization of the respiratory chain after changes in aeration and type of nutrition was also observed. It has been found that, in cells grown heterotrophically, the terminal part of the respiratory chain contained an aa ₃-type oxidase, whereas, during mixotrophic, lithoheterotrophic, and autotrophic growth, aa ₃-type oxidase synthesis was inhibited, and the synthesis of a cbb ₃-type oxidase, which is induced under microaerobic conditions, was activated. The gene of the catalytic subunit CcoN of the cbb ₃-type oxidase was sequenced and proved to be highly homologous to the corresponding genes of other proteobacteria.__________Translated from Mikrobiologiya, Vol. 74, No. 4, 2005, pp. 452–459.Original Russian Text Copyright © 2005 by Muntyan, Grabovich, Patritskaya, Dubinina.     

Lithoautotrophic Growth of the Freshwater Colorless Sulfur Bacterium Beggiatoa “leptomitiformis”D-402

The freshwater colorless sulfur bacterium Beggiatoa leptomitiformisD-402 was shown to be capable of lithoautotrophic growth in a batch culture under microoxic conditions at O₂concentrations in the medium of no higher than 0.5 mg/l. The cell yield was maximum at a dissolved oxygen concentration of 0.15 mg/l. A high activity level of key enzymes of the Calvin cycle and of enzymes involved in dissimilatory oxidation of thiosulfate was recorded in the cells. The high rate of CO₂assimilation (112–139 nmol/(min mg protein)) and the cell yield (12 mg dry cells/mmol thiosulfate oxidized), 91–92% of which was accounted for by CO₂carbon, were close to those typical of autotrophic bacteria. Thiosulfate was oxidized almost completely to sulfate, and the fraction of intracellular sulfur in the final products did not exceed 0.2–1.7% of the thiosulfate sulfur. The cell membrane fraction contained cytochromes (b + o) and two cytochromes cwith M _rof 23 and 26 kDa; the soluble fraction contained cytochrome cwith M _rof 12 kDa.     

Dependence of Malate Dehydrogenase Structure on the Type of Metabolism in Freshwater Filamentous Colorless Sulfur Bacteria of the Genus Beggiatoa

Major pathways of carbon metabolism were studied in strains D-402 and D-405 of freshwater colorless sulfur bacteria of the genus Beggiatoa grown organotrophically and mixotrophically. The bacteria were found to possess all the enzymes of the tricarboxylic acid (TCA) and glyoxylate cycles. When organotrophic growth changed to mixotrophic growth, the activity of the TCA cycle enzymes decreased 2- to 3-fold, but the activity of enzymes of the glyoxylate cycle increased threefold. It follows that, in the oxidation of thiosulfate, organic compounds no longer play the leading part in the energy metabolism, and most of electrons that enter the electron transport chain (ETC) derive from inorganic sulfur compounds. A connection was established between the structure and kinetic characteristics of malate dehydrogenase—an enzyme of the TCA and glyoxylate cycles—and the type of carbon metabolism in the strains studied. Malate dehydrogenase in organotrophically grown cells of strains D-402 and D-405 is dimeric, whereas in strain D-402 grown mixotrophically it is tetrameric.