Eva Pakostová

Cellular ATP Changes in Acidithiobacillus ferrooxidans Cultures Oxidizing Ferrous Iron and Elemental Sulfur

Cellular ATP content of Acidithiobacillus ferrooxidans cultures was determined with a bioluminescence assay in relation to batch growth and oxidation of ferrous iron and elemental sulfur. Inhibitory effects of inorganic substrates and products on luciferase were eliminated. Extracellular ATP levels were negligible. The ATP content of sulfur-grown cells decreased anomalously due to a culture pH increase at the stationary phase. Although the rates of growth and sulfur oxidation reached the original levels, the ATP content of the culture remained constant because of gradual decrease in the cellular ATP. The maximum ATP levels in A. ferrooxidans grown with Fe2+ and S0 were 1.16 and 0.33 amol per cell, respectively. The results defined conditions under which biomass growth could be monitored by the ATP assay to study biogeochemical activities of acidophilic iron- and sulfur-oxidizing bacteria.

Are there multiple mechanisms of anaerobic sulfur oxidation with ferric iron in Acidithiobacillus ferrooxidans

To clarify the pathway of anaerobic sulfur oxidation coupled with dissimilatory ferric iron reduction in Acidithiobacillus ferrooxidans strain CCM 4253 cells, we monitored their energy metabolism gene transcript profiles. Several genes encoding electron transporters involved in aerobic iron and sulfur respiration were induced during anaerobic growth of ferrous iron-grown cells. Most sulfur metabolism genes were either expressed at the basal level or their expression declined. However, transcript levels of genes assumed to be responsible for processing of elemental sulfur and other sulfur intermediates were elevated at the beginning of the growth period. In contrast, genes with predicted functions in formation of hydrogen sulfide and sulfate were significantly repressed. The main proposed mechanism involves: outer membrane protein Cyc2 (assumed to function as a terminal ferric iron reductase); periplasmic electron shuttle rusticyanin; c4-type cytochrome CycA1; the inner membrane cytochrome bc1 complex I; and the quinone pool providing connection to the sulfur metabolism machinery, consisting of heterodisulfide reductase, thiosulfate:quinone oxidoreductase and tetrathionate hydrolase. However, an alternative mechanism seems to involve a high potential iron-sulfur protein Hip, c4-type cytochrome CycA2 and inner membrane cytochrome bc1 complex II. Our results conflict with findings regarding the type strain, indicating strain- or phenotype-dependent pathway variation.

Minimum Aeration in Acidithiobacillus ferrooxidans Cultures Required to Maintain Substrate Oxidation without Oxygen Limitation

The volumetric oxygen transfer coefficient (kLa) was used to define the conditions necessary for minimum aeration and to eliminate potential oxygen limitation in bioleaching cultures of Acidithiobacillus ferrooxidans. The Michaelis constants for oxygen were 1.07 and 0.71 μmol O2 l-1 for the oxidation of ferrous iron and elemental sulphur, respectively. The critical oxygen concentration, below which oxygen limitation occurred, was determined to be 6.25 and 3.125 μmol O2 l-1 for the oxidation of ferrous iron and elemental sulphur, respectively. The (kLa)crit values required to maintain oxygen-unlimited substrate oxidation for ferrous iron and elemental sulphur were 7.70 and 4.88 h-1, respectively.