Microdiverse types of Acidithiobacillus ferrooxidans and their interactions with uranium

In uranium mining waste piles a number of acidophilic chemolithoautotrophic bacteria have been identified, which are able to oxidise sulphide minerals, elemental sulfur, ferrous iron, and in presence of uranium minerals, also U(IV). Especially one representative of this group, Acidithiobacillus ferrooxidans, is of particular interest. This organism has been used industrially in metal leaching from ores and decontamination of industrial wastes.
Sequence analysis of the 16S rRNA genes of several reference strains and uranium mining waste pile isolates of this bacterium revealed specific signatures which distinguish three types within the species. This allowed to develop a technique for analysis of the distribution of the A. ferrooxidans eco-types in the soil samples of a uranium mining waste pile.
The technique is based on amplification of 16S rDNA fragments in total soil DNA by the use of two A. ferrooxidans species specific primers 16S458F and 16S1473R [1]. The resulting amplicons were then digested with a frequently cutting enzyme RsaI which produced three different type-specific profiles [2; 3]. Using this direct approach we have demonstrated that one of the A. ferrooxidans types (type I) was predominant in the soil samples studied and was found in more polluted sites, whereas the type II was found in less contaminated samples. The type III was found mostly to coexist with the type II.
The objectives of this work were to determine whether these eco-types differ in their capability to tolerate and accumulate uranium, and also to study the structural complexes formed at the surfaces of A. ferrooxidans eco-types using spectroscopic techniques as Extended X-ray Absorption Fine Structure (EXAFS), Infra Red (IR) and Raman Spectroscopy. In addition, the most efficient desorbing agent for the accumulated uranium was selected.
The uranium accumulation by the three types of A. ferrooxidans was studied at different metal concentrations and at pH 1.5 and 4. The results obtained (Fig.1) demonstrated that the strains from the three different types possess different capability to accumulate uranium. The amount of uranium biosorbed by the three types increased with increasing concentration of uranium and pH.

Fig.1: Biosorption of uranium by three types of A. ferrooxidans

On the basis of the results obtained studying the uranium tolerance by the three eco-types by the determination of the Minimum Inhibitory concentrations (MICs) of uranium for the growth of the strains studied, one may speculate that the strains of the types I and III are more resistant to uranium, probably because they possess a mechanism which limits the uranium binding below the lethal amounts.
The recovery of accumulated uranium by desorption was investigated using different desorbing agents as sodium carbonate, sodium citrate and EDTA at different concentrations. The results obtained demonstrated that sodium carbonate is able to recuperate up to 97% of the uranium sorbed from the cells of A. ferrooxidans type III, and 88.3 and 88.5 % from the cells of the types I and II, respectively.
Using EXAFS analysis we have found that no significant structural differences were observed between the uranium complexes formed by the thee types of A. ferrooxidans. However, the EXAFS spectra are indicating formation of uranium complexes which are different from those formed by bacilli [4; 5] and other bacteria.
References:
[1] De Wulf-Durand, P. et al., (1997) Appl. Environ. Microbiol. 63, 2944-2948
[2] Selenska-Pobell, S. et al., (2000) Antonie van Leewenhuek (in press)
[3] Selenska-Pobell, S. et al., (1999) Report FZR-285, p. 52
[4] Hennig, C. et al., (2000) Radiochemica Acta. (submitted)
[5] Hennig, C. et al., (1999) Report FZR-285, p. 70