Differences in growth and iron oxidation amongThiobacillus ferrooxidans cultures in the presence of some toxic metals

Biochar-bacteria-plant partnerships: Eco-solutions for tackling heavy metal pollution

Over the past 30 years, the ever-rising demands of the modern and growing population have led to the rapid development of agricultural and industrial sectors worldwide. However, this expansion has exposed the environment to various pollutants including heavy metal (HM)s. Almost all HMs are serious toxicants and can pose serious health risks to living organisms in addition to their bioaccumulative and non-biodegradable nature. Different techniques have been developed to restore the ecological functions of the HM-contaminated soil (HMCS). However, the major downfalls of the commonly used remediation technologies are the generation of secondary wastes, high operating costs, and high energy consumption. Phytoremediation is a prominent approach that is more innocuous than the existing remediation approaches. Some microbes-plant interactions enhance the bioremediation process, with heavy metal resistant-plant growth promoting bacteria (HMRPGPB) being widely used to assist phytoremediation of HMs. However, the most common of all major microbial assisted-phytoremediation disturbances is that the HM-contaminated soil is generally deficient in nutrients and cannot sustain the rapid growth of the applied HMRPGPB. In this case, biochar has recently been approved as a potential carrier of microbial agents. The biochar-HMRPGPB-plant association could provide a promising green approach to remediate HM-polluted sites. Therefore, this review addresses the mechanisms through which biochar and HMRPGPB can enhance phytoremediation. This knowledge of biochar-HMRPGPB-plant interactions is significant with respect to sustainable management of the HM-polluted environment in terms of both ecology and economy, and it offers the possibility of further development of new green technologies.

The small heat shock proteins from Acidithiobacillus ferrooxidans: gene expression, phylogenetic analysis, and structural modeling

Background

Acidithiobacillus ferrooxidans is an acidophilic, chemolithoautotrophic bacterium that has been successfully used in metal bioleaching. In this study, an analysis of the A. ferrooxidans ATCC 23270 genome revealed the presence of three sHSP genes, Afe_1009, Afe_1437 and Afe_2172, that encode proteins from the HSP20 family, a class of intracellular multimers that is especially important in extremophile microorganisms.

Results

The expression of the sHSP genes was investigated in A. ferrooxidans cells submitted to a heat shock at 40°C for 15, 30 and 60 minutes. After 60 minutes, the gene on locus Afe_1437 was about 20-fold more highly expressed than the gene on locus Afe_2172. Bioinformatic and phylogenetic analyses showed that the sHSPs from A. ferrooxidans are possible non-paralogous proteins, and are regulated by the σ³² factor, a common transcription factor of heat shock proteins. Structural studies using homology molecular modeling indicated that the proteins encoded by Afe_1009 and Afe_1437 have a conserved α-crystallin domain and share similar structural features with the sHSP from Methanococcus jannaschii, suggesting that their biological assembly involves 24 molecules and resembles a hollow spherical shell.

Conclusion

We conclude that the sHSPs encoded by the Afe_1437 and Afe_1009 genes are more likely to act as molecular chaperones in the A. ferrooxidans heat shock response. In addition, the three sHSPs from A. ferrooxidans are not recent paralogs, and the Afe_1437 and Afe_1009 genes could be inherited horizontally by A. ferrooxidans.

Quorum-sensing system in Acidithiobacillus ferrooxidans involved in its resistance to Cu²⁺

AIMS

 The purpose of this study was to search for the relationship between quorum sensing (QS) and Cu²⁺ resistance in Acidithiobacillus ferrooxidans.

METHODS AND RESULTS

 Resistance to Cu²⁺ of A. ferrooxidans significantly decreased with the treatment dose of a synthetic QS blocker (5Z)-4-bromo-5-(bromomethylene)-2(5H)-furanone (FUR). Relative differences in expression of the QS genes afeI, afeR and Cu²⁺ resistance-associated genes afe0329, afe0454 were examined in the presence of Cu²⁺ and/or FUR compound. The expression of QS genes afeI and afeR increased significantly with 50 mmol l⁻¹ Cu²⁺ in the culture, while for samples treated with both 50 mmol l⁻¹ Cu²⁺ and 0.01 μg ml⁻¹ FUR compound, they showed little changes compared with control, and the expression of afe0329 and afe0454 genes increased slightly either. These results showed that QS system was positively related to the mechanism of Cu²⁺ resistance.

CONCLUSIONS

QS system in A. ferrooxidans involved in its resistance to Cu²⁺.

SIGNIFICANCE AND IMPACT OF THE STUDY

 The mechanisms of Cu²⁺ resistance in A. ferrooxidans could be revealed on a population level rather than on a single-cell level. Our work also provides useful data for further selection of A. ferrooxidans strains with suitable Cu²⁺ resistance that could probably increase the bioleaching efficiency.

Effect of chloride on ferrous iron oxidation by a Leptospirillum ferriphilum-dominated chemostat culture

Biomining is the use of microorganisms to catalyze metal extraction from sulfide ores. However, the available water in some biomining environments has high chloride concentrations and therefore, chloride toxicity to ferrous oxidizing microorganisms has been investigated. Batch biooxidation of Fe(2+) by a Leptospirillum ferriphilum-dominated culture was completely inhibited by 12 g L(-1) chloride. In addition, the effects of chloride on oxidation kinetics in a Fe(2+) limited chemostat were studied. Results from the chemostat modeling suggest that the chloride toxicity was attributed to affects on the Fe(2+) oxidation system, pH homeostasis, and lowering of the proton motive force. Modeling showed a decrease in the maximum specific growth rate (micro(max)) and an increase in the substrate constant (K(s)) with increasing chloride concentrations, indicating an effect on the Fe(2+) oxidation system. The model proposes a lowered maintenance activity when the media was fed with 2-3 g L(-1) chloride with a concomitant drastic decrease in the true yield (Y(true)). This model helps to understand the influence of chloride on Fe(2+) biooxidation kinetics.

Single and combined effects of nickel (Ni(II)) and cobalt (Co(II)) ions on activated sludge and on other aerobic microorganisms: a review

Nickel (N(II)) and cobalt (Co(II)) are often encountered in wastewaters. As conventional wastewater treatment may only partially remove nickel and cobalt, a large fraction of the above metals is released to the aquatic environment. Both metals have been identified as micronutrients, at trace concentrations; however, they are both microbial growth inhibitors, at relatively high concentrations. On the other hand, the combined effects (e.g.: growth stimulation or toxicity) of the above metals have been found to differ from the summation of the effects which occur when the metals are applied individually. Moreover, a number of environmental factors (e.g.: pH, biomedium composition, biomass concentration, presence of other heavy metals) can affect the microbial toxicity of the above metallic species. The present review discusses, in a systematic way, the individual and joint effects of the above heavy metals to the growth of microorganisms grown under aerobic conditions, with focus on the growth of activated sludge. Data on multi-metal toxicity are particularly useful in establishing criteria for heavy metal tolerance levels in the environment.

ribB and ribBA genes from Acidithiobacillus ferrooxidans: expression levels under different growth conditions and phylogenetic analysis

Acidithiobacillus ferrooxidans is a Gram-negative, chemolithoautotrophic bacterium involved in metal bioleaching. Using the RNA arbitrarily primed polymerase chain reaction (RAP-PCR), we have identified several cDNAs that were differentially expressed when A. ferrooxidans LR was submitted to potassium- and phosphate-limiting conditions. One of these cDNAs showed similarity with ribB. An analysis of the A. ferrooxidans ATCC 23270 genome, made available by The Institute for Genomic Research, showed that the ribB gene was not located in the rib operon, but a ribBA gene was present in this operon instead. The ribBA gene was isolated from A. ferrooxidans LR and expression of both ribB and ribBA was investigated. Transcript levels of both genes were enhanced in cells grown in the absence of K2HPO4, in the presence of zinc and copper sulfate and in different pHs. Transcript levels decreased upon exposure to a temperature higher than the ideal 30 degrees C and at pH 1.2. A comparative genomic analysis using the A. ferrooxidans ATCC 23270 genome revealed similar putative regulatory elements for both genes. Moreover, an RFN element was identified upstream from the ribB gene. Phylogenetic analysis of the distribution of RibB and RibBA in bacteria showed six different combinations. We suggest that the presence of duplicated riboflavin synthesis genes in bacteria must provide their host with some benefit in certain stressful situations.

The Effect of Aluminium and Magnesium Sulphate on the Rate of Ferrous Iron Oxidation by Leptospirillum ferriphilum in Continuous Culture

In heap bioleaching the dissolution of gangue minerals from igneous ore materials can lead to the build-up of considerable concentrations of Mg and Al sulphates in the recycled leach solution. This may interfere with microbial ferrous iron oxidation, which drives the oxidation of the target minerals. The kinetics of the oxidation process have been well studied for Leptospirillum and Acidithiobacillus species in tank systems. Although not directly comparable, kinetic parameters derived for tank systems do apply also for heap bioleach conditions. In the present study the effect of solution concentrations of Mg and Al as sulphate at individual concentrations of 0 to 10 g/L and combined concentrations 0 to 16 g/L each has been investigated in continuous culture using Leptospirillum ferriphilum. Increasing the concentrations of the salts increasingly depresses the rate of ferrous iron oxidation and also shifts the viable range more and more into the low potential region. Al significantly reduces the amount of carbon maintained in the reactor (assumed to be commensurate with biomass), whereas Mg actually enhances it at low concentrations. In both cases, however, the rate is always depressed. The results indicate that heap cultures are likely to perform sub-optimally in those operations where build-up of dissolved gangue minerals is not controlled.

Acidithiobacillus ferrooxidans fixation on mercuric surfaces and its application in stripping voltammetry

The adsorption (fixation) of bacteria Acidithiobacillus ferrooxidans on Hg and Cu metallic surfaces was qualitatively studied owing to two independent methods: frequency measurement using a quartz crystal microbalance and light absorption measuring at the Hg/bacterial culture interface. A method using a dropping mercury electrode (DME) allowed quantifying this bacterial fixation. Determining the superficial tension at Hg/bacterial culture interface led to determine bacteria adsorption on Hg through the Gibbs isotherm. A modified stripping voltammetry was proposed taking benefit of both bacterial adsorption on Hg surface and metal fixation capacity on biomass. Metal preconcentration on the biologically modified Hg electrode appeared to improve the measurement sensitivity of differential pulse anodic stripping voltammetry (DPASV). The height of the detected peaks was thus increased of 17.6% for copper, 48.4% for lead, and 132% for cadmium determinations compared to those obtained with an unmodified mercury electrode. Such augmentation depended on bulk bacteria concentration and bacteria preconcentration.

Metal tolerance and antibiotic resistance patterns of a bacterial population isolated from sea water

The total aerobic heterotrophic and metal-resistant bacterial communities were studied in marine water. The resistance patterns, expressed as MICs, for 81 bacterial isolates to eight heavy metals were surveyed by using the agar dilution method. A great proportion of the isolates were sensitive to cadmium (99%), mercury (91%), zinc (84%) and cobalt (83%). On the other hand, 94%, 40%, 35% and 22% were resistant to lead, nickel, arsenate and copper, respectively. The majority of the tested strains (95.06%) were multiple metal-resistant, with pentametal resistance as the major pattern (25.9%). The response of the isolates to 11 tested antibiotics was tested and ranged from complete resistance to total sensitivity and multiple antibiotic resistance was exhibited by 70.38% of the total isolated population. The highest incidence of metal-antibiotic double resistance existed between lead and all antibiotics (100%), copper and penicillin (95%) and nickel and ampicillin (83.3%).