Problematic heavy metal mining: Plants take it upon themselves

The process of extraction of heavy metals of high value or that are toxic in nature, from soil and water, has always drawn attention from engineers, economists and environmentalists. This is because the conventional techniques are either very costly or are environmentally challenging. Hence nowadays biological methods have gained the much needed popularity, as compared to the usual physical and chemical approaches to capture metals, due to their cost effectiveness and eco-friendly nature.

Biological methods can be classified into two categories: biosorption and bioaccumulation (or bioextraction). In biosorption, the physiochemical property of surface adsorption is enhanced by chemically pre-treating some biological specimens. While bioaccumulation uses the innate ability of some living organisms to naturally absorb metals.

When this bioaccumulation is carried out by plants then the process is called phytoaccumulation. Phytoaccumulation is very useful where a valuable/toxic metal is to be directly extracted from soil without disturbing the soil structure. But the most common disadvantage of phytoaccumulation is its low uptake efficiency. Hence, it would be beneficial to develop methods which enable improved efficiency in phytoaccumulation of the target metal.

Of the many commercial and high-value metals, cesium exists at relatively low levels of approximately 3 ppm in the Earth’s crust. Cesium is used industrially for example in the production of drilling fluids and in the manufacture of atomic clocks. Radiocesium-137 has medical and industrial applications. At present, mining of pollucite ore is the major source of cesium production. But mining is conducted in only very few places and only on a small scale. Cesium, an alkali metal with a wide range of commercial applications, occurs relatively rarely in nature and a process for the recycling of the metal has not yet been established. Although it is said that cesium reserves are sufficient for consumption at the present rate, it is vulnerable to depletion in the long term. Moreover, cesium-137, is a radioactive isotope derived from nuclear waste, presents a serious health risk and environmental threat. Many plant species have been tested for hyperaccumulation of Cesium, by varying various parameters, but most have shown low efficiency.

In this current study, a chemical library composing 10,000 synthetic organic compounds was screened, by the authors, for those which promote cesium accumulation in Arabidopsis thaliana. Out of the 10,000 compounds that had been screened for, 14 chemical compounds were isolated finally, and out of those one was characterised as a cysteine derivative, methyl cysteinate. This characterization was done based on various experiments done on Arabidopsis thaliana, like in optimal potassium conditions (1.75 mM K), 0.4 mM cesium hardly confers visible negative effects on plants. However, if a chemical increases cesium accumulation then the plants show cesium-triggered phenotype such as stunting and chlorosis. The cesium accumulator selected was found to be a cysteine derivative, methyl cysteinate (C4H9NO2S), where a hydrogen atom of the carboxyl group is replaced by a methyl group. Methyl cysteinate at a concentration of 25 μM promoted increased cesium accumulation in plants by 22.4% compared to that in non-treated plants. It was observed that by the application of methyl cysteinate, the potassium concentrations were not altered.

In order to confirm whether cysteine or other cysteine derivatives also help plants accumulate cesium, cysteine, cysteine ethyl ester and N-acetylcysteine, were tested. Cysteine showed a cesium accumulation effect but at the lower extent (14.1% increase) and at a much higher concentration (250 μM) compared to methyl cysteinate. Application of cysteine ethyl ester and N-acetylcysteine did not increase cesium accumulation but severely stunted plants and dramatically decreased potassium concentrations.

methyl cystinate
Chemical structures of methyl cysteinate and cysteine.

It is known that cesium and potassium ratios are important determinants of plant performance, so optimal (1.75 mM), suboptimal (0.5 mM) and deficient (25 μM) potassium conditions in the presence and the absence of cesium were analysed. Seedlings grown in optimal and suboptimal potassium conditions were both healthy and almost indistinguishable from one another while growth of those in a deficient potassium condition were visibly stunted. The effects of cesium varied in accordance with the potassium conditions although the concentrations of cesium were constant. Cesium-treated plants showed higher levels of cysteine in the roots in suboptimal and deficient potassium conditions and in the shoots in optimal and suboptimal potassium conditions.

potassium
Phenotype of wild type (Col-0) seedlings grown under optimal (1.75 mM), suboptimal (0.5 mM) or deficient (25 μM) potassium conditions in the presence or absence of 0.3 mM CsCl for 9 days. The scale bar denotes 1 cm.

These results suggest that it is the particular structure of methyl cysteinate that exerts this enhanced cesium accumulation effect rather than the cysteine metabolic pathway as a whole. Methyl cysteinate was found to be more effective in improving cesium accumulation and worked at one order of magnitude lower concentrations compared to cysteine. This might be explained by the higher dipole moment and higher binding energy with cesium for methyl cysteinate.

The results indicate that methyl cysteinate improves cesium phytoaccumulation efficiency through as-yet-unknown mechanism but which could be through promoting cesium adhesion to the root surface or inhibiting cesium extrusion from the plant cells due to a direct binding with cesium. Although there is potential for further research in this field, it has been suggested from the results that external cesium increases internal cysteine levels in plants and is potentially metabolised into methyl cysteinate which contributes to cesium accumulation through binding with cesium.

So, yet again we see that plants come to our rescue and take upon themselves the burden of extraction of heavy/toxic metals, that was conventionally very difficult.

 

 

References

Adams, E., et al., A novel role for methyl cysteinate, a cysteine derivative, in cesium accumulation in Arabidopsis thaliana. Scientific Reports, 2017. 7: p. 43170.

 

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