Insights on Cadmium Removal by Bioremediation: The Case of Haloarchaea

Abstract

Although heavy metals are naturally found in the environment as components of the earth’s crust, environmental pollution by these toxic elements has increased since the industrial revolution. Some of them can be considered essential, since they play regulatory roles in different biological processes; but the role of other heavy metals in living tissues is not clear, and once ingested they can accumulate in the organism for long periods of time causing adverse health effects. To mitigate this problem, different methods have been used to remove heavy metals from water and soil, such as chelation-based processes. However, techniques like bioremediation are leaving these conventional methodologies in the background for being more effective and eco-friendlier. Recently, different research lines have been promoted, in which several organisms have been used for bioremediation approaches. Within this context, the extremophilic microorganisms represent one of the best tools for the treatment of contaminated sites due to the biochemical and molecular properties they show. Furthermore, since it is estimated that 5% of industrial effluents are saline and hypersaline, halophilic microorganisms have been suggested as good candidates for bioremediation and treatment of this kind of samples. These microorganisms, and specifically the haloarchaea group, are of interest to design strategies aiming the removal of polluting compounds due to the efficiency of their metabolism under extreme conditions and their significant tolerance to highly toxic compounds such as heavy metals, bromate, nitrite, chlorate, or perchlorate ions. However, there are still few trials that have proven the bioremediation of environments contaminated with heavy metals using these microorganisms. This review analyses scientific literature focused on metabolic capabilities of haloarchaea that may allow these microbes to tolerate and eliminate heavy metals from the media, paying special attention to cadmium. Thus, this work will shed light on potential uses of haloarchaea in bioremediation of soils and waters negatively affected by heavy metals, and more specifically by cadmium.

Assessment of Haloferax mediterranei Genome in Search of Copper-Molecular Machinery With Potential Applications for Bioremediation

“…However, these types of molecules have not been found in this research. Intracellular chelation/sequestration in haloarchaea is achieved by cysteine-rich metal-chelating proteins such as γ-glutamylcysteine, as has been evidenced in Haloferax volcanii ( Vera-Bernal and Martínez-Espinosa, 2021 ) and whose nucleotide sequence was also found in H. mediterranei genome. Another chelating molecule of copper is inorganic polyphosphate (PolyP).…”

“…Metal-ion specific efflux families of transporters P 1B -ATPase, Cation Diffusion Facilitators (CDF), and ATP-Binding Cassette (ABC) are found in haloarchaea to actively expulse excess of heavy metals to the periplasm or the extracellular space ( Nies, 2003 ; Voica et al, 2016 ; Vera-Bernal and Martínez-Espinosa, 2021 ). In H. mediterranei , chromosomic and plasmidic copper P 1B -type ATPases might oversee exporting copper.…”

“…CDFA transporters are driven by H + or K + gradient ( Haney et al, 2005 ) and are grouped according to metal specificity (Mn 2+ , Zn 2+ /Fe 2+ , and Zn 2+ and other metals than Fe 2+ ; Montanini et al, 2007 ). Their presence has been annotated in some species of the Halobacteria class such as Natronococcus occultus and Haloquadratum walsbyi (NCBI Database), however, the substrate spectrum may not include the transport of Mg 2+ and Cu 2+ due to their smaller ionic radii ( Vera-Bernal and Martínez-Espinosa, 2021 ). Lastly, ABC transporters are proven to unidirectionally import or export toxic metals in plants and are involved in essential ion transport in bacteria ( Napolitano et al, 2012 ).…”

“…Several recent studies state that this species could be a good candidate for the design of bioremediation-based technologies to treat wastewaters, particularly brines or osmosis concentrates showing high salt concentrations and significant concentrations of nitrate, nitrite and (per)chlorate, compounds that are toxic for most living beings ( Nájera-Fernández et al, 2012 ; Martínez-Espinosa et al, 2015 ; Torregrosa-Crespo et al, 2020 ). Besides, molecular machinery for the potential removal of cadmium has also been identified from many haloarchaeal species ( Vera-Bernal and Martínez-Espinosa, 2021 ). For this reason, bioinformatic analysis of the genome of H. mediterranei strain R-4 (ATCC 33500) might contribute to the understanding of its potential role in bioremediation processes aiming at the removal of heavy metals.…”

Bioremediation of Soil Pollution by Polychlorinated Biphenyls in Solid Waste Dismantling

“…It must be equal to the restoration target value. The determination of the restoration target value can only be obtained after comparing the risk control value with the soil pollution screening value, the control value, and the soil background value [13][14].…”