Abstract
Extreme terrestrial and marine hot environments are excellent niches for specialized microorganisms belonging to the domains of Bacteria and Archaea; these microorganisms are considered extreme from an anthropocentric point of view because they are able to populate harsh habitats tolerating a variety of conditions, such as extreme temperature and/or pH, high metal concentration and/or salt; moreover, like all the microorganisms, they are also able to respond to sudden changes in the environmental conditions. Therefore, it is not surprising that they possess an extraordinary variety of dynamic and versatile mechanisms for facing different chemical and physical stresses. Such features have attracted scientists also considering an applicative point of view. In this review we will focus on the molecular mechanisms responsible for survival and adaptation of thermophiles to toxic metals, with particular emphasis on As(V), As(III), Cd(II), and on current biotechnologies for their detection, extraction and removal.
Highlights
Extreme terrestrial and marine hot environments are excellent niches for specialized microorganisms belonging to the domains of Bacteria and Archaea; these microorganisms are considered extreme from an anthropocentric point of view because they are able to populate harsh habitats tolerating a variety of conditions, such as extreme temperature and/or pH, high metal concentration and/or salt; like all the microorganisms, they are able to respond to sudden changes in the environmental conditions
Metal tolerance of thermophilic Bacteria/Archaea is due to several mechanisms, many found in mesophilic counterparts, such as: extracellular barrier, metal ion transport into and outside the cell, the utilization of toxic metal ions in metabolism or the presence of metal resistance genes with different genomic localization [24]
The majority of thermophilic microorganisms are resistant to heavy metals thanks to an active transport and/or efflux of metal ions outside the cells
Summary
Heavy metals are among the most persistent and toxic pollutants in the environment [1]. Many metals are essential for life because they are actively involved in almost all aspects of metabolism: as examples, iron and copper are involved in the electron transport, manganese and zinc influence enzymatic regulations Their excess can disrupt natural biochemical processes and cause toxicity. Metal biotransformation impacts human health through the food chain: examples include the oxidation of Hg(0) to Hg(II), and the subsequent methylation to methylmercury compounds, which can be accumulated by fish and marine mammals in the aquatic environment [19] Despite their relevant toxicity, in a report of the European Commission (named “Critical-Metals in the Path towards the Decarbonisation of the EU Energy Sector”), several heavy metals such as cadmium, chromium and lead are included into the classification of critical raw materials. Common sources of heavy metals in this context include mining and industrial wastes, vehicle emissions, lead-acid batteries, fertilizers, paints, treated timber, aging water supply infrastructures, and microplastics floating in the world's oceans [21, 22]
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