Abstract
Metal(loid)s have a dual biological role as micronutrients and stress agents. A few geochemical and natural processes can cause their release in the environment, although most metal-contaminated sites derive from anthropogenic activities. Actinobacteria include high GC bacteria that inhabit a wide range of terrestrial and aquatic ecological niches, where they play essential roles in recycling or transforming organic and inorganic substances. The metal(loid) tolerance and/or resistance of several members of this phylum rely on mechanisms such as biosorption and extracellular sequestration by siderophores and extracellular polymeric substances (EPS), bioaccumulation, biotransformation, and metal efflux processes, which overall contribute to maintaining metal homeostasis. Considering the bioprocessing potential of metal(loid)s by Actinobacteria, the development of bioremediation strategies to reclaim metal-contaminated environments has gained scientific and economic interests. Moreover, the ability of Actinobacteria to produce nanoscale materials with intriguing physical-chemical and biological properties emphasizes the technological value of these biotic approaches. Given these premises, this review summarizes the strategies used by Actinobacteria to cope with metal(loid) toxicity and their undoubted role in bioremediation and bionanotechnology fields.
Highlights
Metals [e.g., silver (Ag), aluminum (Al), cadmium (Cd), cobalt (Co), chromium (Cr), cesium (Cs), copper (Cu), mercury (Hg), iron (Fe), magnesium (Mg), manganese (Mn), molybdenum (Mo), nickel (Ni), lead (Pb), strontium (Sr), zinc (Zn), uranium (U)] and metalloids [i.e., arsenic (As), selenium (Se), tellurium (Te)], naturally occurring throughout the Earth’s crust, are not generally present in soluble forms, but suspended as colloids, or in association with organic matters, minerals, and rocks [1]
Besides the metal(loid) uptake, bacteria have evolved an extraordinary variety of resistance mechanisms that overall rely on multidrug efflux systems (MES) [232,233], which control the accumulation of molecules and their homeostasis between the intra- and extracellular environment
Several members of the Actinobacteria phylum are capable of withstanding and converting toxic metals into harmless forms ascribed to metabolism-dependent and -independent cellular activities
Summary
Metals [e.g., silver (Ag), aluminum (Al), cadmium (Cd), cobalt (Co), chromium (Cr), cesium (Cs), copper (Cu), mercury (Hg), iron (Fe), magnesium (Mg), manganese (Mn), molybdenum (Mo), nickel (Ni), lead (Pb), strontium (Sr), zinc (Zn), uranium (U)] and metalloids [i.e., arsenic (As), selenium (Se), tellurium (Te)], naturally occurring throughout the Earth’s crust, are not generally present in soluble forms, but suspended as colloids, or in association with organic matters, minerals, and rocks [1]. If the pH, the type of metal(loid), and the nature of the biosorbent material represent important factors that must be accounted for the optimization of biosorption processes, in natural settings, the contemporary presence of a variety of metal(loid)s may lead to their competition for the ligand sites present on the surface of actinobacterial cells, compromising sorption bioprocesses This is the case of Ca2+ that is the main competitor for metal sorption in contaminated soil matrices [46,47,48], yet R. erythropolis A177 showed good sorption capacity towards Cd2+, even in the presence of Ca2+ ions. It has been reported that the cell amount in a specific contaminated site influences the single-cell sorption capacity, likely due to cross-links between functional groups [39,41,50]
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