Biosynthesis of micro- and nanocrystals of Pb (II), Hg (II) and Cd (II) sulfides in four Candida species: a comparative study of in vivo and in vitro approaches

Regulatory roles of APS reductase in Citrobacter sp. XT1-2-2 as a response mechanism to cadmium immobilization in rice

Citrobacter sp. XT1-2-2, a functional microorganism with potential utilization, has the ability to immobilize soil cadmium. In this study, the regulatory gene cysH, as a rate-limiting enzyme in the sulfur metabolic pathway, was selected for functional analysis affecting cadmium immobilization in soil. To verify the effect of APS reductase on CdS formation, the ΔAPS and ΔAPS-com strains were constructed by conjugation transfer. Through TEM analysis, it was found that the adsorption of Cd2+ was affected by the absence of APS reductase in XT1-2-2 strain. The difference analysis of biofilm formation indicated that APS reductase was necessary for cell aggregation and biofilm formation. The p-XRD, XPS and FT-IR analysis revealed that APS reductase played an important role in the cadmium immobilization process of XT1-2-2 strain and promoting the formation of CdS. According to the pot experiments, the cadmium concentration of roots, culms, leaves and grains inoculated with ΔAPS strain was significantly higher than that of wild-type and ΔAPS-com strains, and the cadmium removal ability of ΔAPS strain was significantly lower than that of wild-type strain. The study provided insights into the exploration of new bacterial assisted technique for the remediation and safe production of rice in cadmium-contaminated paddy soils.

Diversity of Microbial Metal Sulfide Biomineralization

Since the emergence of life on Earth, microorganisms have contributed to biogeochemical cycles. Sulfate-reducing bacteria are an example of widespread microorganisms that participate in the metal and sulfur cycles by biomineralization of biogenic metal sulfides. In this work, we review the microbial biomineralization of metal sulfide particles and summarize distinctive features from exemplary cases. We highlight that metal sulfide biomineralization is highly metal- and organism-specific. The properties of metal sulfide biominerals depend on the degree of cellular control and on environmental factors, such as pH, temperature, and concentration of metals. Moreover, biogenic macromolecules, including peptides and proteins, help cells control their extracellular and intracellular environments that regulate biomineralization. Accordingly, metal sulfide biominerals exhibit unique features when compared to abiotic minerals or biominerals produced by dead cell debris.

The Microbiology of Metal Mine Waste: Bioremediation Applications and Implications for Planetary Health

Mine wastes pollute the environment with metals and metalloids in toxic concentrations, causing problems for humans and wildlife. Microorganisms colonize and inhabit mine wastes, and can influence the environmental mobility of metals through metabolic activity, biogeochemical cycling and detoxification mechanisms. In this article we review the microbiology of the metals and metalloids most commonly associated with mine wastes: arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc. We discuss the molecular mechanisms by which bacteria, archaea, and fungi interact with contaminant metals and the consequences for metal fate in the environment, focusing on long-term field studies of metal-impacted mine wastes where possible. Metal contamination can decrease the efficiency of soil functioning and essential element cycling due to the need for microbes to expend energy to maintain and repair cells. However, microbial communities are able to tolerate and adapt to metal contamination, particularly when the contaminant metals are essential elements that are subject to homeostasis or have a close biochemical analog. Stimulating the development of microbially reducing conditions, for example in constructed wetlands, is beneficial for remediating many metals associated with mine wastes. It has been shown to be effective at low pH, circumneutral and high pH conditions in the laboratory and at pilot field-scale. Further demonstration of this technology at full field-scale is required, as is more research to optimize bioremediation and to investigate combined remediation strategies. Microbial activity has the potential to mitigate the impacts of metal mine wastes, and therefore lessen the impact of this pollution on planetary health.

Bioreduction of precious and heavy metals by Candida species under oxidative stress conditions

The aim of the present work was to evaluate whether Candida species can reduce both precious and toxic pure metals from the respective molecular ions. From these results, the nanoparticles formed were studied using scanning electron microscopy with energy-dispersive spectroscopy, Raman spectroscopy, X-ray fluorescence spectroscopy and synchrotron radiation. Our results showed that the metal ions were reduced to their corresponding metallic nanoconglomerate or nanoparticles by Candida species. This is the first report on how yeasts of this genus are capable of achieving homeostasis (resilience) in the presence of metal ions of both precious and toxic metals by reducing them to a metallic state.

Biosynthesis of micro- and nanocrystals of Pb (II), Hg (II) and Cd (II) sulfides in four Candida species: a comparative study of in vivo and in vitro approaches

Nature produces biominerals (biogenic minerals) that are synthesized as complex structures, in terms of their physicochemical properties. These biominerals are composed of minerals and biological macromolecules. They are produced by living organisms and are usually formed through a combination of chemical, biochemical and biophysical processes. Microorganisms like Candida in the presence of heavy metals can biomineralize those metals to form microcrystals (MCs) and nanocrystals (NCs). In this work, MCs and NCs of PbS, HgS or HgCl₂ as well as CdS are synthesized both in vitro (gels) and in vivo by four Candida species. Our in vivo results show that, in the presence of Pb²⁺, Candida cells are able to replicate and form extracellular PbS MCs, whereas in the presence of Hg²⁺ and Cd²⁺, they did synthesize intercellular MCs from HgS or HgCl₂ and CdS NCs respectively. The MCs and NCs biologically obtained in Candida were compared with those PbS, HgS and CdS crystals synthetically obtained in vitro through the gel method (grown either in agarose or in sodium metasilicate hydrogels). This is, to our knowledge, the first time that the biosynthesis of the various MCs and NCs (presented in several species of Candida) has been reported. This biosynthesis is differentially regulated in each of these pathogens, which allows them to adapt and survive in different physiological and environmental habitats.