Metal-tolerant thermophiles: metals as electron donors and acceptors, toxicity, tolerance and industrial applications

Trophic transfer and interfacial impacts of micro(nano)plastics and per-and polyfluoroalkyl substances in the environment

Both micro(nano)plastics (MNPs) and per-and polyfluoroalkyl substances (PFAS) possessed excellent properties and diverse applications, albeit gained worldwide attention due to their anthropogenic, ubiquitous, degradation resistant nature and a wide variety of ecological and human health impacts. MNPs and PFAS discharged from discrete sources and extensively bioaccumulated in the food chain through trophic transfer and their long-distance transport potential assist in their dispersal to pristine but vulnerable ecosystems such as Antarctica. They inevitably interacted with each other in the environment through polarized N-H bond, hydrogen bond, hydrophobic interaction, and weak bond energies such as Van der Waals, electrostatic, and intramolecular forces. During co-exposure, they significantly impact the uptake and bioaccumulation of each other in exposed organisms, which may increase or decrease their bioavailable concentration. Hence, this review compiles the studies on the co-occurrence and adsorption of PFAS and MNPs in the environment, their trophic transfer, combined in vivo and in vitro impacts, and factors influencing the MNP-PFAS interface. A significant proportion of studies were conducted in China, Europe, and the US, while studies are rare from other parts of the world. Freshwater and marine food chains were more prominently investigated for trophic transfers compared to terrestrial food chains. The most notable in vivo effects were growth and reproductive impairment, oxidative stress, neurotoxicity and apoptosis, DNA damage, genotoxicity and immunological responses, behavioral and gut microbiota modifications, and histopathological alterations. Cellular uptake of PFAS and MNPs can impact cell survival and proliferation, photosynthesis and membrane integrity, ROS generation and antioxidant responses, and extracellular polymeric substances (EPS) release in vitro. MNP characteristics, PFAS properties, tissue and species-dependent distribution, and environmental medium properties were the main factors influencing the PFAS and MNP nexus and associated impacts. Last but not least, gaps and future research directions were highlighted to better understand the interplay between these critical persistent chemicals.

A New Strategy for As(V) Biosensing Based on the Inhibition of the Phosphatase Activity of the Arsenate Reductase from Thermus thermophilus

Arsenic (As) pollution is a widespread problem worldwide. In recent years, biosensors based on enzymatic inhibition have been developed for arsenic detection, making the study of the effect of inhibitors on the selected enzymatic activity crucial for their setup. The arsenate reductase of Thermus thermophilus HB27, TtArsC, reduces As(V) into As(III), but is also endowed with phosphatase activity. This work investigates the inhibitory effects of As(V) and As(III) on phosphatase activity by taking advantage of a simple colorimetric assay; the results show that both of them are non-competitive inhibitors affecting the Vmax but not the K_M of the reaction. However, their Ki values are different from each other (15.2 ± 1.6 μM for As(V) and 394.4 ± 40.3 µm with As(III)), indicating a higher inhibitory effect by As(V). Moreover, the inhibition-based biosystem results to be selective for As(V) since several other metal ions and salts do not affect TtArsC phosphatase activity; it exhibits a sensitivity of 0.53 ± 0.03 mU/mg/μM and a limit of detection (LOD) of 0.28 ± 0.02 μM. The good sensitivity and specificity for As(V) point to consider inhibition of TtArsC phosphatase activity for the setup of a novel biosensor for the detection of As(V).

Structural and thermodynamic insights into a novel Mg2+-citrate-binding protein from the ABC transporter superfamily

More than one third of proteins require metal ions to accomplish their functions, making them obligatory for the growth and survival of microorganisms in varying environmental niches. In prokaryotes, besides their involvement in various cellular and physiological processes, metal ions stimulate the uptake of citrate molecules. Citrate is a source of carbon and energy and is reported to be transported by secondary transporters. In Gram-positive bacteria, citrate molecules are transported in complex with divalent metal ions, whereas in Gram-negative bacteria they are translocated by Na⁺/citrate symporters. In this study, the presence of a novel divalent-metal-ion-complexed citrate-uptake system that belongs to the primary active ABC transporter superfamily is reported. For uptake, the metal-ion-complexed citrate molecules are sequestered by substrate-binding proteins (SBPs) and transferred to transmembrane domains for their transport. This study reports crystal structures of an Mg²⁺-citrate-binding protein (MctA) from the Gram-negative thermophilic bacterium Thermus thermophilus HB8 in both apo and holo forms in the resolution range 1.63-2.50 Å. Despite binding various divalent metal ions, MctA possesses the coordination geometry to bind its physiological metal ion, Mg²⁺. The results also suggest an extended subclassification of cluster D SBPs, which are known to bind and transport divalent-metal-ion-complexed citrate molecules. Comparative assessment of the open and closed conformations of the wild-type and mutant MctA proteins suggests a gating mechanism of ligand entry following an `asymmetric domain movement' of the N-terminal domain for substrate binding.

Extremophiles, a Nifty Tool to Face Environmental Pollution: From Exploitation of Metabolism to Genome Engineering

Extremophiles are microorganisms that populate habitats considered inhospitable from an anthropocentric point of view and are able to tolerate harsh conditions such as high temperatures, extreme pHs, high concentrations of salts, toxic organic substances, and/or heavy metals. These microorganisms have been broadly studied in the last 30 years and represent precious sources of biomolecules and bioprocesses for many biotechnological applications; in this context, scientific efforts have been focused on the employment of extremophilic microbes and their metabolic pathways to develop biomonitoring and bioremediation strategies to face environmental pollution, as well as to improve biorefineries for the conversion of biomasses into various chemical compounds. This review gives an overview on the peculiar metabolic features of certain extremophilic microorganisms, with a main focus on thermophiles, which make them attractive for biotechnological applications in the field of environmental remediation; moreover, it sheds light on updated genetic systems (also those based on the CRISPR-Cas tool), which expand the potentialities of these microorganisms to be genetically manipulated for various biotechnological purposes.

Identification of a New Heavy-Metal-Resistant Strain of Geobacillus stearothermophilus Isolated from a Hydrothermally Active Volcanic Area in Southern Italy

Microorganisms thriving in hot springs and hydrothermally active volcanic areas are dynamically involved in heavy-metal biogeochemical cycles; they have developed peculiar resistance systems to cope with such metals which nowadays can be considered among the most permanent and toxic pollutants for humans and the environment. For this reason, their exploitation is functional to unravel mechanisms of toxic-metal detoxification and to address bioremediation of heavy-metal pollution with eco-sustainable approaches. In this work, we isolated a novel strain of the thermophilic bacterium Geobacillus stearothermophilus from the solfataric mud pool in Pisciarelli, a well-known hydrothermally active zone of the Campi Flegrei volcano located near Naples in Italy, and characterized it by ribotyping, 16S rRNA sequencing and mass spectrometry analyses. The minimal inhibitory concentration (MIC) toward several heavy-metal ions indicated that the novel G. stearothermophilus isolate is particularly resistant to some of them. Functional and morphological analyses suggest that it is endowed with metal resistance systems for arsenic and cadmium detoxification.

Single and combined effects of amino polystyrene and perfluorooctane sulfonate on hydrogen-producing thermophilic bacteria and the interaction mechanisms

As a carrier of perfluorooctane sulfonate, nano-plastics are ubiquitous and finally enriched in the sludge, which is widely used as a raw material for the production of bioenergy (hydrogen or methane) by anaerobic digestion. However, there are still many unknowns about their metabolic toxicity to functional microbes (e.g. hydrogen-producing thermophilic bacteria). Therefore, single and combined effects of amino polystyrene (NPS: 70 nm; 0.2 mg/L) and perfluorooctane sulfonate (PFOS: 0.1, 1 and 5 mg/L) on hydrogen-producing thermophilic bacteria were investigated after exposure for 7 days at 55 °C and pH = 5.7. Single NPS exhibited obvious interference to the metabolism of thermophilic bacteria, resulting in a 53.9% reduction in hydrogen production. However, the combined NPS + PFOS produced an antagonistic effect, leading to a 31.6% reduction in hydrogen production. Nonetheless, the single and combined exposure did not alter the type of hydrogen production (acetic acid-type hydrogen fermentation). Moreover, single NPS and combined NPS + PFOS not only induced the changes of the composition of extracellular polymers (EPSs) and π bond structure of the protein in EPSs, but also decreased the activity of hydrolase in EPSs and surface charge of EPSs. Compared to single NPS exposure, NPS + PFOS-exposed thermophilic bacteria was less permeable to a semi-membrane permeable dye and produced less reactive oxygen species, but were still significantly higher than control group. In short, the main mechanisms of single NPS and combined NPS + PFOS were both to increase cell permeability and to induce oxidative stress. The addition of PFOS alleviated the toxic effect of NPS, but did not change its mechanism of toxicity.

Metal biorecovery and bioremediation: Whether or not thermophilic are better than mesophilic microorganisms

Metal mobilization and immobilization catalyzed by microbial action are key processes in environmental biotechnology. Metal mobilization from ores, mining wastes, or solid residues can be used for recovering metals and/or remediating polluted environments; furthermore, immobilization reduces the migration of metals; cleans up effluents plus ground- and surface water; and, moreover, can help to concentrate and recover metals. Usually these processes provide certain advantages over traditional technologies such as more efficient economical and environmentally sustainable results. Since elevated temperatures typically increase chemical kinetics, it could be expected that bioprocesses should also be enhanced by replacing mesophiles with thermophiles or hyperthermophiles. Nevertheless, other issues like process stability, flexibility, and thermophile-versus-mesophile resistance to acidity and/or metal toxicity should be carefully considered. This review critically analyzes and compares thermophilic and mesophilic microbial performances in recent and selected representative examples of metal bioremediation and biorecovery.

Designating ligand specificities to metal uptake ABC transporters in Thermus thermophilus HB8

Acquisition of different metal ions by metal uptake ABC transporters of Thermus thermophilus HB8 for accomplishing its various cellular functions.

Diversity and Niche of Archaea in Bioremediation

Bioremediation is the use of microorganisms for the degradation or removal of contaminants. Most bioremediation research has focused on processes performed by the domain Bacteria; however, Archaea are known to play important roles in many situations. In extreme conditions, such as halophilic or acidophilic environments, Archaea are well suited for bioremediation. In other conditions, Archaea collaboratively work alongside Bacteria during biodegradation. In this review, the various roles that Archaea have in bioremediation is covered, including halophilic hydrocarbon degradation, acidophilic hydrocarbon degradation, hydrocarbon degradation in nonextreme environments such as soils and oceans, metal remediation, acid mine drainage, and dehalogenation. Research needs are addressed in these areas. Beyond bioremediation, these processes are important for wastewater treatment (particularly industrial wastewater treatment) and help in the understanding of the natural microbial ecology of several Archaea genera.

Characterization of a promiscuous cadmium and arsenic resistance mechanism in Thermus thermophilus HB27 and potential application of a novel bioreporter system

Background

The characterization of the molecular determinants of metal resistance has potential biotechnological application in biosensing and bioremediation. In this context, the bacterium Thermus thermophilus HB27 is a metal tolerant thermophile containing a set of genes involved in arsenic resistance which, differently from other microbes, are not organized into a single operon. They encode the proteins: arsenate reductase, TtArsC, arsenic efflux membrane transporter, TtArsX, and transcriptional repressor, TtSmtB.

Results

In this work we show that the arsenic efflux protein TtArsX and the arsenic responsive transcriptional repressor TtSmtB are required to provide resistance to cadmium. We analyzed the sensitivity to Cd(II) of mutants lacking TtArsX, finding that they are more sensitive to this metal than the wild type strain. In addition, using promoter probe reporter plasmids, we show that the transcription of TtarsX is also stimulated by the presence of Cd(II) in a TtSmtB-dependent way. Actually, a regulatory circuit composed of TtSmtB and a reporter gene expressed from the TtarsX promoter responds to variation in Cd(II), As(III) and As(V) concentrations.

Conclusions

Our results demonstrate that the system composed by TtSmtB and TtArsX is responsible for both the arsenic and cadmium resistance in T. thermophilus. The data also support the use of T. thermophilus as a suitable chassis for the design and development of As-Cd biosensors.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0918-7) contains supplementary material, which is available to authorized users.