A low-GC Gram-positiveThermoanaerobacter-like bacterium isolated from an Indian hot spring contains Cr(VI) reduction activity both in the membrane and cytoplasm

A comprehensive review on chromium (Cr) contamination and Cr(VI)-resistant extremophiles in diverse extreme environments

Chromium (Cr) compounds are usually toxins and exist abundantly in two different forms, Cr(VI) and Cr(III), in nature. Their contamination in any environment is a major problem. Many extreme environments including cold climate, warm climate, acidic environment, basic/alkaline environment, hypersaline environment, radiation, drought, high pressure, and anaerobic conditions have accumulated elevated Cr contamination. These harsh physicochemical conditions associated with Cr(VI) contamination damage biological systems in various ways. However, several unique microorganisms belonging to phylogenetically distant taxa (bacteria, fungi, and microalgae) owing to different and very distinct physiological characteristics can withstand extremities of Cr(VI) in different physicochemical environments. These challenging situations offer great potential and extended proficiencies in extremophiles for environmental and biotechnological applications. On these issues, the present review draws attention to Cr(VI) contamination from diverse extreme environmental regions. The study gives a detailed account on the ecology and biogeography of Cr(VI)-resistant microorganisms in inhospitable environments, and their use for detoxifying Cr(VI) and other applications. The study also focuses on physiological, multi-omics, and genetic engineering approaches of Cr(VI)-resistant extremophiles.

Exploration on the Cr(VI) resistance mechanism of a novel thermophilic Cr(VI)-reducing bacteria Anoxybacillus flavithermus ABF1 isolated from Tengchong geothermal region, China

Hexavalent chromium resistance and reduction mechanisms of microorganism provide a critical guidance for Cr(VI) bioremediation. However, related researches are limited in mesophiles and deficient for thermophiles. In this work, a novel alkaline Cr(VI)-reducing thermophile Anoxybacillus flavithermus ABF1 was isolated from geothermal region. The mechanisms of Cr(VI) resistance and reduction were investigated. The results demonstrated that A. flavithermus ABF1 could survive in a wide temperature range from 50°C to 70°C and in pH range of 7.0-9.0. Strain ABF1 showed excellent growth activity and Cr(VI) removal performance when initial Cr(VI) concentration was lower than 200 mg L^-1 . 93.71% of Cr(VI) was removed at initial concentration of 20 mg L^-1 after 72 h. The majority of Cr(VI) was found to be reduced extracellularly by enzymes secreted by cells. XPS and Raman analysis further manifested that Cr₂ O₃ was the product of Cr(VI) reduction. Moreover, the Cr(VI) transportation-related gene cysP and Cr(VI) reduction-related gene azoR of A. flavithermus ABF1 played key roles in inhibiting Cr(VI) entering cells and promoting extracellular Cr(VI) reduction respectively. This work provides novel insight into the mechanisms of Cr(VI) resistance and detoxication of thermophiles, which leads to a promising alternative strategy for heavy metal bioremediation in areas with elevated temperature.

Transcriptome Analysis Reveals Cr(VI) Adaptation Mechanisms in Klebsiella sp. Strain AqSCr

Klebsiella sp. strain AqSCr, isolated from Cr(VI)-polluted groundwater, reduces Cr(VI) both aerobically and anaerobically and resists up 34 mM Cr(VI); this resistance is independent of the ChrA efflux transporter. In this study, we report the whole genome sequence and the transcriptional profile by RNA-Seq of strain AqSCr under Cr(VI)-adapted conditions and found 255 upregulated and 240 downregulated genes compared to controls without Cr(VI) supplementation. Genes differentially transcribed were mostly associated with oxidative stress response, DNA repair and replication, sulfur starvation response, envelope-osmotic stress response, fatty acid (FA) metabolism, ribosomal subunits, and energy metabolism. Among them, genes not previously associated with chromium resistance, for example, cybB, encoding a putative superoxide oxidase (SOO), gltA2, encoding an alternative citrate synthase, and des, encoding a FA desaturase, were upregulated. The sodA gene encoding a manganese superoxide dismutase was upregulated in the presence of Cr(VI), whereas sodB encoding an iron superoxide dismutase was downregulated. Cr(VI) resistance mechanisms in strain AqSCr seem to be orchestrated by the alternative sigma factors fecl, rpoE, and rpoS (all of them upregulated). Membrane lipid analysis of the Cr(IV)-adapted strain showed a lower proportion of unsaturated lipids with respect to the control, which we hypothesized could result from unsaturated lipid peroxidation followed by degradation, together with de novo synthesis mediated by the upregulated FA desaturase-encoding gene, des. This report helps to elucidate both Cr(VI) toxicity targets and global bacterial response to Cr(VI).

Trace Element Concentrations in Hydrothermal Silica Deposits as a Potential Biosignature

Uncovering and understanding the chemical and fossil record of ancient life is crucial to understanding how life arose, evolved, and distributed itself across Earth. Potential signs of ancient life, however, are often challenging to establish as definitively biological and require multiple lines of evidence. Hydrothermal silica deposits may preserve some of the most ancient evidence of life on Earth, and such deposits are also suggested to exist on the surface of Mars. Here we use micron-scale elemental mapping by secondary ion mass spectrometry to explore for trace elements that are preferentially sequestered by microbial life and subsequently preserved in hydrothermal deposits. The spatial distributions and concentrations of trace elements associated with life in such hydrothermal silica deposits may have a novel application as a biosignature in constraining ancient life on Earth as well as the search for evidence of past life on Mars. We find that active microbial mats and recent siliceous sinter deposits from an alkaline hot spring in Yellowstone National Park appear to sequester and preserve Ga, Fe, and perhaps Mn through early diagenesis as indicators of the presence of life during formation.

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.

The bacterial diversity on steam vents from Paricutín and Sapichu volcanoes

Vapor steam vents are prevailing structures on geothermal sites in which local geochemical conditions allow the development of extremophilic microorganisms. We describe the structure of the prokaryotic community able to grow on the walls and rocks of such microecosystems in two terrestrial Mexican volcanoes: Paricutín (PI and PII samples) and its satellite Sapichu (S sample). The investigated samples showed similar diversity indices, with few dominant OTUs (abundance > 1%): 21, 16 and 23, respectively for PI, PII and S. However, each steam vent showed a particular community profile: PI was dominated by photosynthetic bacteria (Cyanobacteria and Chloroflexia class), PII by Actinobacteria and Proteobacteria, and S by Ktedonobacteria class, Acidobacteria and Cyanobacteria phyla. Concerning the predicted metabolic potential, we found a dominance of cellular pathways, especially the ones for energy generation with metabolisms for sulfur respiration, nitrogen fixation, methanogenesis, carbon fixation, photosynthesis, and metals, among others. We suggest a different maturity stage for the three studied fumaroles, from the youngest (PI) to the oldest (S and PII), also influenced by the temperature and other geochemical parameters. Furthermore, four anaerobic strains were isolated, belonging to Clostridia class (Clostridium sphenoides, C. swellfunanium and Anaerocolumna cellulosilytica) and to Bacilli class (Paenibacillus azoreducens).

Microbiological studies of hot springs in India: a review

The earliest microbiological studies on hot springs in India date from 2003, a much later date compared to global attention in this striking field of study. As of today, 28 out of 400 geothermal springs have been explored following both culturable and non-culturable approaches. The temperatures and pH of the springs are 37-99 °C and 6.8-10, respectively. Several studies have been performed on the description of novel genera and species, characterization of different bio-resources, metagenomics of hot spring microbiome and whole genome analysis of few isolates. 17 strains representing novel species and many thermostable enzymes, including lipase, protease, chitinase, amylase, etc. with potential biotechnological applications have been reported by several authors. Influence of physico-chemical conditions, especially that of temperature, on shaping the hot spring microbiome has been established by metagenomic investigations. Bacteria are the predominant life forms in all the springs with an abundance of phyla Firmicutes, Proteobacteria, Actinobacteria, Thermi, Bacteroidetes, Deinococcus-Thermus and Chloroflexi. In this review, we have discussed the findings on all microbiological studies that have been carried out to date, on the 28 hot springs. Further, the possibilities of extrapolating these studies for practical applications and environmental impact assessment towards protection of natural ecosystem of hot springs have also been discussed.

Complete genome sequence of the chromate-reducing bacterium Thermoanaerobacter thermohydrosulfuricus strain BSB-33

Thermoanaerobacter thermohydrosulfuricus BSB-33 is a thermophilic gram positive obligate anaerobe isolated from a hot spring in West Bengal, India. Unlike other T. thermohydrosulfuricus strains, BSB-33 is able to anaerobically reduce Fe(III) and Cr(VI) optimally at 60 °C. BSB-33 is the first Cr(VI) reducing T. thermohydrosulfuricus genome sequenced and of particular interest for bioremediation of environmental chromium contaminations. Here we discuss features of T. thermohydrosulfuricus BSB-33 and the unique genetic elements that may account for the peculiar metal reducing properties of this organism. The T. thermohydrosulfuricus BSB-33 genome comprises 2597606 bp encoding 2581 protein genes, 12 rRNA, 193 pseudogenes and has a G + C content of 34.20 %. Putative chromate reductases were identified by comparative analyses with other Thermoanaerobacter and chromate-reducing bacteria.

Identification of Multiple Soluble Fe(III) Reductases in Gram-Positive Thermophilic Bacterium Thermoanaerobacter indiensis BSB-33

Thermoanaerobacter indiensis BSB-33 has been earlier shown to reduce Fe(III) and Cr(VI) anaerobically at 60°C optimally. Further, the Gram-positive thermophilic bacterium contains Cr(VI) reduction activity in both the membrane and cytoplasm. The soluble fraction prepared from T. indiensis cells grown at 60°C was found to contain the majority of Fe(III) reduction activity of the microorganism and produced four distinct bands in nondenaturing Fe(III) reductase activity gel. Proteins from each of these bands were partially purified by chromatography and identified by mass spectrometry (MS) with the help of T. indiensis proteome sequences. Two paralogous dihydrolipoamide dehydrogenases (LPDs), thioredoxin reductase (Trx), NADP(H)-nitrite reductase (Ntr), and thioredoxin disulfide reductase (Tdr) were determined to be responsible for Fe(III) reductase activity. Amino acid sequence and three-dimensional (3D) structural similarity analyses of the T. indiensis Fe(III) reductases were carried out with Cr(VI) reducing proteins from other bacteria. The two LPDs and Tdr showed very significant sequence and structural identity, respectively, with Cr(VI) reducing dihydrolipoamide dehydrogenase from Thermus scotoductus and thioredoxin disulfide reductase from Desulfovibrio desulfuricans. It appears that in addition to their iron reducing activity T. indiensis LPDs and Tdr are possibly involved in Cr(VI) reduction as well.

Hexavalent chromium removal by a novel Serratia proteamaculans isolated from the bank of Sebou River (Morocco)

The novel Serratia proteamaculans isolated from a chromium-contaminated site was tolerant to a concentration of 500 mg Cr(VI)/l. The optimum pH and temperature for reduction of Cr(VI) by S. proteamaculans were found to be 7.0 and 30 °C, respectively. The Cr(VI) reduction rate decreased with the increase in Cr(VI) concentration from 100 to 400 mg/l, suggesting the enzymatic chromium reduction. Resting and permeabilised cell assays provided the better evidence that chromate reduction in S. proteamaculans is enzymatic. Reduction by cell-free filtrate shows no extracellular chromate-reducing activity, revealing that this activity may be associated to membrane fraction and/or cytosolic fraction. Assays conducted with cytosolic and particulate fraction of S. proteamaculans confirmed the role of membrane-bound proteins in Cr(VI) reduction. Furthermore, chromium reduced by heat-treated cells suggests that membrane-associated chromate reductase activity of S. proteamaculans is preceded by its adsorption on the cell surface.

Evidence for direct electron transfer by a gram-positive bacterium isolated from a microbial fuel cell

Despite their importance in iron redox cycles and bioenergy production, the underlying physiological, genetic, and biochemical mechanisms of extracellular electron transfer by Gram-positive bacteria remain insufficiently understood. In this work, we investigated respiration by Thermincola potens strain JR, a Gram-positive isolate obtained from the anode surface of a microbial fuel cell, using insoluble electron acceptors. We found no evidence that soluble redox-active components were secreted into the surrounding medium on the basis of physiological experiments and cyclic voltammetry measurements. Confocal microscopy revealed highly stratified biofilms in which cells contacting the electrode surface were disproportionately viable relative to the rest of the biofilm. Furthermore, there was no correlation between biofilm thickness and power production, suggesting that cells in contact with the electrode were primarily responsible for current generation. These data, along with cryo-electron microscopy experiments, support contact-dependent electron transfer by T. potens strain JR from the cell membrane across the 37-nm cell envelope to the cell surface. Furthermore, we present physiological and genomic evidence that c-type cytochromes play a role in charge transfer across the Gram-positive bacterial cell envelope during metal reduction.