Metal oxidoreduction by microbial cells

Nano-biogenic heavy metals adsorptive remediation for enhanced soil health and sustainable agricultural production

Hazardous heavy metal (HM) pollution constitutes a pervasive global challenge, posing substantial risks to ecosystems and human health. The exigency for expeditious detection, meticulous monitoring, and efficacious remediation of HM within ecosystems is indisputable. Soil contamination, stemming from a myriad of anthropogenic activities, emerges as a principal conduit for HM ingress into the food chain. Traditional soil remediation modalities for HM elimination, while effective are labor-intensive, susceptible to secondary contamination, and exhibit limited efficacy in regions characterized by low metal toxicity. In response to these exigencies, the eco-friendly paradigm of bioremediation has garnered prominence as a financially judicious and sustainable remedial strategy. This approach entails the utilization of hyperaccumulators, Genetically Modified Microorganisms (GMM), and advantageous microbes. The current review offers a comprehensive elucidation of cutting-edge phyto/microbe-based bioremediation techniques, with a specific emphasis on their amalgamation with nanotechnology. Accentuating their pivotal role in advancing sustainable agricultural practices, the review meticulously dissects the synergistic interplay between plants and microbes, underscoring their adeptness in HM remediation sans secondary contamination. Moreover, the review scrutinizes the challenges intrinsic to implementing bioremediation-nanotechnology interface techniques and propounds innovative resolutions. These discernments proffer auspicious trajectories for the future of agriculture. Through the environmentally conscientious marvels of phyto/microbe bioremediation, an optimistic outlook emerges for environmental preservation and the cultivation of a sustainable, salubrious planet via the conduit of cleaner agricultural production.

The role of iron-rich organic fertilizer in promoting the growth of Chinese cabbage and inhibiting the transformation of cadmium

The issue of heavy metal pollution caused by human production and living activities is progressively worsening. This study explored the effect of iron-rich organic fertilizer on the growth, quality, and cadmium (Cd) absorption of Chinese cabbage under Cd stress. The results showed that iron-rich organic fertilizer could increase the soluble protein content and root length of Chinese cabbage. Meanwhile, it could change the form of Cd to inhibit the enrichment of Cd in Chinese cabbage. The alkali hydrolyzed nitrogen (AN), total potassium (TK), organic matter (OM), and moisture content (MC) of the Z3 treatment group (2 % ferrous sulfate heptahydrate) were significantly higher than those of other treatment groups. The microbial network of Z3 was more complex than the other three groups. PICRUSt analysis and correlation analysis showed that the genes related to protein synthesis (e.g., glutathione S-transferase, zinc and Cd transporter, outer membrane protein, ArsR family transcriptional regulator, catalase, etc.) can also promote microbial absorption. This study aims to provide theoretical insights into soil Cd pollution immobilization techniques.

Utilizing a divalent metal ion transporter to control biogenic nanoparticle synthesis

Biogenic synthesis of inorganic nanomaterials has been demonstrated for both wild and engineered bacterial strains. In many systems the nucleation and growth of nanomaterials is poorly controlled and requires concentrations of heavy metals toxic to living cells. Here, we utilized the tools of synthetic biology to engineer a strain of Escherichia coli capable of synthesizing cadmium sulfide nanoparticles from low concentrations of reactants with control over the location of synthesis. Informed by simulations of bacterially-assisted nanoparticle synthesis, we created a strain of E. coli expressing a broad-spectrum divalent metal transporter, ZupT, and a synthetic CdS nucleating peptide. Expression of ZupT in the outer membrane and placement of the nucleating peptide in the periplasm focused synthesis within the periplasmic space and enabled sufficient nucleation and growth of nanoparticles at sub-toxic levels of the reactants. This strain synthesized internal CdS quantum dot nanoparticles with spherical morphology and an average diameter of approximately 3.3 nm.

ONE-SENTENCE SUMMARY

Expression of a metal ion transporter regulates synthesis of cadmium sulfide nanoparticles in bacteria.

Modulation in Biofertilization and Biofortification of Wheat Crop by Inoculation of Zinc-Solubilizing Rhizobacteria

Zinc is an important micronutrient needed for the optimum growth and development of plants. Contrary to chemical zinc fertilizers, the use of zinc-solubilizing bacteria is an environmentally friendly option for zinc enrichment in edible parts of crops. This study was conducted with the objective of selecting potential zinc-solubilizing rhizobacteria from the rhizosphere of chickpea grown in soils of eastern Uttar Pradesh and further assessing their impact on the magnitude of zinc assimilation in wheat crops. Among 15 isolates, CRS-9, CRS-17, CRS-30, and CRS-38 produced net soluble zinc in broth to the tune of 6.1, 5.9, 5.63, and 5.6 μg ml-1, respectively, in zinc phosphate with the corresponding pH of 4.48, 5.31, 5.2, and 4.76. However, the bacterial strains CRS-17, CRS-30, CRS-38, and CRS-9 showed maximum zinc phosphate solubilization efficiency of 427.79, 317.39, 253.57, and 237.04%, respectively. The four bacterial isolates were identified as Bacillus glycinifermentans CRS-9, Microbacterium oxydans CRS-17, Paenarthrobacter nicotinovorans CRS-30, and Bacillus tequilensis CRS-38 on the basis of morphological and biochemical studies and 16S rRNA gene sequencing. Bacterial inoculants significantly colonized the roots of wheat plants and formed a biofilm in the root matrix. These strains significantly increased seed germination (%) and vigor indices in wheat grown under glasshouse conditions. After 30 days of sowing of wheat under microcosm conditions, eight zinc transporter (TaZIP) genes were expressed maximally in roots, with concomitant accumulation of higher zinc content in the bacterially treated plant compared to the absolute control. Out of the four strains tested, two bacteria, B. tequilensis CRS-38 and P. nicotinovorans CRS-30, improved seed germination (%), vigor indices (2-2.5 folds), plant biomass, grain yield (2.39 g plant-1), and biofortificated grains (54.25 μg g-1Zn) of wheat. To the best of our knowledge, this may be the first report on the presence of zinc solubilization trait in B. glycinifermentans CRS-9, M. oxydans CRS-17, and P. nicotinovorans CRS-30.

Biofertilizers: An ecofriendly technology for nutrient recycling and environmental sustainability

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Agriculture plays an important role in a country's economy. In modern intensive agricultural practices, chemical fertilizers and pesticides are applied on large scale to increase crop production in order to meet the nutritional requirements of the ever-increasing world population. However, rapid urbanization with shrinking agricultural lands, dramatic change in climatic conditions and extensive use of agrochemicals in agricultural practices has been found to cause environmental disturbances and public health hazards affecting food security and sustainability in agriculture. Besides this, agriculture soils are continuously losing their quality and physical properties as well as their chemical (imbalance of nutrients) and biological health due to indiscriminate use of agrochemicals. Plant-associated microbes with their plant growth- promoting traits have enormous potential to solve these challenges and play a crucial role in enhancing plant biomass and crop yield under greenhouse and field conditions. The beneficial mechanisms of plant growth improvement include enhanced availability of nutrients (i.e., N, P, K, Zn and S), phytohormone modulation, biocontrol of phytopathogens and amelioration of biotic and abiotic stresses. This plant-microbe interplay is indispensable for sustainable agriculture and these microbes may perform essential role as an ecological engineer to reduce the use of chemical fertilizers. Various steps involved for production of solid-based or liquid biofertilizer formulation include inoculum preparation, addition of cell protectants such as glycerol, lactose, starch, a good carrier material, proper packaging and best delivery methods. In addition, recent developments of formulation include entrapment/microencapsulation, nano-immobilization of microbial bioinoculants and biofilm-based biofertilizers. Thus, inoculation with beneficial microbes has emerged as an innovative eco-friendly technology to feed global population with available resources. This review critically examines the current state-of-art on use of microbial strains as biofertilizers in different crop systems for sustainable agriculture and in maintaining soil fertility and enhancing crop productivity. It is believed that acquisition of advanced knowledge of plant-PGPR interactions, bioengineering of microbial communities to improve the performance of biofertilizers under field conditions, will help in devising strategies for sustainable, environment-friendly and climate smart agricultural technologies to deliver short and long terms solutions for improving crop productivity to feed the world in a more sustainable manner.

Modern intensive agricultural practices face numerous challenges that pose major threats to global food security. In order to address the nutritional requirements of the ever-increasing world population, chemical fertilizers and pesticides are applied on large scale to increase crop production. However, the injudicious use of agrochemicals has resulted in environmental pollution leading to public health hazards. Moreover, agriculture soils are continuously losing their quality and physical properties as well as their chemical (imbalance of nutrients) and biological health. Plant-associated microbes with their plant growth- promoting traits have enormous potential to solve these challenges and play a crucial role in enhancing plant biomass and crop yield. The beneficial mechanisms of plant growth improvement include enhanced nutrient availability, phytohormone modulation, biocontrol of phytopathogens and amelioration of biotic and abiotic stresses. Solid-based or liquid bioinoculant formulation comprises inoculum preparation, addition of cell protectants such as glycerol, lactose, starch, a good carrier material, proper packaging and best delivery methods. Recent developments of formulation include entrapment/microencapsulation, nano-immobilization of microbial bioinoculants and biofilm-based biofertilizers. This review critically examines the current state-of-art on use of microbial strains as biofertilizers and the important roles performed by these beneficial microbes in maintaining soil fertility and enhancing crop productivity.

Dynamic release and transformation of metallic copper colloids in flooded paddy soil: Role of soil reducible sulfate and temperature

Mobile metal Cu colloids can be formed in periodically flooded paddy soils, potentially aggravating the risks to rice cultivated in these soils. Here, we investigated the formation and fate of Cu colloids in flooded soil as influenced by soil reducible sulfate and temperature. In microcosms with different initial sulfate availability (1.30, 5.34, or 7.38 mmol/kg), we found the treatments with higher sulfate concentrations showed the greater and faster release of metal colloids. Sulfate reduction resulted in the transformation of copper in the colloids from Cu(0) to Cu_xS, and the percentage of Cu_xS in the colloid phase increased with increasing sulfate content according to the Cu K-edge EXAFS spectra. The batch experiments incubated at 5, 25 or 35 °C proved that high temperature enhanced the microbial activity and released more Cu colloids during flooding. The colloid formation was delayed at low temperature but persisted longer in the soil, which led to greater particle average size because of slow growth and uniform agglomeration. Low temperature appeared to only influence the formation and growth but not the speciation of Cu colloids. Our results highlight the importance of soil reducible sulfate and temperature in mediating the dynamics of colloidal metals in flooded soil.

Mining Saline Soils to Manifest Plant Stress-Alleviating Halophilic Bacteria

The connection between soil and microbes plays a critical role in soil health and quality and can be elastic with the application of soil amendments and/or crop rotations. Inappropriate management of soil and application of impermissible levels of fertilizers ruptures the overriding connection between the soil and microbes. This is currently evidenced in the degraded soils (i.e., saline soils of India) which are caused by modern agricultural practices. Reclamation of saline soils with a saturated package of practices and conventional breeding methods requires biological intervention. Shortfall of nutrients is one of the chief constraints for plant growth in salt-affected soils. In the present investigation, we have observed an arsenal of fifty halophilic bacteria carrying an absolute requirement of 3% NaCl for solubilizing the insoluble minerals (ZnCO₃, ZnO, Mica and tri-calcium phosphate) under in vitro conditions; however, increasing the amount of NaCl over and above resulted in loss of solubilization capacity. Of the isolates solubilizing zinc carbonate and zinc oxide at 3% NaCl concentration, there were 29 isolates; at 10% concentration, 10 isolates were positive for the presence of zinc carbonate. At 3% NaCl concentration, HB-5 showed 23.16 mm zinc carbonate solubilization, HB-20 showed 13.3 mm Zinc oxide solubilization, and HB-7 showed 13.4 mm tri-calcium phosphate solubilization. Mica solubilization was peaked at 6% NaCl and maximum solubilization was observed in HB-27 (18.03 mm). When compared to the zinc carbonate solubilization, zinc oxide solubilization was slow to reach desired levels. Solubilization lasted for up to 9 days and ceased thereafter in all the tests. Eight elite isolates were identified as Bacillus albus, Bacillus safensis, Pseudomonas stutzeri (2), Lysinibacillus sphaericus, Staphylococcus xylosus, and Bacillus cereus (2) based on 16S rRNA analysis.

Biogenic Control of Manganese Doping in Zinc Sulfide Nanomaterial Using Shewanella oneidensis MR-1

Bacteria naturally alter the redox state of many compounds and perform atom-by-atom nanomaterial synthesis to create many inorganic materials. Recent advancements in synthetic biology have spurred interest in using biological systems to manufacture nanomaterials, implementing biological strategies to specify the nanomaterial characteristics such as size, shape, and optical properties. Here, we combine the natural synthetic capabilities of microbes with engineered genetic control circuits toward biogenically synthesized semiconductor nanomaterials. Using an engineered strain of Shewanella oneindensis with inducible expression of the cytochrome complex MtrCAB, we control the reduction of manganese (IV) oxide. Cytochrome expression levels were regulated using an inducer molecule, which enabled precise modulation of dopant incorporation into manganese doped zinc sulfide nanoparticles (Mn:ZnS). Thereby, a synthetic gene circuit controlled the optical properties of biogenic quantum dots. These biogenically assembled nanomaterials have similar physical and optoelectronic properties to chemically synthesized particles. Our results demonstrate the promise of implementing synthetic gene circuits for tunable control of nanomaterials made by biological systems.

Perspectives of Microbial Inoculation for Sustainable Development and Environmental Management

How to sustainably feed a growing global population is a question still without an answer. Particularly farmers, to increase production, tend to apply more fertilizers and pesticides, a trend especially predominant in developing countries. Another challenge is that industrialization and other human activities produce pollutants, which accumulate in soils or aquatic environments, contaminating them. Not only is human well-being at risk, but also environmental health. Currently, recycling, land-filling, incineration and pyrolysis are being used to reduce the concentration of toxic pollutants from contaminated sites, but too have adverse effects on the environment, producing even more resistant and highly toxic intermediate compounds. Moreover, these methods are expensive, and are difficult to execute for soil, water, and air decontamination. Alternatively, green technologies are currently being developed to degrade toxic pollutants. This review provides an overview of current research on microbial inoculation as a way to either replace or reduce the use of agrochemicals and clean environments heavily affected by pollution. Microorganism-based inoculants that enhance nutrient uptake, promote crop growth, or protect plants from pests and diseases can replace agrochemicals in food production. Several examples of how biofertilizers and biopesticides enhance crop production are discussed. Plant roots can be colonized by a variety of favorable species and genera that promote plant growth. Microbial interventions can also be used to clean contaminated sites from accumulated pesticides, heavy metals, polyaromatic hydrocarbons, and other industrial effluents. The potential of and key processes used by microorganisms for sustainable development and environmental management are discussed in this review, followed by their future prospects.

Copper Mobilization and Immobilization along an Organic Matter and Redox Gradient-Insights from a Mofette Site

Mofettes (natural geogenic CO₂ exhalations) represent excellent sites to study the behavior of Cu in soils and the co-occurrence of different mobilization and immobilization processes since they exhibit both a gradient in redox conditions (oxic to permanently anoxic) and in soil organic matter (SOM; low to high contents). Soil and pore water samples from an 18 m-transect over a mofette showed a complex behavior of Cu, with highest mobility in the transition between oxic and anoxic conditions. Cu(II) sorption experiments on SOM-rich topsoil revealed that Cu mobility under oxic conditions was confined by adsorption to SOM while in the oxygen-free mofette center reduction and precipitation of sulfides was the dominating Cu-sequestering process. In transition areas with low amounts of oxygen (<10%), there was no mineral precipitation, instead high dissolved-to-soil organic carbon ratios strongly increased Cu mobility. Our results show that low stability of SOM formed under oxygen-limited conditions leads to increased Cu mobility unless sulfur-reducing conditions cause Cu sequestration by sulfide precipitation. The interplay of these (im)mobilization processes and especially the unexpectedly high mobility under suboxic conditions have to be considered when assessing Cu mobility along spatial or temporal redox gradients, e.g., at contamination sites or periodically flooded soils.

Contribution of Zinc Solubilizing Bacteria in Growth Promotion and Zinc Content of Wheat

Zinc is an imperative micronutrient required for optimum plant growth. Zinc solubilizing bacteria are potential alternatives for zinc supplementation and convert applied inorganic zinc to available forms. This study was conducted to screen zinc solubilizing rhizobacteria isolated from wheat and sugarcane, and to analyze their effect on wheat growth and development. Fourteen exo-polysaccharides producing bacterial isolates of wheat were identified and characterized biochemically as well as on the basis of 16S rRNA gene sequences. Along these, 10 identified sugarcane isolates were also screened for zinc solubilizing ability on five different insoluble zinc sources. Out of 24, five strains, i.e., EPS 1 (Pseudomonas fragi), EPS 6 (Pantoea dispersa), EPS 13 (Pantoea agglomerans), PBS 2 (E. cloacae) and LHRW1 (Rhizobium sp.) were selected (based on their zinc solubilizing and PGP activities) for pot scale plant experiments. ZnCO3 was used as zinc source and wheat seedlings were inoculated with these five strains, individually, to assess their effect on plant growth and development. The effect on plants was analyzed based on growth parameters and quantifying zinc content of shoot, root and grains using atomic absorption spectroscopy. Plant experiment was performed in two sets. For first set of plant experiments (harvested after 1 month), maximum shoot and root dry weights and shoot lengths were noted for the plants inoculated with Rhizobium sp. (LHRW1) while E. cloacae (PBS 2) increased both shoot and root lengths. Highest zinc content was found in shoots of E. cloacae (PBS 2) and in roots of P. agglomerans (EPS 13) followed by zinc supplemented control. For second set of plant experiment, when plants were harvested after three months, Pantoea dispersa (EPS 6), P. agglomerans (EPS 13) and E. cloacae (PBS 2) significantly increased shoot dry weights. However, significant increase in root dry weights and maximum zinc content was recorded for Pseudomonas fragi (EPS 1) inoculated plants, isolated from wheat rhizosphere. While maximum zinc content for roots was quantified in the control plants indicating the plant's inability to transport zinc to grains, supporting accelerated bioavailability of zinc to plant grains with zinc solubilizing rhizobacteria.

Bacterial endotoxin (lipopolysaccharide) stimulates the rate of iron oxidation

Bacterial endotoxin (lipopolysaccharide) has affinity for a number of cations, including iron. Previous investigations have demonstrated that lipopolysaccharide can affect the oxidation rate of iron; heme-bound ferrous iron in hemoglobin is oxidized to ferric iron when hemoglobin binds lipopolysaccharide. In the present study, we directly examined the interaction between lipopolysaccharide and iron. Lipopolysaccharide caused a concentration-dependent increase in the rate of iron oxidation, with up to a 23-fold increase in oxidation in the presence of 200 µg/ml Escherichia coli lipopolysaccharide. This effect was seen both with several carbohydrate-rich smooth lipopolysaccharides and also with carbohydrate-poor rough lipopolysaccharide. Extensively deacylated rough lipopolysaccharide had no effect, suggesting a role of the fatty acid components of lipopolysaccharide in this process. Purified lipid A produced inconsistent results: some preparations stimulated iron oxidation and others did not. A series of sugars, starches and a preparation of purified O-chain polysaccharide (the carbohydrate portion of the lipopolysaccharide macromolecule) had no effect on the rate of iron oxidation, whereas phospholipid-enriched brain tissue extracts (similar to the lipid A component of lipopolysaccharide) stimulated oxidation. We conclude that the lipid moiety of bacterial lipopolysaccharide is responsible for the stimulation of iron oxidation. This process may contribute to the ability of lipopolysaccharide to cause oxidation of heme-bound iron in hemoglobin.

Prospective Zinc Solubilising Bacteria for Enhanced Nutrient Uptake and Growth Promotion in Maize (Zea mays L.)

Zinc (Zn) is one of the essential micronutrients required for optimum plant growth. Substantial quantity of applied inorganic zinc in soil is converted into unavailable form. Zinc solubilising bacteria are potential alternates for zinc supplement. Among 10 strains screened for Zn solubilisation, P29, P33, and B40 produced 22.0 mm clear haloes on solid medium amended with ZnCO₃. Similarly, P17 and B40 showed 31.0 mm zone in ZnO incorporated medium. P29 and B40 showed significant release of Zn in broth amended with ZnCO₃ (17 and 16.8 ppm) and ZnO (18 and 17 ppm), respectively. The pH of the broth was almost acidic in all the cases ranging from 3.9 to 6.1 in ZnCO₃ and from 4.1 to 6.4 in ZnO added medium. Short term pot culture experiment with maize revealed that seed bacterization with P29 @ 10 g·kg⁻¹ significantly enhanced total dry mass (12.96 g) and uptake of N (2.268%), K (2.0%), Mn (60 ppm), and Zn (278.8 ppm).

Zn biomineralization processes and microbial biofilm in a metal-rich stream (Naracauli, Sardinia)

Several decades after the closure of the Ingurtosu mine (SW Sardinia), a variety of seasonal Zn biomineralizations occurs. In this work, waters, microbial consortia, and seasonal precipitates from the Naracauli stream were sampled to investigate chemical composition of stream waters and biominerals, and microbial strain identity. Molecular and morphological analysis revealed that activity of dominant cyanobacterium Leptolyngbya frigida results in precipitation of Zn silicate. The activity of the cyanobacterium was associated to other bacteria and many kind of diatoms, such as Halamphora subsalina and Encyonopsis microcephala, which are trapped in the process of biomineral growth. In this work, the precipitation process is shown to be the result of many different parameters such as hydrologic regime, microbial community adaptation, and biological mediation. It results in a decrease of dissolved Zn in the stream water, and is a potential tool for Zn pollution abatement.

Attenuation of chromium toxicity by bioremediation technology

Chromium is an important toxic environmental pollutant. Chromium pollution results largely from industrial activities, but other natural and anthropogenic sources also contribute to the problem. Plants that are exposed to environmental contamination by chromium are affected in diverse ways, including a tendency to suffer metabolic stress. The stress imposed by Cr exposure also extends to oxidative metabolic stress in plants that leads to the generation of active toxic oxygen free radicals. Such active free radicals degrade essential biomolecules and distort plant biological membranes. In this chapter, we describe sources of environmental chromium contamination, and provide information about the toxic impact of chromium on plant growth and metabolism. In addition, we address different phytoremediation processes that are being studied for use worldwide, in contaminated regions, to address and mitigate Cr pollution. There has been a long history of attempts to successfully mitigate the toxic effects of chromium-contaminated soil on plants and other organisms. One common approach, the shifting of polluted soil to landfills, is expensive and imposes environmental risks and health hazards of its own. Therefore, alternative eco-friendly bioremediation approaches are much in demand for cleaning chromium-polluted areas. To achieve its cleaning effects, bioremediation utilizes living organisms (bacteria, algae, fungi, and plants) that are capable of absorbing and processing chromium residues in ways which amend or eliminate it. Phytoremediation (bioremediation with plants) techniques are increasingly being used to reduce heavy metal contamination and to minimize the hazards of heavy metal toxicity. To achieve this, several processes, viz., rhizofiltration, phytoextraction, phytodetoxification, phytostabilization, and phytovolatilization, have been developed and are showing utility in practice, or promise. Sources of new native hyperaccumulator plants for use at contaminated sites are needed and constitute a key goal of ongoing phytoremediation research programs. Such new plants are needed to enhance the attractiveness of phytoremediation as an effective, affordable, and eco-friendly technique to achieve successful clean-up of metal-contaminated sites worldwide.

Effects of electron transport inhibitors on iron reduction in Aeromonas hydrophila strain KB1

The aim of this study was to determine the influence of respiratory chain inhibitors upon iron (III) reduction in Aeromonas hydrophila strain KB1. Optimal conditions of the reduction process were established by determining the amount of biomass, optimal pH, temperature and substrate concentration. The obtained results allowed us to determine Hill equation coefficients (K(m)=1.45+/-0.18 mM; V(max)=83.40+/-2.70 microM/min, and h=0.7+/-0.03). The value of h points to Michaelis-like kinetics of the process. The substrate concentration used in our study was such as to allow the maximum iron reduction rate. The reaction was mesophilic. The participation of electron carriers in the iron reduction process was investigated using respiratory chain inhibitors. Rotenone and capsaicin were used to study Q sites of the respiratory chain complex I. Dicumarol was used as an inhibitor of the quinone loop, while quinacrine was used to inhibit alloxazine centers. Additionally, complex III inhibitors, such as antimycin A, myxothiazole and 2-heptyl-4-hydroxy-quinoline N-oxide (HQNO) were used. Azide was used to inhibit complex IV. The observed inhibition of iron reduction by rotenone and capsaicin may suggest the existence of Q sites in formate reductase, analogous to those in complex I. Inhibition of quinones, isoalloxazine centers and complex III suggests participation of these carriers in the electron transport during iron reduction. Lack of inhibition of iron reduction by azide suggests that complex IV does not participate in this process.

Intracellular sequestration of manganese and phosphorus in a metal-resistant fungus Cladosporium cladosporioides from deep-sea sediment

A heavy metal resistant fungus was isolated from the sediment of Pacific Ocean, and identified to be Cladosporium cladosporioides. It grew normally in a medium containing 60 mM Mn(2+) and could endure 1,200 mM as the highest concentration tested. Quantification analysis confirmed a high accumulation of Mn which was 58 mg/g in dried biomass. Under transmission electron microscope, many intracellular crystals were observed in the cytoplasm of the hypha cells grown in a Mn-rich medium, and varied from a few nanometers to 200 nm in length. Energy dispersive X-ray (EDX) analysis showed that the crystals were composed of manganese and phosphorus in atomic ratio of 1.6:1 (Mn/P). Further, factors which might influence the resistance of this fungus were investigated. As a result, its high resistance to Mn(2+) was found dependent on the presence of Mg(2+), and could be further enhanced by phosphate. However, the effect of phosphate was not observed without the presence of Mg(2+). In addition, the resistance was also influenced by pH of the medium, which was lost above pH 8. This is the first report on a fungus which showed a hyper resistance to manganese by forming a large quantity of intracellular Mn/P crystals.

Uranyl precipitation by Pseudomonas aeruginosa via controlled polyphosphate metabolism

The polyphosphate kinase gene from Pseudomonas aeruginosa was overexpressed in its native host, resulting in the accumulation of 100 times the polyphosphate seen with control strains. Degradation of this polyphosphate was induced by carbon starvation conditions, resulting in phosphate release into the medium. The mechanism of polyphosphate degradation is not clearly understood, but it appears to be associated with glycogen degradation. Upon suspension of the cells in 1 mM uranyl nitrate, nearly all polyphosphate that had accumulated was degraded within 48 h, resulting in the removal of nearly 80% of the uranyl ion and >95% of lesser-concentrated solutions. Electron microscopy, energy-dispersive X-ray spectroscopy, and time-resolved laser-induced fluorescence spectroscopy (TRLFS) suggest that this removal was due to the precipitation of uranyl phosphate at the cell membrane. TRLFS also indicated that uranyl was initially sorbed to the cell as uranyl hydroxide and was then precipitated as uranyl phosphate as phosphate was released from the cell. Lethal doses of radiation did not halt phosphate secretion from polyphosphate-filled cells under carbon starvation conditions.

Effect of arsenic and mercury speciation on inhibition of respiration rate in activated sludge systems

The inhibition effect of arsenite, As(III), arsenate, As(V), inorganic mercury, Hg(II) and methylmercury, MeHg, on the respiration rate of activated heterotrophic sludge microorganisms was evaluated. As(III) and MeHg were much more toxic to activated sludge than As(V) and Hg(II) respectively. The effect of various experimental parameters on the toxicity, such as sludge age, concentration of suspended solids and exposure time, was investigated. An increase of sludge age or the concentration of suspended solids reduces the observed inhibition. Longer exposure seems to dramatically increase the inhibition of As(III), MeHg and Hg(II) during the first hours of exposure, while the later inhibition increases at a slower rate. On the contrary, in the presence of As(V), 24 hours after exposure, the respiration rate was similar to that of the control biomass.

Solubilization of zinc salts by a bacterium isolated from the air environment of a tannery

Airborne bacteria isolated from a tannery air environment were screened for the property of solubilization of insoluble zinc oxide and zinc phosphate. Out of 10 strains tested, a strain of Pseudomonas aeruginosa (CMG 823) showed the best solubilization and solubilized both zinc oxide and zinc phosphate. Colonies of the bacterium produced clear haloes on solid medium which contained these insoluble metal compounds, but only when glucose was provided as a carbon source. Solubilization of zinc oxide and phosphate was accompanied by an increase in the H+ concentration of the medium, probably a consequence of the production of 2-ketogluconic acid.

Copper status, serum cholesterol, and milk fatty acid profile in Holstein cows fed varying concentrations of copper

An experiment was conducted to determine the effects of dietary copper (Cu) on Cu status and lipid metabolism in Holstein cows. Three primiparous and 21 multiparous Holstein cows were utilized in this experiment. Groups of three cows similar in parity, days in milk, and milk yield were assigned randomly to one of the following three treatments: 1) control (no supplemental Cu), 2) 10 mg of Cu/kg of DM from Cu sulfate (CuSO4), and 3) 40 mg of Cu/kg of DM from CuSO4. Liver Cu concentrations were higher in Cu supplemented cows at the end of the 61-d study. Cows receiving 40 mg of Cu/kg of DM had higher liver Cu concentrations than cows receiving 10 mg of Cu. Plasma Cu concentrations were similar across treatments. Total serum cholesterol concentrations were higher in cows receiving supplemental Cu. Cows receiving 40 mg of Cu/kg of DM had higher serum cholesterol concentrations than cows receiving 10 mg of Cu. Dry matter intake, average daily milk production, and milk lipid, protein, and somatic cell numbers were similar across treatments. On d 61, milk fatty acids C18:1 trans and C18-conjugated dienes were lower in cows receiving supplemental Cu relative to the nonsupplemented controls. Cows receiving 40 mg of Cu/kg of DM had higher C12:0 and lower C18:2 and total polyunsaturated fatty acids in milk than cows receiving 10 mg of Cu/kg of DM. These results indicate that Cu supplementation alters lipid metabolism in high producing dairy cows and that Cu supplementation at 40 mg/kg of DM for 61 d can elevate liver Cu concentrations to levels considered to be marginally toxic in dairy cattle.

Interactions of chromium with microorganisms and plants

Chromium is a highly toxic non-essential metal for microorganisms and plants. Due to its widespread industrial use, chromium (Cr) has become a serious pollutant in diverse environmental settings. The hexavalent form of the metal, Cr(VI), is considered a more toxic species than the relatively innocuous and less mobile Cr(III) form. The presence of Cr in the environment has selected microbial and plant variants able to tolerate high levels of Cr compounds. The diverse Cr-resistance mechanisms displayed by microorganisms, and probably by plants, include biosorption, diminished accumulation, precipitation, reduction of Cr(VI) to Cr(III), and chromate efflux. Some of these systems have been proposed as potential biotechnological tools for the bioremediation of Cr pollution. In this review we summarize the interactions of bacteria, algae, fungi and plants with Cr and its compounds.

Microbial resistance to metals in the environment

Many microorganisms demonstrate resistance to metals in water, soil and industrial waste. Genes located on chromosomes, plasmids, or transposons encode specific resistance to a variety of metal ions. Some metals, such as cobalt, copper, nickel, serve as micronutrients and are used for redox processes, to stabilize molecules through electrostatic interactions, as components of various enzymes, and for regulation of osmotic pressure. Most metals are nonessential, have no nutrient value, and are potentially toxic to microorganisms. These toxic metals interact with essential cellular components through covalent and ionic bonding. At high levels, both essential and nonessential metals can damage cell membranes, alter enzyme specificity, disrupt cellular functions, and damage the structure of DNA. Microorganisms have adapted to the presence of both nutrient and nonessential metals by developing a variety of resistance mechanisms. Six metal resistance mechanisms exist: exclusion by permeability barrier, intra- and extra-cellular sequestration, active transport efflux pumps, enzymatic detoxification, and reduction in the sensitivity of cellular targets to metal ions. The understanding of how microorganisms resist metals can provide insight into strategies for their detoxification or removal from the environment.

Anaerobic killing of oral streptococci by reduced, transition metal cations

Reduced, transition metal cations commonly enhance oxidative damage to cells caused by hydroperoxides formed as a result of oxygen metabolism or added externally. As expected, the cations Fe2+ and Cu+ enhanced killing of Streptococcus mutans GS-5 by hydroperoxides. However, unexpectedly, they also induced lethal damage under fully anaerobic conditions in a glove box with no exposure to O2 or hydroperoxides from initial treatment with the cations. Sensitivities to anaerobic killing by Fe2+ varied among the organisms tested. The oral streptococci Streptococcus gordonii ATCC 10558, Streptococcus rattus FA-1, and Streptococcus sanguis NCTC 10904 were approximately as sensitive as S. mutans GS-5. Enterococcus hirae ATCC 9790, Actinomyces viscosus OMZ105E, and Actinomyces naeslundii WVU45 had intermediate sensitivity, while Lactobacillus casei ATCC 4646 and Escherichia coli B were insensitive. Killing of S. mutans GS-5 in response to millimolar levels of added Fe2+ occurred over a wide range of temperatures and pH. The organism was able to take up ferrous iron, but ferric reductase activity could not be detected. Chelators, uric acid, and thiocyanate were not effective inhibitors of the lethal damage. Sulfhydryl compounds, ferricyanide, and ferrocyanide were protective if added prior to Fe2+ exposure. Fe2+, but not Fe3+, acted to reduce the acid tolerance of glycolysis by intact cells of S. mutans. The reduction in acid tolerance appeared to be related directly to Fe2+ inhibition of F-ATPase, which could be assayed with permeabilized cells, isolated membranes, or F1 enzyme separated from membranes. Cu+ and Cu2+ also inhibited F-ATPase and sensitized glycolysis by intact cells to acid. All of these damaging actions occurred anaerobically and thus did not appear to involve reactive oxygen species.