Inhibition of the human thioredoxin system. A molecular mechanism of mercury toxicity

Mercury toxicity mediated by different forms of mercury is a major health problem; however, the molecular mechanisms underlying toxicity remain elusive. We analyzed the effects of mercuric chloride (HgCl(2)) and monomethylmercury (MeHg) on the proteins of the mammalian thioredoxin system, thioredoxin reductase (TrxR) and thioredoxin (Trx), and of the glutaredoxin system, glutathione reductase (GR) and glutaredoxin (Grx). HgCl(2) and MeHg inhibited recombinant rat TrxR with IC(50) values of 7.2 and 19.7 nm, respectively. Fully reduced human Trx1 bound mercury and lost all five free thiols and activity after incubation with HgCl(2) or MeHg, but only HgCl(2) generated dimers. Mass spectra analysis demonstrated binding of 2.5 mol of Hg(2+) and 5 mol of MeHg(+)/mol of Trx1 with the very strong Hg(2+) complexes involving active site and structural disulfides. Inhibition of both TrxR and Trx activity was observed in HeLa and HEK 293 cells treated with HgCl(2) or MeHg. GR was inhibited by HgCl(2) and MeHg in vitro, but no decrease in GR activity was detected in cell extracts treated with mercurials. Human Grx1 showed similar reactivity as Trx1 with both mercurial compounds, with the loss of all free thiols and Grx dimerization in the presence of HgCl(2), but no inhibition of Grx activity was observed in lysates of HeLa cells exposed to mercury. Overall, mercury inhibition was selective toward the thioredoxin system. In particular, the remarkable potency of the mercury compounds to bind to the selenol-thiol in the active site of TrxR should be a major molecular mechanism of mercury toxicity.     

Formation of Methyl Mercury by Bacteria

Twenty-three Hg2+-resistant cultures were isolated from sediment of the Savannah River in Georgia; of these, 14 were gram-negative short rods belonging to the genera Escherichia and Enterobacter, six were gram-positive cocci (three Staphylococcus sp. and three Streptococcus sp.) and three were Bacillus sp. All the Escherichia, Enterobacter, and the Bacillus strain were more resistant to Hg2+ than the strains of staphylococci and streptococci. Adaptation using serial dilutions and concentration gradient agar plate techniques showed that it was possible to select a Hg2+-resistant strain from a parent culture identified as Enterobacter aerogenes. This culture resisted 1,200 mug of Hg2+ per ml of medium and produced methyl mercury from HgCl2, but was unable to convert Hg2+ to volatile elemental mercury (Hg0). Under constant aeration (i.e., submerged culture), slightly more methyl mercury was formed than in the absence of aeration. Production of methyl mercury was cyclic in nature and slightly decreased if DL-homocysteine was present in media, but increased with methylcobalamine. It is concluded that the bacterial production of methyl mercury may be a means of resistance and detoxification against mercurials in which inorganic Hg2+ is converted to organic form and secreted into the environment.     

Functional dissection of a mercuric ion transporter, MerC, from Acidithiobacillus ferrooxidans

Topological analysis with a phoA gene fusion suggested that Acidithiobacillus ferrooxidans MerC, a mercury transporter, has two periplasmic loops and four transmembrane domains. Cys-23 and Cys-26 of the protein were involved in Hg(2+)-recognition/uptake, but Cys-132 and Cys-137 were not. Escherichia coli cells producing the MerC were hypersensitive to CdCl(2). In this case, mutation of His72 rendered the host cells less CdCl(2) sensitive, whereas none of the Cys residues affected it. E. coli cells expressing the gene encoding a mercuric ion transporter (merC)-deletion mutant, in which the coding-sequence of the carboxy-terminal cytoplasmic region was removed, retained Hg(2+) hypersensitivity and showed about 55% HgCl(2) uptake ability compared to that of the one expressing the intact merC, indicating that the region is not essential for Hg(2+) uptake. Coexpression of A. ferrooxidans the gene encoding mercuric reductase (merA) and the merC deletion mutation conferred HgCl(2) tolerance to E. coli host cells. Under this condition, the merC deletion gene product was exclusively present as a monomer.     

Volatilization of mercuric chloride by mercury-resistant plasmid-bearing strains of Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa

Several bacterial strains carrying genes determining mercury resistance on naturally occurring plasmids will convert 10(-5)m Hg(2+) (chloride) to a form of mercury which is volatile and soluble in organic solvents. The volatilization activity is induced by exposure to HgCl(2).     

A methylene blue-mediated enzyme electrode for the determination of trace mercury(II), mercury(I), methylmercury, and mercury-glutathione complex

A methylene blue-mediated enzyme biosensor has been developed for the detection of inhibitors including mercury(II), mercury(I), methylmercury, and mercury-glutathione complex. The inhibition to horseradish peroxidase was apparently reversible and noncompetitive in the presence of HgCl2 in less than 8 s and irreversibly inactivated when incubated with different concentrations of HgCl2 for 1-8 min. The binding site of horseradish peroxidase with HgCl2 probably was a cysteine residue SH. Mercury compounds can be assayed amperometrically with the detection limits 0.1 ng ml(-1) Hg for HgCl2 and methylmercury, 0.2 ng ml(-1) Hg for Hg2(NO3)2 and 1.7 ng ml(-1) Hg for mercury glutathione complex. Inactivation of the immobilized horseradish peroxidase was displayed in the AFM images of the enzyme membranes.     

Ferrous iron-dependent volatilization of mercury by the plasma membrane of Thiobacillus ferrooxidans

Of 100 strains of iron-oxidizing bacteria isolated, Thiobacillus ferrooxidans SUG 2-2 was the most resistant to mercury toxicity and could grow in an Fe(2+) medium (pH 2.5) supplemented with 6 microM Hg(2+). In contrast, T. ferrooxidans AP19-3, a mercury-sensitive T. ferrooxidans strain, could not grow with 0.7 microM Hg(2+). When incubated for 3 h in a salt solution (pH 2.5) with 0.7 microM Hg(2+), resting cells of resistant and sensitive strains volatilized approximately 20 and 1.7%, respectively, of the total mercury added. The amount of mercury volatilized by resistant cells, but not by sensitive cells, increased to 62% when Fe(2+) was added. The optimum pH and temperature for mercury volatilization activity were 2.3 and 30 degrees C, respectively. Sodium cyanide, sodium molybdate, sodium tungstate, and silver nitrate strongly inhibited the Fe(2+)-dependent mercury volatilization activity of T. ferrooxidans. When incubated in a salt solution (pH 3.8) with 0.7 microM Hg(2+) and 1 mM Fe(2+), plasma membranes prepared from resistant cells volatilized 48% of the total mercury added after 5 days of incubation. However, the membrane did not have mercury reductase activity with NADPH as an electron donor. Fe(2+)-dependent mercury volatilization activity was not observed with plasma membranes pretreated with 2 mM sodium cyanide. Rusticyanin from resistant cells activated iron oxidation activity of the plasma membrane and activated the Fe(2+)-dependent mercury volatilization activity of the plasma membrane.     

Volatilization of mercury by an iron oxidation enzyme system in a highly mercury-resistant Acidithiobacillus ferrooxidans strain MON-1

A highly mercury-resistant strain Acidithiobacillus ferrooxidans MON-1, was isolated from a culture of a moderately mercury-resistant strain, A. ferrooxidans SUG 2-2 (previously described as Thiobacillus ferrooxidans SUG 2-2), by successive cultivation and isolation of the latter strain in a Fe2+ medium with increased amounts of Hg2+ from 6 microM to 20 microM. The original stain SUG 2-2 grew in a Fe2+ medium containing 6 microM Hg2+ with a lag time of 22 days, but could not grow in a Fe2+ medium containing 10 microM Hg2+. In contrast, strain MON-1 could grow in a Fe2+ medium containing 20 microM Hg2+ with a lag time of 2 days and the ability of strain MON-1 to grow rapidly in a Fe2+ medium containing 20 microM Hg2+ was maintained stably after the strain was cultured many times in a Fe2+ medium without Hg2+. A similar level of NADPH-dependent mercury reductase activity was observed in cell extracts from strains SUG 2-2 and MON-1. By contrast, the amounts of mercury volatilized for 3 h from the reaction mixture containing 7 microM Hg2+ using a Fe(2+)-dependent mercury volatilization enzyme system were 5.6 nmol for SUG 2-2 and 67.5 nmol for MON-1, respectively, indicating that a marked increase of Fe(2+)-dependent mercury volatilization activity conferred on strain MON-1 the ability to grow rapidly in a Fe2+ medium containing 20 microM Hg2+. Iron oxidizing activities, 2,3,5,6-tetramethyl-p-phenylenediamine (TMPD) oxidizing activities and cytochrome c oxidase activities of strains SUG 2-2 and MON-1 were 26.3 and 41.9 microl O2 uptake/mg/min, 15.6 and 25.0 microl O2 uptake/mg/min, and 2.1 and 6.1 mU/mg, respectively. These results indicate that among components of the iron oxidation enzyme system, especially cytochrome c oxidase activity, increased by the acquisition of further mercury resistance in strain MON-1. Mercury volatilized by the Fe(2+)-dependent mercury volatilization enzyme system of strain MON-1 was strongly inhibited by 1.0 mM sodium cyanide, but was not by 50 nM rotenone, 5 microM 2-n-heptyl-4-hydroxy-quinoline-N-oxide (HQNO), 0.5 microM antimycin A, or 0.5 microM myxothiazol, indicating that cytochrome c oxidase plays a crucial role in mercury volatilization of strain MON-1 in the presence of Fe2+.     

Biotoxicity of mercury as influenced by mercury(II) speciation

Integration of physicochemical procedures for studying mercury(II) speciation with microbiological procedures for studying the effects of mercury on bacterial growth allows evaluation of ionic factors (e.g., pH and ligand species and concentration) which affect biotoxicity. A Pseudomonas fluorescens strain capable of methylating inorganic Hg(II) was isolated from sediment samples collected at Buffalo Pound Lake in Saskatchewan, Canada. The effect of pH and ligand species on the toxic response (i.e., 50% inhibitory concentration [IC50]) of the P. fluorescens isolated to mercury were determined and related to the aqueous speciation of Hg(II). It was determined that the toxicities of different mercury salts were influenced by the nature of the co-ion. At a given pH level, mercuric acetate and mercuric nitrate yielded essentially the same IC50s; mercuric chloride, on the other hand, always produced lower IC50s. For each Hg salt, toxicity was greatest at pH 6.0 and decreased significantly (P = 0.05) at pH 7.0. Increasing the pH to 8.0 had no effect on the toxicity of mercuric acetate or mercuric nitrate but significantly (P = 0.05) reduced the toxicity of mercuric chloride. The aqueous speciation of Hg(II) in the synthetic growth medium M-IIY (a minimal salts medium amended to contain 0.1% yeast extract and 0.1% glycerol) was calculated by using the computer program GEOCHEM-PC with a modified data base. Results of the speciation calculations indicated that complexes of Hg(II) with histidine [Hg(H-HIS)HIS+ and Hg(H-HIS)2(2+)], chloride (HgCl+, HgCl2(0), HgClOH0, and HgCl3-), phosphate (HgHPO4(0), ammonia (HgNH3(2+), glycine [Hg(GLY)+], alanine [Hg(ALA)+], and hydroxyl ion (HgOH+) were the Hg species primarily responsible for toxicity in the M-IIY medium. The toxicity of mercuric nitrate at pH 8.0 was unaffected by the addition of citrate, enhanced by the addition of chloride, and reduced by the addition of cysteine. In the chloride-amended system, HgCl+, HgCl2(0), and HgClOH0 were the species primarily responsible for observed increases in toxicity. In the cysteine-amended system, formation of Hg(CYS)2(2-) was responsible for detoxification effects that were observed. The formation of Hg-citrate complexes was insignificant and had no effect on Hg toxicity.     

Biotoxicity of mercury as influenced by mercury(II) speciation.

Integration of physicochemical procedures for studying mercury(II) speciation with microbiological procedures for studying the effects of mercury on bacterial growth allows evaluation of ionic factors (e.g., pH and ligand species and concentration) which affect biotoxicity. A Pseudomonas fluorescens strain capable of methylating inorganic Hg(II) was isolated from sediment samples collected at Buffalo Pound Lake in Saskatchewan, Canada. The effect of pH and ligand species on the toxic response (i.e., 50% inhibitory concentration [IC50]) of the P. fluorescens isolated to mercury were determined and related to the aqueous speciation of Hg(II). It was determined that the toxicities of different mercury salts were influenced by the nature of the co-ion. At a given pH level, mercuric acetate and mercuric nitrate yielded essentially the same IC50s; mercuric chloride, on the other hand, always produced lower IC50s. For each Hg salt, toxicity was greatest at pH 6.0 and decreased significantly (P = 0.05) at pH 7.0. Increasing the pH to 8.0 had no effect on the toxicity of mercuric acetate or mercuric nitrate but significantly (P = 0.05) reduced the toxicity of mercuric chloride. The aqueous speciation of Hg(II) in the synthetic growth medium M-IIY (a minimal salts medium amended to contain 0.1% yeast extract and 0.1% glycerol) was calculated by using the computer program GEOCHEM-PC with a modified data base. Results of the speciation calculations indicated that complexes of Hg(II) with histidine [Hg(H-HIS)HIS+ and Hg(H-HIS)2(2+)], chloride (HgCl+, HgCl2(0), HgClOH0, and HgCl3-), phosphate (HgHPO4(0), ammonia (HgNH3(2+), glycine [Hg(GLY)+], alanine [Hg(ALA)+], and hydroxyl ion (HgOH+) were the Hg species primarily responsible for toxicity in the M-IIY medium. The toxicity of mercuric nitrate at pH 8.0 was unaffected by the addition of citrate, enhanced by the addition of chloride, and reduced by the addition of cysteine. In the chloride-amended system, HgCl+, HgCl2(0), and HgClOH0 were the species primarily responsible for observed increases in toxicity. In the cysteine-amended system, formation of Hg(CYS)2(2-) was responsible for detoxification effects that were observed. The formation of Hg-citrate complexes was insignificant and had no effect on Hg toxicity.     

Transformations of inorganic mercury by Candida albicans and Saccharomyces cerevisiae

Saccharomyces cerevisiae and Candida albicans were incubated with 0.25, 0.5, or 0.75 micrograms of Hg (as HgCl2) per ml of Nelson's medium in the presence of trace amounts of oxygen at 28 degrees C for 12 days. Two control media were used, one without added Hg and one without yeast inoculum. Yeast cell growth was estimated after 1, 2, 3, and 8 days of incubation. The contents of organomercury in the system and of elemental mercury released from the media and collected in traps were determined at the end of the experiments. The results were as follows. (i) C. albicans was the more mercury-resistant species, but both yeast species failed to grow in the media containing 0.75 micrograms of Hg per ml. (ii) The amounts of organomercury produced by the two species were proportional to the amount of HgCl2 added to the medium. In all cases C. albicans produced considerably larger amounts of methylmercury than S. cerevisiae. (iii) The amounts of elemental Hg produced were inversely proportional to the HgCl2 level added in the case of S. cerevisiae but were all similar in the case of C. albicans. (iv) Neither organomercury nor elemental Hg was produced in any of the control media.     

Transformations of inorganic mercury by Candida albicans and Saccharomyces cerevisiae.

Saccharomyces cerevisiae and Candida albicans were incubated with 0.25, 0.5, or 0.75 micrograms of Hg (as HgCl2) per ml of Nelson's medium in the presence of trace amounts of oxygen at 28 degrees C for 12 days. Two control media were used, one without added Hg and one without yeast inoculum. Yeast cell growth was estimated after 1, 2, 3, and 8 days of incubation. The contents of organomercury in the system and of elemental mercury released from the media and collected in traps were determined at the end of the experiments. The results were as follows. (i) C. albicans was the more mercury-resistant species, but both yeast species failed to grow in the media containing 0.75 micrograms of Hg per ml. (ii) The amounts of organomercury produced by the two species were proportional to the amount of HgCl2 added to the medium. In all cases C. albicans produced considerably larger amounts of methylmercury than S. cerevisiae. (iii) The amounts of elemental Hg produced were inversely proportional to the HgCl2 level added in the case of S. cerevisiae but were all similar in the case of C. albicans. (iv) Neither organomercury nor elemental Hg was produced in any of the control media.     

Long-term performance of bioreactors cleaning mercury-contaminated wastewater and their response to temperature and mercury stress and mechanical perturbation

The long-term performance of bioreactors retaining mercury from contaminated industrial wastewater was analyzed at the laboratory scale, and its response to mechanical perturbations (gas bubbles and shaking) as well as to physical (increased temperature and hydraulic load) and chemical stresses (increased mercury concentration) likely to occur during on site operation was studied. Two packed-bed bioreactors with 80-cm(3) lava chips as biofilm carrier were inoculated with nine Hg(II)-resistant natural isolates of alpha- and gamma-proteobacteria. Chloralkali wastewater containing ionic mercury (3.0 to 9.7 mg/L Hg(2+)), amended with sucrose and yeast extract, flowed through the bioreactors at 160 mL/h. During the 16-month investigation the bioreactors showed no sign of depleted performance in terms of mercury-retaining capacity. After 16 months, both bioreactors still retained 96% of the mercury load. The performance of the bioreactors was sensitive to mechanical perturbations (e.g., sheer forces of gas bubbles). Shifts to higher Hg(2+) inflow concentrations initially decreased the mercury retention efficacy slightly. However, the bioreactors could adapt to Hg(2+) concentrations of up to 7.6 mg/L within several days. Old biofilms were less affected than the younger ones. The performance of the bioreactors was not affected by an increase in temperature up to 41 degrees C and an increased volumetric load (up to 240 mL/h). The bioreactors regained activity spontaneously after the stress had stopped. Recovery could be accelerated by increased nutrient concentration, although this may lead to blocking of the packed bed.     

Effect of catabolite repression on the mer operon

The plasmid-determined mer operon, which provides resistance to inorganic mercury compounds, was subject to a 2.5-fold decrease in expression when glucose was administered at the same time as the inducer HgCl2. This glucose-mediated transient repression of the operon was overcome by the addition of cyclic AMP. Permanent catabolite repression of the operon was observed in the 1.6- to 1.9-fold decrease in expression in mutants lacking either adenyl cyclase (cya) or the catabolite activator protein (crp). The effect of the cya mutation on mer expression could be overcome by the addition of cyclic AMP at the time of induction, In addition to these effects on the whole cells of a wild-type strains, we examined the effect of catabolite repression on the expression of the mercuric ion [Hg(II)] reductase enzyme, assayable in cell extracts, and on the Hg(II) uptake system, assayable in a mutant strain which lacked reductase activity. There was a two- to threefold effect of repression on the Hg(II) reductase enzyme assayable in vitro after induction under catabolite repressing conditions (either with glucose or in the crp and cya mutants). We did not find a similar repressing effect on the induction of the Hg(II) uptake system, which is also determined by the mer operon.     

Mercury and Organomercurial Resistances Determined by Plasmids in Pseudomonas

Mercury and organomercurial resistance determined by genes on ten Pseudomonas aeruginosa plasmids and one Pseudomonas putida plasmid have been studied with regard to the range of substrates and the range of inducers. The plasmidless strains were sensitive to growth inhibition by Hg(2+) and did not volatilize Hg(0) from Hg(2+). A strain with plasmid RP1 (which does not confer resistance to Hg(2+)) similarly did not volatilize mercury. All 10 plasmids determine mercury resistance by way of an inducible enzyme system. Hg(2+) was reduced to Hg(0), which is insoluble in water and rapidly volatilizes from the growth medium. Plasmids pMG1, pMG2, R26, R933, R93-1, and pVS1 in P. aeruginosa and MER in P. putida conferred resistance to and the ability to volatilize mercury from Hg(2+), but strains with these plasmids were sensitive to and could not volatilize mercury from the organomercurials methylmercury, ethylmercury, phenylmercury, and thimerosal. These plasmids, in addition, conferred resistance to the organomercurials merbromin, p-hydroxymercuribenzoate, and fluorescein mercuric acetate. The other plasmids, FP2, R38, R3108, and pVS2, determined resistance to and decomposition of a range of organomercurials, including methylmercury, ethylmercury, phenylmercury, and thimerosal. These plasmids also conferred resistance to the organomercurials merbromin, p-hydroxymercuribenzoate, and fluorescein mercuric acetate by a mechanism not involving degradation. In all cases, organomercurial decomposition and mercury volatilization were induced by exposure to Hg(2+) or organomercurials. The plasmids differed in the relative efficacy of inducers. Hg(2+) resistance with strains that are organomercurial sensitive appeared to be induced preferentially by Hg(2+) and only poorly by organomercurials to which the cells are sensitive. However, the organomercurials p-hydroxymercuribenzoate, merbromin, and fluorescein mercuric acetate were strong gratuitous inducers but not substrates for the Hg(2+) volatilization system. With strains resistant to phenylmercury and thimerosal, these organomercurials were both inducers and substrates.     

Effect of mercury on selenium utilization and selenoperoxidase activity in LNCaP cells

Formation of stable complexes with protein thiols is the best-known mechanism of mercury toxicity. However, the solubility product of Hg(2+) with sulfides, although very low, is higher than that with selenides, suggesting that the fully reduced form of selenium might also be a relevant target for Hg(2+). In cells, selenide is the suggested intermediate for selenoprotein biosynthesis and selenoenzymes, in turn, contain reduced selenium as the catalytic moiety. Thus, inhibition of biological functions of selenium could be seen as a different mechanism of Hg(2+) toxicity. To address this issue, we investigated selenoperoxidase (SeGPx) activity in LNCaP cells exposed to HgCl(2). Cells growing in standard medium express a low GPx activity, which increases on addition of selenium donors such as selenite, selenomethionine, or methyl-Se-cysteine. HgCl(2) added to the medium has different effects depending on the type of Se donor. A progressive decrease of SeGPx activity is observed in cells grown in standard medium exposed to HgCl(2), while coadministration of suprastoichiometric amounts of HgCl(2) prevents the increase of SeGPx activity only when selenite, but not selenomethionine or methyl-Se-cysteine, is the selenium source. From this evidence we conclude that HgCl(2): (a) does not inhibit directly SeGPxs, as confirmed on isolated enzymes; (b) does not interfere with the intermediates of the metabolic pathway of selenoprotein synthesis; and (c) decreases the bioavailability of selenium only when ionic complexes can be formed.     

Enhanced mercury biosorption by bacterial cells with surface-displayed MerR

The metalloregulatory protein MerR, which exhibits high affinity and selectivity toward mercury, was exploited for the construction of microbial biosorbents specific for mercury removal. Whole-cell sorbents were constructed with MerR genetically engineered onto the surface of Escherichia coli cells by using an ice nucleation protein anchor. The presence of surface-exposed MerR on the engineered strains enabled sixfold-higher Hg(2+) biosorption than that found in the wild-type JM109 cells. Hg(2+) binding via MerR was very specific, with no observable decline even in the presence of 100-fold excess Cd(2+) and Zn(2+). The Hg(2+) binding property of the whole-cell sorbents was also insensitive to different ionic strengths, pHs, and the presence of metal chelators. Since metalloregulatory proteins are currently available for a wide variety of toxic heavy metals, our results suggest that microbial biosorbents overexpressing metalloregulatory proteins may be used similarly for the cleanup of other important heavy metals.     

Effect of glucose on pyruvate utilization by Rhodotorula glutinis

Summary The utilization of glucose and pyruvate by the yeast Rhodotorula glutinis in a medium containing both carbon sources has been studied. Glucose is readily consumed whereas the uptake of pyruvate is completely blocked by the presence of the sugar.The content of pyruvate kinase and phosphoenolpyruvate carboxykinase in R. glutinis cells growing on glucose plus pyruvate are drastically affected with time by the disappearance of the sugar from the culture medium. After complete exhaustion of glucose, the level of pyruvate kinase drops sharply down to a minimum whereas that of phosphoenolpyruvate carboxykinase rises abruptly up to a maximum.Feeding experiments with labelled compounds show that glucose affects the utilization of the amino acids alanine and aspartate, and conversely that the amino acids influence the utilization of the sugar. Glucose breakdown and its incorporation into polysaccharides is controlled by the amino acids and gluconeogenesis from the amino acids is controlled by the sugar.     

Plasmid-encoded mercuric reductase in Mycobacterium scrofulaceum.

A Chesapeake Bay water isolate of Mycobacterium scrofulaceum containing a 115-megadalton plasmid (pVT1) grew in the presence of 100 microM HgCl2 and converted soluble 203Hg2+ to volatile mercury at a rate of 50 pmol/10(8) cells per min. Cell extracts contained a soluble mercuric reductase whose activity was not dependent on exogenously supplied thiol compounds. The enzyme displayed nearly identical activity when either NADH or NADPH served as the electron donor. A spontaneously cured derivative lacking pVT1 failed to grow in the presence of 100 microM HgCl2 and possessed no detectable mercuric reductase activity.     

不同汞化合物对水稻、小麦的影响及作物对汞的吸收积累

本试验研究了5种汞化合物(HgS,HgO,CH_3HgCl,HgCl_2,C_8H_8O_2Hg)对水稻、小麦生长发育的影响及作物对汞的吸收、积累。结果表明,C_8H_8O_2Hg对作物的危害比HgCl_2和CH_3HgCl大,HgS的危害最轻。不同汞化合物对水稻蒸腾作用的抑制程度看出,C_8H_8O_2Hg的毒性大,HgS的毒性最小;抑制小麦光合作用的程度看出,HgCl_2的毒性大、HgS的毒性小。不同汞化合物处理的土壤中,水稻、小麦的含汞量是随着汞化合物的浓度增加而增加,以C_8H_8O_2Hg处理的土壤,作物吸收的汞最多,转移到地上部的汞最多,HgS处理的土壤,汞转移到地上部最少;小麦吸收的汞大部积累在根中,地上部(茎叶)的含汞量显著比水稻少;各处理的土壤总汞含量与水稻的含汞量相关性显著。土壤中的HgS、HgCl_2可以转化为CH_3HgCl,并转运到植物体各器官。 本试验是用盆栽试验的方法,土壤用不同浓度不同汞化台物处理。 用的“称重法”测定了水稻的蒸腾作用。用FQW-CO_2红外气体分析仪测定了小麦的光合强度。用F-732测汞仪测定了水稻、小麦不同器官和土壤中的总汞含量。用巯基棉气相色谱法测定了甲基汞的含量。     

Cyanobacteria as a biosorbent for mercuric ion

The biosorption of Hg(2+) by two strains of cyanobacteria, Spirulina platensis and Aphanothece flocculosa, was studied under a batch stirred reaction system. Essential process parameters, including pH, biomass concentration, initial metal concentration, and presence of co-ions were shown to influence the Hg(2+) uptake. Hg(2+) uptake was optimal at pH 6.0 for both strains. The maximum loading capacities per gram of dry biomass were found to be 456 mg Hg(2+) for A. flocculosa and 428 mg Hg(2+) for S. platensis. At an initial concentration of 10 ppm Hg(2+), A. flocculosa was able to remove more than 98% of the mercury ion from solution. The biosorption kinetics of both strains showed that the metal uptake is bi-phasic, exhibiting a rapid initial uptake followed by a slower absorption process. The presence of dissolved Co(2+), Ni(2+), and Fe(3+) were found to play a synergistic role for Hg(2+) uptake by both strains. Regeneration of the biomass was examined by treating Hg(2+)-loaded samples with HCl and NH(4)Cl over four cycles of sorption and desorption.