Mercury methylation by dissimilatory iron-reducing bacteria

The Hg-methylating ability of dissimilatory iron-reducing bacteria in the genera Geobacter, Desulfuromonas, and Shewanella was examined. All of the Geobacter and Desulfuromonas strains tested methylated mercury while reducing Fe(III), nitrate, or fumarate. In contrast, none of the Shewanella strains produced methylmercury at higher levels than abiotic controls under similar culture conditions. Geobacter and Desulfuromonas are closely related to known Hg-methylating sulfate-reducing bacteria within the Deltaproteobacteria.     

Mercury methylation and hydrogen sulfide production among unexpected strains isolated from periphyton of two macrophytes of the Amazon

The periphyton of macrophytes had previously been identified as important spots for mercury methylation in the Amazon basin, but the microorganisms that facilitate methylation in such compartment are still to be identified. Here, bacteria were isolated from periphyton associated with Eichhornia crassipes and Polygonum densiflorum in Widdel and Pfennig medium and tested for mercury methylation with a stable isotope tracer technique using (198)HgCl, hydrogen sulfide production and molybdate inhibition. Three Pleomorphomona spp., one unidentified Deltaproteobacteria, two Klebsiella spp., and one Tolumonas sp. were isolated. All except Tolumonas sp. were able to methylate mercury (up to 5% of the (198)HgCl added) and produce up to 4 mM of H(2)S, while the Deltaproteobacteria was also able to demethylate methylmercury. Although these bacteria may not be as strong mercury methylators as sulfate-reducing bacteria, they have the potential to contribute to methylmercury accumulation in the system.     

Tributyltin-resistant bacteria from estuarine and freshwater sediments.

Resistance to tributyltin (TBT) was examined in populations from TBT-polluted sediments and nonpolluted sediments from an estuary and from fresh water as well as in pure cultures isolated from those sediments. The 50% effective concentrations (EC50s) for populations were higher at a TBT-polluted freshwater site than at a site without TBT, suggesting that TBT selected for a TBT-resistant population. In contrast, EC50s were significantly lower for populations from a TBT-contaminated estuarine site than for those from a site without TBT, suggesting that other factors in addition to TBT determine whether populations become resistant. EC50s for populations from TBT-contaminated freshwater sediments were nearly 30 times higher than those for populations from TBT-contaminated estuarine sediments. We defined a TBT-resistant bacterium as one which grows on trypticase soy agar containing 8.4 microM TBT, a concentration which prevented the growth of 90% of the culturable bacteria from these sediments. The toxicity of TBT in laboratory media was influenced markedly by the composition of the medium and whether it was liquid or solid. Ten TBT-resistant isolates from estuarine sediments and 19 from freshwater sediments were identified to the genus level. Two isolates, each a Bacillus sp., may be the first gram-positive bacteria isolated from fresh water in the presence of a high concentration of TBT. There was a high incidence of resistance to heavy metals: metal resistance indices were 0.76 for estuarine isolates and 0.68 for freshwater isolates.(ABSTRACT TRUNCATED AT 250 WORDS)     

Tributyltin-resistant bacteria from estuarine and freshwater sediments.

Resistance to tributyltin (TBT) was examined in populations from TBT-polluted sediments and nonpolluted sediments from an estuary and from fresh water as well as in pure cultures isolated from those sediments. The 50% effective concentrations (EC50s) for populations were higher at a TBT-polluted freshwater site than at a site without TBT, suggesting that TBT selected for a TBT-resistant population. In contrast, EC50s were significantly lower for populations from a TBT-contaminated estuarine site than for those from a site without TBT, suggesting that other factors in addition to TBT determine whether populations become resistant. EC50s for populations from TBT-contaminated freshwater sediments were nearly 30 times higher than those for populations from TBT-contaminated estuarine sediments. We defined a TBT-resistant bacterium as one which grows on trypticase soy agar containing 8.4 microM TBT, a concentration which prevented the growth of 90% of the culturable bacteria from these sediments. The toxicity of TBT in laboratory media was influenced markedly by the composition of the medium and whether it was liquid or solid. Ten TBT-resistant isolates from estuarine sediments and 19 from freshwater sediments were identified to the genus level. Two isolates, each a Bacillus sp., may be the first gram-positive bacteria isolated from fresh water in the presence of a high concentration of TBT. There was a high incidence of resistance to heavy metals: metal resistance indices were 0.76 for estuarine isolates and 0.68 for freshwater isolates.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hydrocarbon accumulation in freshwater sediments of an urban catchment

The top and bottom of two sediment cores collected from an urban receiving basin in NW London, and stormwater samples from the attendant catchment, have been analysed for their hydrocarbon content. In surface sediments, basal sediments and stormwater, total aliphatic hydrocarbon levels are 445–690 μg g⁻¹ dry wt., 43–224 μg g⁻¹ and 0.36–1.10 mg l⁻¹, respectively; and total levels of polyaromatic hydrocarbons (PAHs) are 780–1 100 μg g⁻¹, 310–640 μg g¹ and 5.83–18.21 mg l⁻¹, respectively. Biodegradation of aliphatics is assessed by phytane:n-C₁₈ and pristane: n-C₁₇ ratios. Hydrocarbon sources are determined from phytane: pristane ratios, odd: even carbon chain length ratios, the presence of an unresolved complex mixture, and by comparison of the amount of methyl-substituted PAH s with that of the parent compounds. Comparison of total levels between surface and basal sediments shows a 1 to 3 fold increase in total PAHs and a 3 to 10 fold increase in aliphatic hydrocarbons over a 120 year period.     

Dominance of Geobacteraceae in BTX-degrading enrichments from an iron-reducing aquifer

Microbial community structure was linked to degradation potential in benzene-, toluene- or xylene- (BTX) degrading, iron-reducing enrichments derived from an iron-reducing aquifer polluted with landfill leachate. Enrichments were characterized using 16S rRNA gene-based analysis, targeting of the benzylsuccinate synthase-encoding bssA gene and phospholipid fatty acid (PLFA) profiling in combination with tracking of labelled substrate. 16S rRNA gene analysis indicated the dominance of Geobacteraceae, and one phylotype in particular, in all enrichments inoculated with polluted aquifer material. Upon cultivation, progressively higher degradation rates with a concomitant decrease in species richness occurred in all primary incubations and successive enrichments. Yet, the same Geobacteraceae phylotype remained common and dominant, indicating its involvement in BTX degradation. However, the bssA gene sequences in BTX degrading enrichments differed considerably from those of Geobacter isolates, suggesting that the first steps of toluene, but also benzene and xylene oxidation, are carried out by another member of the enrichments. Therefore, BTX would be synthrophically degraded by a bacterial consortium in which Geobacteraceae utilized intermediate metabolites. PLFA analysis in combination with (13)C-toluene indicated that the enriched Geobacteraceae were assimilating carbon originally present in toluene. Combined with previous studies, this research suggests that Geobacteraceae play a key role in the natural attenuation of each BTX compound in situ.     

A heme-C-containing enzyme complex that exhibits nitrate and nitrite reductase activity from the dissimilatory iron-reducing bacterium Geobacter metallireducens

Nitrate reduction in the dissimilatory iron-reducing bacterium Geobacter metallireducens was investigated. Nitrate reductase and nitrite reductase activities in nitrate-grown cells were detected only in the membrane fraction. The apparent K _m values for nitrate and nitrite were determined to be 32 and 10 μM, respectively. Growth on nitrate was not inhibited by either tungstate or molybdate at concentrations of 1 mM or less, but was inhibited by both at 10 and 20 mM. Nitrate and nitrite reductase activity in the membrane fraction was not, however, affected by dialysis with 20 mM tungstate. An enzyme complex that exhibited both nitrate and nitrite reductase activity was solubilized from membrane fractions with CHAPS and was partially purified by preparative gel electrophoresis. It was found to be composed of four different polypeptides with molecular masses of 62, 52, 36, and 16 kDa. The 62-kDa polypeptide [a low-midpoint potential (–207 mV), multiheme cytochrome c] exhibited nitrite reductase activity under denaturing conditions. No molybdenum was detected in the complex by plasma-emission mass spectrometry. Received: 26 March 1999 / Accepted: 16 August 1999     

Toxicity of pentachlorophenol for micromycetes isolated from freshwater sediments

In order to evaluate the impact of xenobiotics on fungal communities of sediments, lentic ecosystems have been reconstituted in the laboratory and examined before as well as after contamination.Microcosms (2 litres) have been treated with three different concentrations of pentachlorophenol (PCP) (3, 100, and 1000 mg l^-1) at 22 degC. The fungal strains present in the sediments were first isolated and identified before contamination with PCP. Most of the strains belonged to Deuteromycetes (mainly Mucedinaceæ and Dematiaceæ) with a predominance of the genus Penicillium, and Ascomycetes were found in exceptional abundance. New isolations and identifications were made on days 1, 2, 4, 8, and 15 after contamination with PCP. The sensitivity of the various species were determined as a function of the level of PCP and the duration of contamination. Ascomycetes was particularly resistant to high concentrations of PCP. The toxicity of the xenobiotic towards strains belonging to various taxonomic groups is discussed, as well as the possibility that some of them may be useful for biodegradation and/or bioremediation processes, or as bioindicators of polluted sites.Groupe pour l'Étude du Devenir des Xénobiotiques dans l'Environnement (GEDEXE)Laboratoire de Botanique, Cryptogamie, Biologie Cellulaire et Génétique.     

Isolation of anaerobic oxalate-degrading bacteria from freshwater lake sediments

Enrichment cultures that anaerobically degraded oxalate were obtained from lake sediment inocula. From these, 5 pure cultures of anaerobic oxalate-degrading bacteria were isolated and partially characterized. The isolates were Gram-negative, non-sporeforming, non-motile, obligate anaerobes. Oxalate was required for growth and was stoichiometrically converted to formate; ¹⁴CO₂ was also recovered when ¹⁴C-oxalate was added. Maximal growth occurred when the oxalate concentration was 50 mM. Acetate stimulated growth in the presence of oxalate, however, ¹⁴C-experiments indicated that acetate was only utilized for cell carbon.The isolates were either spiral-shaped or rod-shaped organisms. The first morphotype grew much more slowly than the second and exhibited 13-fold lower cell yields. These isolates represent a new strain of oxalate-degrading bacteria. The second morphotype was similar to the anaerobic oxalate-degrading bacteria previously found in rumen. This report extends the known habitats in which anaerobic oxalate-degrading organisms have been found to include aquatic sediments.     

Rheinheimera aquatica sp. nov., an antimicrobial activity producing bacterium isolated from freshwater culture pond

A bacterial strain designated GR5(T) was isolated from freshwater culture pond in Taiwan during the screening of bacteria for antimicrobial compounds and characterized using the polyphasic taxonomic approach. Strain GR5(T) was Gram-negative, aerobic, greenish-yellow colored, rod-shaped, and motile by means of a polar single flagellum. Growth occurred at 10-40 degrees C (optimum, 35 degrees C), at pH 7.0-8.0 (optimum pH 8.0) and with 0-2.0% NaCl (optimum, 0.5-1.0%). The major fatty acids were C(16:1) omega7c (36.3%), C(16:0) (16.6%), C(12:0) 3-OH (12.5%) and C(18:1) omega7c (9.1%). The major respiratory quinone was Q-8. The DNA G+C content of the genomic DNA was 51.9 mol%. Phylogenetic analyses based on 16S rRNA gene sequences showed that strain GR5(T) belonged to the genus Rheinheimera and its most closely neighbours were Rheinheimera texasensis A62-14B(T) and Rheinheimera tangshanensis JA3-B52(T) with sequence similarities of 98.1 and 97.5%, respectively. The sequence similarities to any other recognized species within Gammaproteobacteria were less than 96.5%. The mean level of DNA-DNA relatedness between strain GR5(T) and R. texasensis A62-14B(T), the strain most closely related to the isolate, was 26.5 +/- 7.6 %. On the basis of the phylogenetic and phenotypic data, strain GR5(T) should be classified as representing a novel species, for which the name Rheinheimera aquatica sp. nov. is proposed. The type strain is GR5(T) (= BCRC 80081(T) = LMG 25379(T)).     

Isolation of magnetotactic bacterium WM-1 from freshwater sediment and phylogenetic characterization

The magnetotactic bacterium was isolated from freshwater sediment from North Lake of Wuhan. The isolate, designated WM-1, was Gram-negative, helical shaped, and studied by means of electron microscopy. The strain WM-1 was 0.2-0.4 μm in diameter and 3–4 μm in length. The DNA G + C content was found to be 65.7 mol%. Phylogenetic analysis of the 16S rDNA gene (Accession number DQ899734 in GeneBank) revealed that this isolate was a member ofαsubdivision of the Proteobacteria. Strain WM-1 was closely related (97.7%) to Magnetospirillum sp. AMB-1. Randomly amplified polymorphic DNA analysis showed that these two strains were in fact different strains. Electron diffraction patterns of WM-1 magnetosomes indicated that the magnetosomes were composed of magnetite. The magnetosomes from WM-1 were cuboidal in shape as observed by electron microscopy. Statistical analysis of magnetite crystals from WM-1 showed narrow asymmetric size distribution. The average number of magnetosomes in each WM-1 bacterium was 8 ± 3.4. The average length of magnetosomes in WM-1 was 54 ± 12.3 nm and the average width is 43 ± 10.9 nm. These data showed that the grains in WM-1 were single-domain crystals.     

汞甲基化细菌研究进展

汞甲基化细菌在厌氧条件下将无机汞(Hg)转化成最高毒性的甲基汞(MeHg),通过生物富集以及在食物链中的生物放大造成人类甲基汞暴露.本文综述了水环境中汞甲基化细菌的种类、系统发生、甲基化机理、甲基汞生成的空间位置和影响因素.水环境中汞甲基化主要发生在海洋、海湾、河流和湖泊的厌氧沉积物中.硫酸盐还原菌和铁还原菌是主要的汞甲基化细菌,它们的种类、群落结构和分布制约了甲基汞的生成,从而影响人体健康.汞甲基化的生化机理的研究表明,甲基汞可能产生于不同的代谢途径,但是对于汞甲基化机理仍没有一致的认识.沉积物中汞甲基化细菌的分布影响甲基汞生成的空间位置和甲基化率.因此,水环境中的地球化学因素影响甲基化细菌的分布、甲基化率和甲基汞的生成.     

Isolation and characterization of Chlorella viruses from freshwater sources in Korea

We isolated 23 Chlorella viruses from 9 Korean cities. The viruses were initially amplified in the Chlorella strain NC64A. Pure isolates were obtained by repeated plaque isolations. A SDS-PAGE analysis revealed similar but distinct protein patterns, both among the group of purified viruses and in comparison with the prototype Chlorella virus PBCV-1. Digestions of the 330- to 350-kb genomic DNAs with 10 restriction enzymes revealed different restriction fragment patterns among the isolates. One isolate, SS-1, was resistant to digestion with HindIII, PvuII, AluI, and HaeIII, indicating methylation at the AGCT or GC sequences. Some isolates reacted with antiserum against PBCV-1. The others that did not react to this PBCV-1 antibody reacted to the antibody that was raised against purified HS-2 virion. The tRNA-coding regions of 8 Chlorella viruses were cloned and sequenced. These viruses contained 14-16 tRNA genes within a 1.2- to 2-kb region, except for the SS-1 isolate, which had a 1039-bp spacer in a cluster of 11 tRNA genes. The SS-1 spacer contained an open-reading frame (ORF) of 294 amino acids. This ORF had a 51% amino acid sequence similarity to the PBCV-1 ORF A478L. A Southern blot analysis suggested that it was a novel gene that lacked a homologue in PBCV-1.     

Effects of acidification on mercury methylation, demethylation, and volatilization in sediments from an acid-susceptible lake

The effect of experimental acidification on mercury methylation, demethylation, and volatilization was examined in surficial sediment samples from a weakly buffered northern Wisconsin lake. All mercury transformations were measured with radioisotopic tracers. Acidification of sediment pH with H2SO4, HCl, or HNO3 significantly decreased 203Hg(II) methylation. Acidification of pH 6.1 (ambient) sediments to pH 4.5 with either H2SO4 or HCl inhibited methylation by over 65%. The decreased methylation was due to the increased hydrogen ion concentration because methylation was not affected by concentrations of Na2SO4 or NaCl equimolar to the amount of acid added. Inhibition of methylation was observed even after prolonged acidification of sediments to pH 5.0 for up to 74 days. Acidification of sediments to pH 5.5, 4.5, and 3.5 with HNO3 resulted in a near complete inhibition of methylation at each pH. Similarly, the addition of equimolar amounts of NaNO3 resulted in a near complete inhibition of methylation, indicating that the inhibition was due to the nitrate ion rather than to the acidity. Demethylation of methyl mercury was not affected by pHs between 8.0 and 4.4, but sharply decreased below pH 4.4. Volatilization of 203Hg(II) from surface sediments was less than 2% of methylation activity and was not significantly different from that in killed sediments. This study indicated that acidification of sediments inhibits mercury methylation and that the observed increase in the mercury burdens in fish from low pH lakes is not due to increased production of methylmercury in sediments.     

Spirosoma xylofaga sp. nov., an oligotrophic pleomorphic bacterium from a myco-bacterial community of freshwater ecosystems

A novel aerobic bacterial strain Z-0088 was assigned to the genus Spirosoma on the basis of 16S rRNA analysis; it was isolated from a bacterial community of moderately acidic (pH 5.0) dystrophic, slightly humified water formed by xylotrophic fungi grown on decaying spruce wood. The cells are nonmotile, gramnegative, straight or curved rods, 0.5–1.5 × 1.0–6.0 μm; they may also be toroidal. The cells are usually single but can form spiral filaments containing from 4 to 13 coils; their reproduction is by division. Strain Z-0088 is an organoheterotroph utilizing xylan, inulin, xylose, sucrose, and N-acetylglucosamine as organic growth substrates. The bacterium is oligotrophic (the optimum substrate concentration is 0.5 g/L). It is characterized by high sensitivity to NaCl concentration; growth was completely suppressed at 1% NaCl. The strain grows in a pH range of 3.8–7.5 with the optimum at pH 5.5–6.5. The temperature range for growth was 13–35°C with the optimum at 28°C. The DNA G+C base content was 50.2 mol %. The ecophysiological features of strain Z-0088, such as oligotrophic, mesophilic, moderate acidophilic properties, and sensitivity to NaCl, support its designation as a representative of ombrophilic dissipotrophs. The strain is assigned to a novel species Spirosoma xylofaga sp. nov.