Amending cultures of selenium-resistant bacteria with dimethyl selenone

A possible biological intermediate in the reduction and methylation of selenium oxyanions, dimethyl selenone, was synthesized, and the first experiments involving the amendment of selenium resistant bacterial cultures with this compound are reported. The amount of volatile, reduced selenium-containing species released from these cultures into the headspace is significantly more than that produced in analogous experiments involving sodium selenate amended cultures. Dimethyl selenone is reduced in the presence of dimethyl sulfide and dimethyl disulfide in a complex growth medium, trypticase soy broth with 0.1% nitrate. This reduction occurs whether or not the reduced sulfur compounds are biologically produced.     

Confusion between dimethyl selenenyl sulfide and dimethyl selenone released by bacteria

Dimethyl selenone [(CH₃)₂SeO₂] has been reported in the literature as a metabolite released by bacteria in contact with selenium metal or selenium salts. In this study, mass spectral, chromatographic, and boiling-point data are presented that show that dimethyl selenone has been confused with dimethyl selenenyl sulfide (CH₃SeSCH₃). In addition, the headspaces above monocultures of selenium-resistant bacteria were examined using gas chromatography followed by fluorine-induced chemiluminescence detection. A number of alkyl sulfur and selenium species were detected, along with dimethyl selenenyl sulfide. A pathway from oxidized selenium salts to reduced methylated selenides and dimethyl selenenyl sulfide is also presented.     

Confirmation of the aerobic production of trimethylstibine by Scopulariopsis brevicaulis

The filamentous fungus Scopulariopsis brevicaulis produces volatile trimethylstibine, found in the culture headspace, when grown in an antimony(III)‐rich medium under aerobic conditions. The trimethylstibine was purged from cultures using a continuous flow of compressed air and trapped in a U‐shaped tube containing Supelcoport SP 2100 at −78 °C. The trap contents were determined by using GC–ICP–MS methodology. Typically between 60 and 500 pg of trimethylstibine was trapped during sampling (12 h) from cultures containing 1000 g Sb ml⁻¹ as potassium antimony tartrate. The total production of trimethylstibine over 18 days of growth was estimated at 10 ng. Trimethylarsine was produced in greater quantities than trimethylstibine, even though no arsenic compounds were added to the medium. Copyright © 1999 John Wiley & Sons, Ltd.     

Speciation of volatile antimony compounds in culture headspace gases of Cryptococcus humicolus using solid phase microextraction and gas chromatography–mass spectrometry

Direct analysis of the volatile antimony compounds stibine (SbH₃), monomethylantimony, dimethylantimony (Me₂Sb) and trimethylantimony (Me₃Sb) using solid phase microextraction (SPME) with polydimethylsiloxane fibres and gas chromatography–mass spectrometry (GC–MS) is described. The best analyte to background signal ratio was achieved using a 20 min extraction time. Antimony species were separated using a 3% phenylmethylsilicone capillary column operated at a column pressure of 70 kPa, a flow rate of 1.4 ml min^?1 and temperature ramping from 30 to 36 °C at 0.1 °C min^?1. Cryogenic focusing of desorbed species was required to achieve resolution of antimony species. The optimized SPME–GC–MS method was applied to the analysis of headspace gases from cultures of Cryptococcus humicolus incubated with inorganic antimony(III) and (V) substrates. The headspace gases from biphasic (aerobic–anaerobic) biomass‐concentrated culture incubations revealed the presence of SbH₃, Me₂Sb and Me₃Sb. Stibine was the major antimony species detected in cultures amended with inorganic antimony(V). Me₃Sb was the sole volatile antimony species detected when cultures were amended with antimony(III). Copyright © 2002 John Wiley & Sons, Ltd.     

Transformations of mercury species in the presence of Elbe river bacteria

The influence of Elbe river bacteria isolated from suspended particulate matter (SPM) on dynamic species transformation of mercury was investigated. Experiments were carried out in the presence of bacteria (batch cultures) and in sterile tapwater as a control. For the methylation of inorganic mercury ions by bacteria several cofactors are under discussion. In this work, methylcobalamin, methyl iodide and S-adenosylmethionine were tested as biogenic methyl donors and trimethyl-lead chloride, trimethyltin chloride and dimethylarsenic acid as abiotic methyl donors. Transmethylation reactions as examples of abiotic methyl transfers have higher effectiveness in the formation of methylmercury (CH₃Hg⁺) than methylation with biogenic compounds. This result was observed in batch cultures as well as in sterile water. SPM-bacteria inhibit methyl transfer to mercury(II) ions. This is not only due to passive adsorption processes of mercury(II) to bacterial cell walls; methylmercury is also decomposed very rapidly by SPM-bacteria and is immobilized as mercury(II) by the cells.     

Biotransformation of butyltin compounds using pure strains of microorganisms

The debutylation of tributyltin chloride by several strains of fungi, yeasts and bacteria is described. Under standard conditions and with low initial concentration of substrate, significant biotic degradation of tributyltin (6–32%) was detected after five days at 28°C. Dibutyltin and monobutyltin were formed in all cases, with higher yields of the latter. Two microorganisms catalysed the transformation of monobutyltin to dimethyltin and trimethytin whereas all microorganisms were able to methylate inorganic tin(IV) to trimethyltin. Our results suggest that tributyltin biodegradation by microorganisms is generally possible, provided sufficiently low concentrations of substrate are used.     

Confirmation of the Biomethylation of Antimony Compounds

We have evidence that an organic and an inorganic salt of antimony were reduced and methylated biologically by microorganisms in laboratory experiments. The organoantimony compound produced was trimethylstibine [(CH₃)₃Sb] and was detected in a culture headspace. This was confirmed by matching the compound's retention time in capillary gas chromatography, as detected by fluorine-induced chemiluminescence, with a com- mercial standard and by its mass spectrum determined with gas chromatography/ mass spectrometry (GC–MS). (CH₃)₃Sb was detected in the headspace of soil samples amended with either potassium antimonyltartrate or potassium hexahydroxyantimonate and augmented with any one of three different nitrate-containing growth media. The identity of the microorganisms in soil that accomplished this are as yet unknown. Of 48 soil samples amended with these two compounds, 24 produced trimethylstibine. Bioreduction of trimethyldibromoantimony was also detected in a liquid monoculture of Pseudomonas fluorescens K27 which also produced tri- methylstibine. This headspace production of (CH₃)₃Sb was determined to be linked to the culture's cell population as measured by optical density. This microbe, however, did not biomethylate either potassium antimonyltartrate or potassium hexahydroxyantimonate in any experiments we performed. © 1997 John Wiley & Sons, Ltd.     

The methylation of arsenic in marine sediments

Laboratory studies have shown that microorganisms present in both natural marine sediments and sediments contaminated with mine-tailings are capable of methylating arsenic under aerobic and anaerobic conditions. Incubation of sediments with culture media produced volatile arsines [including AsH₃, (CH₃)AsH₂, and (CH₃)₃As] as well as the methylarsenic(V) compounds (CH₃)_nAs(O) (OH)_3?n (n = 1, 2, 3). The concentration of the arsines increased and then decreased in a growth and decay pattern reminiscent of the methylation and demethylation of mercury. Thus, arsenic speciation varied with time, being controlled by the biochemical activity of the dominant microbe(s) at the time of sampling, and changing in response to the ecological succession within the microbial community. The analysis of the interstitial waters of sediments collected from several British Columbia (Canada) coastal sites gave results that were consistent with the culture experiments, in that the methylarsenicals were ubiquitous, but present only in small amounts. It is estimated that methylarsenic(V) species account for less than 1% of the arsenic present in porewaters. The actual proportion was dependent on a number of factors but, contrary to prevailing viewpoints, there was no relationship to the organic content of the sediments, nor did methylation occur only in the presence of high arsenic concentrations. Instead, all of the evidence was consistent with in situ microbial methylation and demethylation processes that are similar to the arsenic transformations that occur in soil ecosystems. The results are discussed in terms of the cycling of arsenic in the marine environment and within the marine food web.     

Methylation of inorganic arsenic by arsenic-tolerant freshwater algae

Five arsenic-resistant freshwater algae which had been isolated from an arsenic-polluted environment were studied for the biotransformation of arsenic compounds accumulated by them from the aqueous phase. The algal cells bioaccumulating arsenic were digested by 2 mol dm^?3 NaOH at 95°C, the As? C bonds except for As? CH₃ were cleaved by the treatment and the methylated arsenic compounds were reduced to the corresponding arsines by sodium borohydride (hydride generation). The arsines were chromatographically separated on the basis of their boiling-point difference and determined by atomic absorption spectrophotometry. Methylated arsenic compounds were found in all algal cells. The predominant arsenic species in the cells, however, were non-methylated arsenic compounds which were mainly present in the residue of a chloroform–methanol extract. The non-methylated arsenic compounds were found to be not present in the free inorganic arsenic substrate and to be bound strongly with proteins or polysaccharides in the cells. Methylated arsenic compounds were found mainly in the lipid-soluble fractions and the major form was a dimethylarsenic compound. Trimethyl- and monomethyl-arsenic compounds were detected but at very low level. The dimethylarsinic acid was not present in the free form in the lipid-soluble fraction and should be bound with a lipid molecule. It was also found that the accumulation of arsenic by Nostoc occurred only in living cells.     

Biomethylation of arsenic and its excretion by the alga Chlorella vulgaris

Experimental results in this paper lead to the following conclusions. (1) Cell homogenates of Chlorella vulgaris adsorbed the inorganic arsenic compound Na₂HAsO₄ but no methylation of the arsenic occurred in vitro. (2) A small part of the arsenic bioaccumulated by C. vulgaris was methylated in vivo. The quantity of arsenic methylated in the cell increased with an increase of arsenic concentration in the medium. (3) When the arsenic-accumulating cells were transferred into arsenic-free media, the arsenic was excreted and the relative quantity of the methylated arsenic in the excrement was larger than that in the cell. (4) In the growth phase of C. vulgaris, a small fraction of the arsenic accumulated in the cell was first transformed to monomethyl and dimethyl-arsenic compounds during the early exponential phase, and after a short time a fraction was transformed to trimethylarsenic species.