Ralph R. Turner

Application of a mer-lux biosensor for estimating bioavailable mercury in soil

Abstract A previously described bioassay using a mer-lux gene fusion for detection of bioavailable mercury was applied for the estimation of the bioavailable fraction of mercury in soil. The bioavailable fraction is defined here as being part of the water leachable fraction. Due to masking of light emission of soil particles leachates had to be cleaned prior to assays. Filtration of leachates through nitro-cellulose filters using pressure resulted in an underestimation of bioavailable mercury. Gravity filtration and centrifugation showed elevated (as compared with untreated leachate) and very similar responses. The utility of the mer-lux biosensor assay was tested by relating measurements of bioavailable and total mercury to the response of the soil microbial community to mercury exposure. Two different soil types (an agricultural and a beech forest soil) were spiked with 2.5 μg Hg(II) g−1 in microcosms and the frequency of mercury resistant heterotrophs and changes in community diversity, defined as the number of different 16S rDNA bands observed in DGGE gels, were monitored. In the agricultural soil the initial concentration of bioavailable mercury was estimated to be 40 ng g−1. This concentration did not change during the first 3 d and coincided with increased degrees of resistance and a decrease in diversity. The concentration of bioavailable mercury decreased subsequently rapidly and remained just above the detection level (0.2 ng g−1) for the remainder of the experiment. As a possible consequence of the decreased selection pressure of mercury, the resistance and diversity gradually returned to pre-exposure amounts. In the beech forest soil the concentration of bioavailable mercury was found to be about 20 ng g−1 throughout the experiment. This concentration did not at any time result in changes in resistance or diversity. This study showed that the bioassay using the mer-lux biosensor is a useful and sensitive tool for estimation of bioavailable mercury in soil.

Contribution of coexisting sulfate and iron reducing bacteria to methylmercury production in freshwater river sediments

We investigated microbial methylmercury (CH(3)Hg) production in sediments from the South River (SR), VA, an ecosystem contaminated with industrial mercury (Hg). Potential Hg methylation rates in samples collected at nine sites were low in late spring and significantly higher in late summer. Demethylation of (14)CH(3)Hg was dominated by (14)CH(4) production in spring, but switched to producing mostly (14)CO(2) in the summer. Fine-grained sediments originating from the erosion of river banks had the highest CH(3)Hg concentrations and were potential hot spots for both methylation and demethylation activities. Sequencing of 16S rRNA genes of cDNA recovered from sediment RNA extracts indicated that at least three groups of sulfate-reducing bacteria (SRB) and one group of iron-reducing bacteria (IRB), potential Hg methylators, were active in SR sediments. SRB were confirmed as a methylating guild by amendment experiments showing significant sulfate stimulation and molybdate inhibition of methylation in SR sediments. The addition of low levels of amorphous iron(III) oxyhydroxide significantly stimulated methylation rates, suggesting a role for IRB in CH(3)Hg synthesis. Overall, our studies suggest that coexisting SRB and IRB populations in river sediments contribute to Hg methylation, possibly by temporally and spatially separated processes.

The relationships of Hg(II) volatilization from a freshwater pond to the abundance ofmer genes in the gene pool of the indigenous microbial community.

The role of biological activities in the reduction and volatilization of Hg(II) from a polluted pond was investigated. Elemental mercury was evolved from pond water immediately following spiking with203Hg(NO3)2, whereas an acclimation period of 36 hours was required in control samples collected from a nearby, unpolluted river before onset of volatilization. Genes encoding the bacterial mercuric reductase enzyme (mer genes) were abundant in DNA fractions extracted from biomass of the pond microbial community, but not in samples extracted from control communities. Thus, evolution of Hg0 was probably due to activities mediated by the bacterial mercuric reductase. Of four characterizedmer operons, the system encoded by transposon 501 (mer(Tn501)) dominated and likely contributed to the majority of the observed Hg(II) volatilization. Thus,mer-mediated reduction and volatilization could be used to reduce Hg(II) concentrations in polluted waters, in turn decreasing rates of methylmercury formation by limiting substrate availability.

Atmospheric Deposition of Metals to Forest Vegetation

Atmospheric deposition during the growing season contributes one-third or more of the estimated total flux of lead, zinc, and cadium from the forest canopy to soils beneath an oak stand in the Tennessee Valley but less than 10 percent of the flux of manganese. The ratio of the wet to dry deposition flux to the vegetation during this period ranges from 0.1 for manganese to 0.8 for lead to ∼ 3 to 4 for cadmium and zinc. Interactions between metal particles deposited on dry leaf surfaces and subsequent acid precipitation can result in metal concentrations on leaves that are considerably higher than those in rain alone.

Oxidation state of arsenic in coal ash leachate.

D.; Prentice, B. A. Enuiron. Sci. Technol. 1979,13,455. ( 7 ) Fisher, G. L.; Prentice, B. A.; Silberman, D.; Ondov, J. M.; Ragaini, R. C.; Biermann, A. H.; McFarland, A. R.; Pawley, J. B. “SizeDependence of the Physical and Chemical Properties of Coal Fly Ash”. In Proceedings of the Division of Fuel Chemisty Symposium on Properties of Coal Ash, 2nd Joint Chemical Institute of CanadalAmerican Chemical Society Meeting, Montreal, Canada, May 1977 (reprints available from G. L. Fisher). (8) Silberman, D.; Fisher, G. L. Anal. Chim. Acta 1979,106,299. (9) McFarland, A. R.; Bertch, R. W.; Fisher, G. L.; Prentice, B. A. Enuiron. Sci. Technol. 1977,11,781. (10) Fisher, G. L.; Prentice, B. A.; Silberman, D.; Ondov, J. M.; Bierman, A. H.; Ragaini, R. C.; McFarland, A. R. Enuiron. Sci. Technol. 1978,12,447. (11) Hansen, L. D.; Fisher, G. L. Enuiron. Sci. Technol. 1978,14, 1111. (12) Hulett, L. D.; Weinberger, A. J.; Ferguson, N. M.; Northcutt, K. J.; Lyon, W. S. “Chemical Speciation Studies of Fly Ash”; Final Report to the Electric Power Research Institute, July 1979, Part 1, Section 1.4.1. (13) Wilson, A. D. Analyst (London) 1960,85,823-7. (14) Weast, R. C., Ed. “Handbook of Chemistry and Physics”, 55th

Formation of freshwater Fe-Mn coatings on gravel and the behavior of 60Co, 90Sr, and 137Cs in a small watershed

Iron and manganese oxide coatings are actively forming on stream substrates in the White Oak Creek watershed in East Tennessee. Although oxidizing conditions are required for Fe-Mn oxide precipitation, coatings accumulate only if dissolved iron and manganese exceed 50 μg/L. Below this, coatings are lost by abrasion as fast as or faster than they form. Annual rates of formation of 3 mg/g on substrate (gravel) were observed. Manganese is dissolved from coatings between Eh values of 100 to 300 mV and below 1 mg/L dissolved O2 at pH 6.5 to 7.5. Iron oxides can be precipitated under these conditions. Uncontaminated gravels with oxide coatings (composed of illite, quartz, and feldspar) adsorbed radionuclides rapidly from waters below MPC (Maximum Permissible Concentration) levels. Contaminated gravels placed in uncontaminated waters lost 60Co by abrasion in oxidizing conditions and by dissolution of manganese coatings in reducing conditions. Exchangeable 90Sr was completely lost after one month whereas nonexchangeable 90Sr was lost more slowly; 137Cs was totally retained by the gravels. Gravels such as these can be used to monitor the radionuclide content of waters in the environment.

Some observations of organic constituents in rain above and below a forest canopy.

Some organic components of rain have been determined in samples collected on an event basis above and below the canopy of a deciduous forest. Plasticizers and chlorohydrocarbons that were identified were attributed to sources external to the vegetation. Organic acids and high molecular weight waxy components identified were attributed to the foliage itself. Approximately equal amounts of material were ascribed to each of these sources. Low molecular weight organic acids were not found generally. Event sampling is advocated as the best approach to use for identifying sources of organic components in rain reaching the forest floor.

Form of mercury in stream fish exposed to high concentrations of dissolved inorganic mercury

The form of mercury predominating in mercury-contaminated fish from both pristine and industrialized waters in North America and Europe has almost universally been methylmercury. Sunfish (Lepomis auritus) living in a stream contaminated with 0.5-1 micrograms/L dissolved inorganic mercury accumulated greater concentrations of total mercury at headwater sites, where the dissolved mercury concentrations were greatest, than they did at downstream sites. However, despite evidence from laboratory studies that dissolved inorganic mercury is rapidly accumulated by fish without transformation to methylmercury, methylmercury constituted 85% or more of the total mercury concentration in fish at all sites.

Mercury biotransformations and their potential for remediation of mercury contamination

Bacterially mediated ionic mercury reduction to volatile Hg0 was shown to play an important role in the geochemical cycling of mercury in a contaminated freshwater pond. This process, and the degradation of methylmercury, could be stimulated to reduce the concentration of methylmercury that is available for accumulation by biota. A study testing the utility of this approach is described.

Bioaccessibility of Mercury in Soils

The initial risk assessment for the East Fork Poplar Creek (EFPC) floodplain in Oak Ridge, Tennessee, a superfund site heavily contaminated with mercury, was based on a reference dose for mercuric chloride. Mercuric chloride, however, is a soluble mercury compound not expected to be present in the floodplain, which is frequently saturated with water. Previous investigations had suggested mercury in the EFPC floodplain was less soluble and therefore potentially less bioavailable than mercuric chloride, possibly making the results of the risk assessment unduly conservative. A bioaccessibility study, designed to measure the amount of mercury available for absorption in a childs digestive tract (the most critical risk pathway endpoint), was performed on 20 soils from the EFPC floodplain. The average bioac-cessible mercury for the 20 soils was 5.3%, compared with 100% of the mercuric chloride subjected to the same conditions. The alteration of the procedure to more closely mimic conditions in the digestive tract did not significantly change the results. Therefore, the use of a reference dose for mercuric chloride at EFPC, and potentially at other mercury-contaminated sites, without incorporating a corresponding bioavailability adjustment factor may overestimate the risk posed by the site.