Mercury Analysis of Acid- and Alkaline-Reduced Biological Samples: Identification of meta -Cinnabar as the Major Biotransformed Compound in Algae

Abstract

The biotransformation of Hg II in pH-controlled and aerated algal cultures was investigated. Previous researchers have observed losses in Hg detection in vitro with the addition of cysteine under acid reduction conditions in the presence of SnCl 2 . They proposed that this was the effect of Hg-thiol complexing. The present study found that cysteine-Hg, protein and nonprotein thiol chelates, and nucleoside chelates of Hg were all fully detectable under acid reduction conditions without previous digestion. Furthermore, organic (R-Hg) mercury compounds could not be detected under either the acid or alkaline reduction conditions, and only ␤-HgS was detected under alkaline and not under acid SnCl 2 reduction conditions. The blue-green alga Limnothrix planctonica biotransformed the bulk of Hg II applied as HgCl 2 into a form with the analytical properties of ␤-HgS. Similar results were obtained for the eukaryotic alga Selenastrum minutum. No evidence for the synthesis of organomercurials such as CH 3 Hg ؉ was obtained from analysis of either airstream or biomass samples under the aerobic conditions of the study. An analytical procedure that involved both acid and alkaline reduction was developed. It provides the first selective method for the determination of ␤-HgS in biological samples. Under aerobic conditions, Hg II is biotransformed mainly into ␤-HgS (meta-cinnabar), and this occurs in both prokaryotic and eukaryotic algae. This has important implications with respect to identification of mercury species and cycling in aquatic habitats.

Biotransformation of Hg(II) by Cyanobacteria

“…Of the volatilized Hg, 98.8% passed through a Carbotrap tube to be trapped by a tube of silvered Chromosorb P. Analysis of cultures using CdCl 2 -catalyzed alkaline reduction gave no evidence of organomercurials in these aerobic cultures, which therefore appear to volatilize Hg only as Hg(0) (24).…”

“…This decrease was not matched by a similar loss in alkaline-reducible Hg. The difference between alkaline (total inorganic)-and acid-reducible Hg (Hg 2ϩ , thiol-complexed Hg 2ϩ , and/or nucleoside-bound Hg 2ϩ ) represents ␤-HgS, or meta-cinnabar (24). The difference between the initial Hg application and alkaline-reducible Hg represents the maximum for volatile Hg (10 ppb Ϯ 4 ppb; triplicate determinations).…”

“…Daniels and Wigfield (10) found that in the presence of a sulfhydryl reagent (L-cysteine), an acid reduction-resistant mercurial compound or complex was fully reduced to Hg(0) only under alkaline conditions. Our previous studies indicated that this was overcome by increasing incubation periods (24). Extraction followed by either thin-layer or column chromatography of a major mercurial component found in our treated cyanobacteria showed that it comigrated with meta-cinnabar (␤-HgS) standards and that it was distinct from all other common mercurials.…”

“…The detection limit was 0.l ng in 1-ml samples. Analytical methods, including quality assurance and quality control, are described in detail by Kelly and colleagues (24). Briefly, 100-l samples were reduced under alkaline conditions in 6 ml of 10% SnCl 2 dissolved in 10% HNO 3 alkalinized to a net equivalent of 10% KOH, giving an estimate for all mercury species in the cyanobacteria and medium.…”

Studies of biouptake and transformation of mercury by a typical unicellular diatom Phaeodactylum tricornutum

“…Surface-bound and intracellular Hg amounts were obtained as follows: the P. tricornutum cells were collected from 25 mL culture solution via centrifugation at 4000×g for 10 min, and the Hg remaining in the supernatant was determined using CVG-AFS. The P. tricornutum cell pellets obtained were washed three times using the natural seawater to remove any substrate and then resuspended for 10 min in 10 mL 0.01 mol L -1 DTE solution to remove the Hg adsorbed on the cell walls [19], the Hg in the supernatant after centrifugation at 5000×g for 15 min and digestion with HNO 3 was finally determined using CVG-AFS to obtain the amount of surface-bound Hg. The P. tricornutum cells obtained after washing by DTE were subjected to HNO 3 digestion and then determined using CVG-AFS to obtain the amount of intracellular Hg.…”