EFFECTS OF CHEMICAL SPECIATION IN GROWTH MEDIA ON THE TOXICITY OF MERCURY(II)

The toxicity of metals, including mercury, is expressed differently in different media, and the addition of soluble organics to the growth medium can have a significant impact on bioassay results. Although the effect of medium composition on metal toxicity is generally attributed to its effect on metal speciation (i.e., the chemical form in which the metal occurs), the importance of individual metal-ligand species remains largely unclear. Here, we report the results of a study that investigated, both experimentally and from a modeling perspective, the effects of complex soluble organic supplements on the acute toxicity (i.e., 50% inhibitory concentration [IC50]) of mercury to a Pseudomonas fluorescens isolate in chemically well-defined synthetic growth media (M-IIX). The media consisted of a basal inorganic salts medium supplemented with glycerol (0.1%, vol/vol) and a variety of common protein hydrolysates (0. 1%, vol/vol), i.e., Difco beef extract (X = B), Casamino Acids (X = C), peptone (X = P), soytone (X = S), tryptone (X = T), and yeast extract (X = Y). These were analyzed to obtain cation, anion, and amino acid profiles and the results were used to compute the aqueous speciation of Hg(II) in the media. Respirometric bioassays were performed and IC50s were calculated. Medium components varied significantly in their effects on the acute toxicity of Hg(II) to the P. fluorescens isolate. IC50s ranged from 1.48 to 14.54 mug of Hg ml-2, and the acute toxicity of Hg(II) in the different media decreased in the order M-IIC much-greater-than M-IIP > M-IIB much-greater-than M-IIT > M-IIS >>> M-IIY. The calculated IC50s were related to the aqueous speciation of Hg(II), and a significant negative correlation (r = -0.904) (P = 0.05) between the IC50 and the mole fraction of Hg(II) bound in Hg-chloride complexes was observed. This was particularly noteworthy because the only source of chloride in the M-IIX media was the organic supplements. There also was a significant positive correlation (r = 0.831) (P = 0.05) between the IC50 and the mole fraction of free Ca2+ plus Mg2+ in the media, suggesting that these ions have a moderating effect on the toxicity of Hg(II). Overall, differences in the mole fractions of Hg-chloride complexes (primarily HgCl+, HgCl20 and HgClOH0) and free Ca2+ plus Mg2+ accounted for about 96% of the variation in the acute toxicity of mercury (i.e., R2 = 0.955) (P = 0.01).