Effects of bismuth contamination on the growth and activity of soil microorganisms using thiols as model compounds

The present study was undertaken to obtain information about the effects of Bi contamination on soil microbial growth and activity using a series of Bi complexes with thiol compounds, including mercaptoethanol, thioglycerol, mercaptoethylamine, thioglycolic acid, thiomallic acid, reduced glutathione, 2-mercaptopropionic acid, and L-cysteine. We found that Bi complexes with mercaptoethanol, thioglycerol, and mercaptoethylamine, all of which showed lipophilicity, markedly inhibited bacterial growth in 1/10 TSB liquid media in both Eutric Cambisol (brown forest soil) and Eutric Fluvisol (brown lowland soil), with relative CFU of less than 2% at 50 micro M Bi and 6% at 25 micro M Bi. However, none of the Bi-thiols, including Highly lipophilic complexes, at 200 micro M Bi in rosebengal agar medium inhibited fungal growth, possibly because fungi have a metabolic system that protects against Bi uptake or detoxifies Bi compounds. When soil was contaminated experimentally with Bi-thiol, these complexes suppressed soil dehydrogenase activity, particularly in brown forest soil, which contains large amounts of easily decomposable organic matter. These results indicate that the effects of Bi on soil microbes may depend on mutual reactions with organic matter in the environment.