Mercury is one of the most hazardous contaminants that may be present in the aquatic environment, but its ecological and toxicological effects are strongly dependent on the chemical species present. Species distribution and transformation processes in natural aquatic systems are controlled by various physical, chemical, and biological factors. Depending on the prevailing environmental conditions, inorganic mercury species may be converted to many times more toxic methylated forms such as methylmercury, a potent neurotoxin that is readily accumulated by aquatic biota. Despite a considerable amount of literature on the subject, the behavior of mercury and many of the transformation and distribution mechanisms operating in the natural aquatic environment are still poorly understood. This review examines the current state of knowledge on the physicochemical behavior of mercury in the aquatic environment, and in particular the environmental factors influencing its transformation into highly toxic methylated forms.

Mercury in the Aquatic Environment: A Review of Factors Affecting Methylation

Pharmacology of oleanolic acid and ursolic acid

Substrate-electron acceptor combinations and specific metabolic inhibitors were applied to anoxic saltmarsh sediment spiked with mercuric ions (Hg2+) in an effort to identify, by a direct approach, the microorganisms responsible for the synthesis of hazardous monomethylmercury. 2-Bromoethane sulfonate (30 mM), a specific inhibitor of methanogens, increased monomethylmercury synthesis, whereas sodium molybdate (20 mM), a specific inhibitor of sulfate reducers, decreased Hg2+ methylation by more than 95%. Anaerobic enrichment and isolation procedures yielded a Desulfovibrio desulfuricans culture that vigorously methylated Hg2+ in culture solution and also in samples of presterilized sediment. The Hg2+ methylation activity of sulfate reducers is fully expressed only when sulfate is limiting and fermentable organic substrates are available. To date, sulfate reducers have not been suspected of Hg2+ methylation. Identification of these bacteria as the principal methylators of Hg2+ in anoxic sediments raises questions about the environmental relevance of previous pure culture-based methylation work.

Sulfate-Reducing Bacteria: Principal Methylators of Mercury in Anoxic Estuarine Sediment

Organotin compounds are ubiquitous contaminants in the environment. The high biological activity of some compounds toward aquatic organisms lead to deleterious impacts in aquatic ecosystems. Here, the aquatic ecotoxicology of organotins is reviewed based on a multidisciplinary approach involving environmental chemical, toxicological, and ecological aspects. Basic results were obtained both with field and laboratory studies, and some of the most important recent results and conclusions are critically reviewed. The contamination of and fate in aquatic systems is reported and linked with effects at different levels of biological organization. Major emphasis is placed on the development of a concept of ecotoxicology that encompasses not only effect assessment alone, but also integrates environmental chemistry with aquatic toxicology. Thereby, the influence of speciation for bioavailability, basic modes of toxic action, and aquatic toxicity are discussed. This case study on organotins allows to a certain extent generalizations to ecotoxicology in general.

Ecotoxicology of organotin compounds.

Anthocyanins and their role in cancer prevention

Methylation of mercury by humic substances in an aquatic environment

Findings on methylation of tin, mercury, and arsenic in the environment are reviewed. The development of separation and detection systems in trace analysis has made it possible to determine the concentration of these methylated forms in the environment. Abiological methylation mechanisms are similar for these three metals, and methylcobalamin and methyl iodide are thought to be the major methyl donors in the environment. Photochemical reaction and transalkylation produce methylmetals. Humic and fulvic acids are the factors affecting methylation. The research on biological methylation started from incubation with polluted water and sediments, and some exceptional reports cast doubt as to whether it was really biomethylation. Some organisms that can form methyl metals from their metal forms have been separated, and the mechanism has been investigated using the pure culture. Methylation of tin and mercury increases the toxicity of their original metal forms, while methylation of arsenic lowers its t...

Formation, distribution, and ecotoxicity of methylmetals of tin, mercury, and arsenic in the environment

Separation and purification of (Cd, Cu, Zn)-metallothionein in carp hepato-pancreas.

We investigated the inhibitory effect of eggplant (Solanum melongena var. marunasu) extract on human fibrosarcoma HT-1080 cell invasion of reconstituted basement membrane [Matrigel (MG)]. We found that the effective component of the plant extract was delphinidin, a flavonoid pigment contained in the peel. The extract and delphinidin did not affect tumor cell adhesion to MG or haptotactic migration to MG. HT-1080 secretes matrix metalloproteinase(MMP)-2 and MMP-9, which degrade extracellular matrix as part of the invasive process. Delphinidin slightly inhibited the activity of MMPs, which may have been responsible, in part, for the inhibition of tumor cell invasiveness.

Takahiko Sato

Papers

Methylation of mercury by humic substances in an aquatic environment

Formation, distribution, and ecotoxicity of methylmetals of tin, mercury, and arsenic in the environment

Separation and purification of (Cd, Cu, Zn)-metallothionein in carp hepato-pancreas.

Inhibitory effect of delphinidin from Solanum melongena on human fibrosarcoma HT-1080 invasiveness in vitro.

Mercury methylation by compounds in humic material