The Chemical Cycle and Bioaccumulation of Mercury paper by: Francois Morel, Anne Kraepiel, and Marc Amyot presented by: Roxanne Myshkowec class: CE 468 3/2/00

Introduction b The purpose of the paper was to answer the question: “How do concentrations of parts per trillion of mercury in water yield concentrations of parts per million in fish?” b The answer is reached by examining the chemical and biological mechanisms that control the cycle.

Topics of Discussion b Background information on Mercury b Describe the global mercury cycle b Discuss chemical and biological influences on the cycle b Discuss bioaccumulation of mercury in fish b Conclusions

Background Information b common species : elemental mercury: (Hg o ), ionic mercury: (Hg +2 ) complexes, and organic mercury: (CH 3 Hg + )elemental mercury: (Hg o ), ionic mercury: (Hg +2 ) complexes, and organic mercury: (CH 3 Hg + ) b sources : burning of fossil fuels, industrial, pesticides, volcanoes, amalgams burning of fossil fuels, industrial, pesticides, volcanoes, amalgams b health risks : depends on species: inorganic- kidney, organic- CNS, possible carcinogendepends on species: inorganic- kidney, organic- CNS, possible carcinogen b MCL: 2 micrograms/liter

Global Mercury Cycle b Long range transport as result of residence time b coupled precipitation/volatilization cycle b 2/3 mercury in atmosphere result of anthropogenic sources b inputs tripled in last 150yrs

Depth profiles

Chemical and Biological Influences b OXIC WATERS: pM total Hg b reduction of Hg +2 : photoreduction vs. microbial reduction (dependent upon concentration)photoreduction vs. microbial reduction (dependent upon concentration) b oxidation of Hg o thought to be negligiblethought to be negligible b demethylation: CH 3 Hg + + H + = CH 4 + Hg +2 requires enzymatic or photochemical catalystrequires enzymatic or photochemical catalyst b methylation unknown microbial processunknown microbial process

Chemical and Biological Influences Cont. ANOXIC WATERS: Hg +2 high affinity for sulfide dominates b reduction of Hg +2 : microbial vs. humic substances pathwaysmicrobial vs. humic substances pathways b methylation: photochemical vs. microbial catalyzed pathwaysphotochemical vs. microbial catalyzed pathways

Aquatic Cycle

Bioaccumulation b microbial uptake: transported into cell by mer T transport proteintransported into cell by mer T transport protein transported by diffusion through lipid membranetransported by diffusion through lipid membrane b speciation and K ow effect uptake uncharged species have higher uptake yieldsuncharged species have higher uptake yields b reactivity in cell effects accumulation. Organic mercury better retained since more reactive than Hg +2Organic mercury better retained since more reactive than Hg +2

Bioaccumulation

Conclusions b answer to question- bioaccumulation b bioaccumulation and thus toxicity dependent on speciation b by examining mercury cycle can begin to understand the chemical and biological influences that control speciation b in future better examine water-sediment interface reaction and unknown microbial interactions