Speciation of aqueous methylmercury influences uptake by a freshwater alga (Selenastrum capricornutum).
Uptake of methylmercury (MeHg) by the alga Selenastrum capricornutum was measured in freshwater batch culture bioassays. The concentration of MeHg in the alga increased rapidly (within 15 min), reached a maximum by 6 h, and then declined because of growth dilution. The alga's rapid growth rate (doubling time, approximately 10 h) contributed to the importance of growth dilution. Conditional first-order rate constants were calculated for uptake (k1 = 6.95 x 10(-9) L/cell/h) and growth (kG = 0.07/h). A competitive synthetic ligand, disodium ethylenediaminetetra-acetate, formed strong complexes with MeHg and reduced MeHg uptake, consistent with the biotic ligand model. A conditional equilibrium formation constant (K) for the MeHg-algae complex was estimated to be approximately 10(16) and was used to model the influence of natural ligands on MeHg bioavailability. Model results suggested MeHg would be most bioavailable at concentrations of dissolved organic matter (DOM) less than 10 mg/L and increasingly unavailable at higher DOM concentrations for the specific humic acid modeled. Similarly, at molar concentrations of sulfide (and, possibly, metal-sulfide clusters) equal to approximately half the MeHg concentration, MeHg was predicted to be unavailable to algae because of the formation of strong 2:1 MeHg-sulfide complexes.
Gorski
Patrick R
PR
University of Wisconsin-Madison, Environmental Chemistry and Technology Program, Water Science and Engineering Laboratory, 660 North Park Street, Madison, Wisconsin 53706, USA. patrick.gorski@madison.ppdi.com
Armstrong
David E
DE
Hurley
James P
JP
Shafer
Martin M
MM
D001681:Biological Assay; D001682:Biological Availability; D002614:Chelating Agents / Q000737:chemistry; D000460:Chlorophyta / Q000737:chemistry* / Q000254:growth & development; D004492:Edetic Acid / Q000737:chemistry; D007700:Kinetics; D008024:Ligands; D008767:Methylmercury Compounds / Q000737:chemistry* / Q000493:pharmacokinetics*; D008962:Models, Theoretical
D002614:Chelating Agents; D008024:Ligands; D008767:Methylmercury Compounds; D004492:Edetic Acid