Speciation of aqueous methylmercury influences uptake by a freshwater alga (Selenastrum capricornutum)

Abstract

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.

The Great Salt Lake's monimolimnion and its importance for mercury bioaccumulation in brine shrimp (Artemia franciscana)

“…Both chloride (85 g L 21 ) and DOC (42 mg L 21 ) are very high in the mixed layer, and both of these influence Hg speciation and uptake, but not always in predictable ways (Aiken et al 2003;Pickhardt and Fisher 2007;Luengen et al 2012). Although DOC can help maintain Hg in solution, its reactivity and concentration influence biotic uptake of MeHg (Gorski et al 2006). Consequently, more work will need to be done to understand whether the MeHg accumulated by Artemia is from particles delivered from the deep brine layer, or by reactions in the mixed layer that transform the dissolved advected Hg into particles or molecules that can be taken up by these organisms.…”

Bioaccumulation and Biomagnification of Mercury through Food Webs

“…Intensive fishing has been suggested as a way of decreasing Hg bioaccumulation in aquatic food webs (Schindler et al, 1995), since removal of fish has been shown to diminish competition and increase food availability, which in turn has led to higher fish growth rates and greater dilution of Hg (Verta, 1990;Therien et al, 2003;Mailman et al, 2006;Lepak et al, 2009). Similarly, rapid growth of algal colonies and greater densities of algal cells have been associated with dilution of their Hg concentrations (Pickhardt et al, 2002;Hill and Larsen, 2005;Gorski et al, 2006). Conversely, in areas of low primary productivity and poor nutrient quality, slow rates of growth are correlated with higher concentrations of Hg in algae, which are transferred to higher trophic level organisms such as zooplankton (Pickhardt et al, 2005;Karimi et al, 2007).…”

Mercury and methylmercury incidence and bioaccumulation in plankton from the central Pacific Ocean

“…The resultant formation of natural complexes influences uptake of many metals by biota. Prior research suggests that phytoplankton MeHg uptake occurs primarily via passive diffusion of neutral complexes across a cell membrane (Fisher and Reinfelder, 1995;Gorski et al, 2006;Lawson and Mason, 1998;Mason et al, 1996;Pickhardt and Fisher, 2007). Passive uptake of both species could also result after sorption of Hg and MeHg to cell surfaces (Fisher, 1985).…”