6 The Bioaccumulation of Mercury, Methylmercury, and Other Toxic Elements into Pelagic and Benthic Organisms Robert P Mason CONTENTS 6.1 Introduction 6.2 Bioaccumulation in Pelagic Food Webs 6.3 Bioaccumulation in Benthic Organisms 6.4 Membrane Transport Processes 6.5 Summary Acknowledgments References 6.1 INTRODUCTION Many elements are toxic to organisms, but often only in a specific chemical form For example, although inorganic mercury (Hg) is toxic to organisms at low concentrations, it is the organic form of Hg, monomethylmercury (MMHg), that is highly bioaccumulative and accounts for the wildlife and health concerns resulting from the consumption of fish with elevated MMHg burdens.1 For other metals and metalloids, such as cadmium (Cd), lead (Pb), arsenic (As), and selenium (Se), it is also often specific chemical forms, such as the free ion, e.g., Cd2+, or the methylated or reduced species, e.g., mono- and dimethylAs or As(III), that are the most toxic.2 Thus, knowledge of the total concentration (i.e., the sum of all chemical forms) of a potentially toxic element in the environment is insufficient to assess its toxicity accurately Furthermore, it is accepted that contaminants must be in solution to be taken up directly from water3,4 and, as a result, it is the competitive binding of ©2002 CRC Press LLC contaminants to dissolved organic and inorganic ligands, colloids, and to particulate phases that ultimately controls the availability of an element in an aquatic system.4 In sediments, it is also the specific composition of the solid matrix, such as the amount of organic carbon or sulfide (acid volatile sulfide, or AVS; or pyrite), that determines the amount in solution in the sediment pore water,4,5 as well as the bioavailability of the contaminants in the solid phase For example, Lawrence and Mason6 showed that the MMHg bioaccumulation factor (BAF) for amphipods living in sediment was a function of the sediment particulate organic content (POC) Additionally, a number of studies have shown that the particulate–water distribution coefficient (Kd) for Hg and MMHg is a function of POC.5,7,8 Metal concentrations in sediments away from point source inputs are often strongly correlated with sedimentary parameters such as POC, AVS, or Fe,4,5 and, as these parameters are often co-correlated, it is difficult to determine the controlling phase Nonetheless, the binding strength of a metal or metalloid to the sediment influences its bioavailability and bioaccumulation in benthic organisms This has been shown for copper (Cu), as well as other metals and organic contaminants.9,10 Recently, Lawrence et al.11 also showed that the bioavailability of Hg and MMHg to benthic organisms during digestion depended on the organic content of the sediment and further studies have extended these ideas to other metals.12 Understanding the sources, fate, and bioaccumulation of Hg and MMHg in the environment has received heightened attention primarily as a result of human and wildlife concerns resulting from the consumption of fish with elevated Hg.1,13,14 In the United States, the U.S EPA has targeted anthropogenic sources of Hg for regulation1,15 to reduce Hg inputs to the atmosphere It is apparent that future regulatory policies will focus on other metals and metalloids, such as As, Se, and Cd, that are volatilized to the atmosphere during high temperature combustion processes.16 Each element has a particular anthropogenic source inventory, e.g., coal combustion and waste incineration (both medical and municipal) for Hg; coal combustion for Se; waste incineration for Cd; and smelting and other industrial activities for As.17 It has been estimated that the input of metals to the atmosphere as a result of human activities has increased emissions by a factor of for Cd, 1.6 for As, and for Hg Selenium anthropogenic inputs are about 60% of natural inputs.18–20 In addition to anthropogenic inputs to the atmosphere, metal and metalloids are also introduced directly into the aquatic environment as a result of activities such as mining and smelting and other industrial processes These elements are typically retained within watersheds,21,22 and postindustrialization activities have likely resulted in a general increase in their burden in surface soils, lake sediments, and other aquatic systems For example, studies in contaminated environments such as the Clark Fork Superfund Site in Montana23 have documented the bioaccumulation of metals in stream biota and have documented the environmental perturbation resulting from elevated metals in sediments and water The knowledge that chemical speciation controls bioavailability has become the guiding principle for research into the toxicity and bioaccumulation of inorganic contaminants in the environment.4,5,24 However, while recent research has advanced the knowledge of the important differences in toxicity and fate of inorganic species, corresponding environmental regulations, especially for coastal ©2002 CRC Press LLC waters, are typically still based on total dissolved concentrations of the contaminant in water or on total concentrations in sediments To some degree, the lack of change in the regulatory framework is the result of the fact that, while much has been learned about the impact of chemical speciation on bioaccumulation and fate, knowledge is incomplete This chapter provides a review of what is currently known about the factors controlling the bioaccumulation and fate of inorganic Hg and MMHg and other toxic metals and metalloids, such as As, Se, Cd, and Pb, in estuarine and coastal environments 6.2 BIOACCUMULATION IN PELAGIC FOOD WEBS For most trace metals, the largest bioconcentration occurs between water and phytoplankton/microorganisms,13,24–27 and it is uptake at the base of the food chain that likely exerts the primary control on the amount of contaminant reaching higher trophic levels.24,25,28 A comparison between the bioaccumulation of inorganic Hg, MMHg, As, Se, Cd, zinc (Zn), and silver (Ag) (Table 6.1) shows that although all these elements are concentrated in fish above their concentration in water, it is only MMHg that bioaccumulates at each stage of the food chain.25–28 The mechanism of accumulation plays a significant role in determining the magnitude of the accumulated concentration and the fate of the elements during trophic transfer.25,29,30 For many metals, it is thought that the accumulation into phytoplankton and microbes is controlled by the free metal ion concentration in solution,24,31 and a large body of research has documented this for both essential and potentially toxic metals, such as Cu, Zn, Cd, and Fe In most cases, it is the free metal ion that is the form taken up in an active process through specific ion channels in the membrane Metals are either specifically taken up, because they are essential for growth (e.g., Fe, Zn, and Cu), or inadvertently (e.g., Cd, Cu), as the transport sites are not entirely element specific For example, Cd2+ and Pb2+ have been shown to be taken up through the Ca2+ channels in membranes,4,30 whereas As, which exists TABLE 6.1 Representative BAFs for a Variety of Elements for Both Phytoplankton and Piscivorous Fish Element Zn Cd Ag Hg MMHg As Se BAF Algaea BAF Fisha 4.7 3.7 5.0 4.5 5.0 — — 3.2 3.0 2.7 3.8 6.3 3.0 3.8 a Values estimated from a variety of sources including Mason et al.,25 Watras and Bloom,26 and Reinfelder and Fisher.29,30 ©2002 CRC Press LLC as an oxyanion in water, is thought to be an analogue for phosphate, and therefore, is transported into the cell Metals such as Cu are required at low concentration by phytoplankton but can be toxic at high concentration.24 There are a number of excellent reviews of trace metal uptake by microorganisms,4,24,30 and the topic will not be dealt with in detail here All these mechanisms involve energy and are therefore considered active processes.24 For Hg, Mason et al.25,32 demonstrated passive uptake, likely by diffusion through the lipid bilayer, of neutral inorganic complexes of both inorganic Hg and MMHg by the estuarine diatom, Thalassiosira weisflogii In these experiments, uptake was most efficient for the neutral chloride complexes, HgCl2 and CH3HgCl, and it was shown that these complexes have octanol–water partition coefficients (Kow) that were one to two orders of magnitude higher than those of the neutral hydroxide complexes (Hg(OH)2, CH3HgOH), which were not taken up as efficiently (Table 6.2) Overall, at a given chloride concentration, the fraction of Hg or MMHg present as chlorocomplexes decreases with increasing pH Further studies with both diatoms33 and sulfate-reducing bacteria34 have similarly shown that neutral complexes with sulfide — HgS and CH3HgSH — and with organic thiols, such as cysteine, are also efficiently taken up and have Kow values that are higher than those of the chloride complexes (Table 6.2) These results suggest that in the presence of neutral inorganic or simple organic complexes, passive accumulation of Hg and MMHg occurs by partitioning of these complexes into the cell membrane Passive accumulation of neutral inorganic complexes has also been demonstrated for other metals For Ag, it has been shown that the complex AgCl has a higher Kow than the free metal, and that it is taken up by phytoplankton more rapidly TABLE 6.2 Estimated Octanol–Water Partition Coefficients for Neutrally Charged Inorganic and Organic Complexes of Metals Metal Hg MMHg Cd Ag Cu Pb Inorganic Complexes HgCl2 HgOHCl Hg(OH)2 HgS CH3HgCl CH3HgOH CH3HgSH CdCl2 AgCl — — — Kow 3.3 1.2 0.05 26 1.7 0.07 28 0.002 0.09 Organic Complexes Hg(cysteine)2 Hg(thiourea)2 CH3Hg(cysteine) CH3Hg(thiourea) Cd(dithiocarbamate)2 — Cu(oxine)2 Cu(dithiocarbamate)2 Pb(dithiocarbamate)2 Kow 3.7 4.6 50 630 1000 400 630 10,000 Adapted from Mason et al.,25 Lawson and Mason,33 Benoit et al.,34 Phinney and Bruland,38 and Reinfelder and Chang.35 ©2002 CRC Press LLC than the free metal ion.35 The Kow is, however, much lower than that of HgCl2, and is of similar magnitude to that of Hg(OH)2 (Table 6.2) Additionally, uptake rates of AgCl, when normalized to exposure concentration, are similar to those for Hg(OH)2, and both are less than that for HgCl2 The differences in Kow between the complexes are expected based on the relative “ionic” character of the complexes Similarly, CdCl2 has a low Kow compared with HgCl2.35 Uptake of CdCl2 by artificial membranes was examined by Gutknecht,36 who showed that CdCl2 was taken up much more rapidly than Cd2+, but the rate for CdCl2 was many orders of magnitude less than that for HgCl2 under the same conditions,37 in accordance with the measured differences in Kow These results confirm that there is the potential for uptake of neutral inorganic metal complexes by passive diffusion into phytoplankton However, for complexes with substantial “ionic” character, other mechanisms likely dominate as the uptake rates are relatively slow It is only for relatively “covalent” complexes such as HgCl2 and CH3HgCl, and the corresponding sulfide complexes, which have significant Kow values, that passive diffusion rates are significant compared with other accumulation mechanisms Overall, except for Hg and MMHg, the neutral inorganic complexes of metals not appear to be rapidly taken up by passive processes This is not true for neutral organic complexes The accumulation by diatoms of Hg and MMHg as thiol complexes (e.g., with cysteine and thiourea) has been demonstrated.33 These complexes have relatively high Kow values (Table 6.2) Additionally, Phinney and Bruland38 showed that, although Cu, Cd, and Pb were not accumulated as charged complexes of these metals with EDTA, all were taken up if complexed to other organic ligands (e.g., oxine, dithiocarbamate) that formed neutral complexes with substantial Kow values (Table 2) In all cases, initial accumulation rates were much higher for the neutrally complexed metal than they were in the absence of the ligand The studies with Cu-oxine have been repeated with a variety of algae.39 The observed permeability of the complex varied little across species as expected for a passive accumulation process However, the observed permeability of the Cu-oxine complex was similar to that of HgCl2 even though the Kow of the Cu complex is two orders of magnitude higher.38 As demonstrated by others, the size of the molecule is an important consideration as diffusion through the cell membrane likely limits the accumulation rate by passive processes for large compounds, even if they are highly lipophilic.40 Bioaccumulation is further complicated by the presence of dissolved organic carbon (DOC) in most natural systems Dissolved concentrations of Hg and MMHg in natural waters are often positively correlated with DOC, but are negatively correlated with the BAF for phytoplankton, invertebrates, and fish6,41–45 in the same systems In this chapter, the BAF is defined as the concentration of the contaminant in the organism relative to the concentration of the medium in which it resides or upon which it feeds (e.g., water or sediment) These Hg–DOC relationships suggest that organic matter complexation makes Hg and MMHg much less bioavailable, so that positive relationships between lake DOC and MMHg in fish43,44 cannot be solely explained by enhanced uptake of MMHg at the base of the food chain Recent measurements of octanol–water partitioning of Hg in the presence of DOC extracted from the Florida Everglades confirm that the Hg–DOC complexes not partition ©2002 CRC Press LLC into octanol to any significant degree46 and, consequently, will not passively diffuse across the cell membrane However, when complexed to DOC, Hg and MMHg are still taken up by phytoplankton, albeit less efficiently than the neutral complexes.33 Additionally, as discussed below, complexation to organic matter does not hinder accumulation across the gut lining of higher organisms These observations suggest that Hg and MMHg–DOC complexes are taken up across membranes by other processes as well Overall, in the aquatic environment, DOC affects MMHg bioaccumulation into fish via a number of conflicting interactions as it increases dissolved concentrations while decreasing bioavailability of Hg to methylating bacteria, phytoplankton, and to benthic invertebrates For other metals and metalloids, the influence of DOC is likely less marked than it is for Hg For Cd, which binds relatively weakly to DOC,47 the influence of DOC on water column speciation is small This is not so for Cu, which is almost entirely bound to organic matter in seawater.24 Demonstration of the influence of DOC on metal accumulation in phytoplankton is limited to synthetic ligands, as discussed above It has recently been suggested that zebra mussels can take up DOC directly and that metals could be similarly assimilated.48 Such a pathway has not been considered previously, and its general applicability needs to be demonstrated For fish, studies of uptake across gills49 have shown that addition of DOC can reduce Cu accumulation, but that realistic DOC additions have little influence on Cd uptake These studies also confirm the interaction between Cd and Ca, and demonstrate that Cd is taken up by higher organisms through Ca ion channels but Cu is not Bioavailability of metalloids that exist as oxyanions in aqueous solution is not directly influenced by DOC Both As and Se are present in surface waters in two oxidation states primarily because the reduced state, As(III) or Se(IV), is kinetically relatively stable to oxidation As discussed above, As(V) is taken up as a phosphate analogue and is either incorporated into organic compounds within phytoplankton or released, either as As(III) or as methylated As compounds.50,51 For Se, preferential uptake of Se(IV) over Se(VI) has been shown52 and phytoplankton also excrete organo-Se compounds Selenium is required at low levels by most organisms and is incorporated into protein, but it is toxic at higher levels Chemical speciation modeling allows estimation of the impact of DOC on metal bioavailability and toxicity if the binding constants for the metal to DOC are known In these estimations it is assumed that the metal–DOC complex is not taken up and this assumption is for the most part true although there is the potential for absorption and/or competitive exchange reactions occurring between the metal complex and surface active sites on the membrane.4,47 There is increasing evidence6,28,33 that adsorption may be an important mechanism for the accumulation of Hg and MMHg as accumulation into both algae and higher organisms occurs under conditions where equilibrium speciation modeling indicates that all the metal should be bound to DOC There is the potential for such interactions with other metals as well Although all metals enter phytoplankton cells, more efficient trophic transfer of some constituents leads to their enhanced bioaccumulation upon grazing by primary consumers.25,29,30 Of all the toxic metals, MMHg transfer from diatoms to copepods is the greatest and this coincides with the relatively greater sequestration of MMHg ©2002 CRC Press LLC in the diatom cytoplasm compared with binding to cellular membranes Similarly, more MMHg is associated with the soft tissue of copepods, and this correlates with the higher assimilation of MMHg over inorganic Hg by fish feeding on these organisms.33 A similar trend is found for other metals and elements.53 Overall, Ag, Cd, and Hg behave similarly with relatively low assimilation efficiencies (90%) in predatory fish tissues is MMHg Thus, the fraction of body burden Hg as MMHg in fish is a function of trophic position This is also true for invertebrates Riisgård and Famme62 observed 73% MMHg in carnivorous shrimp compared with 17% in suspension-feeding mussels collected from the same area, and Mason et al.55 found 60 to 100% MMHg in predatory insect larvae compared with