There is increasing recognition of the quantitative importance of metal accumulation from dietary pathways in different marine food webs. With a simple kinetic model requiring mea- surements of metal assimilation efficiency (AE), metal efflux rate and ingestion activity of the rele- vant animals, it is possible to predict the transfer (and potential biomagnification) of metals along diverse marine food chains. In this paper, food chain transfer in different marine food chains (plank- tonic and benthic) is reviewed, and any potential biomagnification of metals is predicted using the simple kinetic equation. It is well recognized that metal concentrations in animals are probably not related to the trophic level in the food chain, but some metals (Hg and Zn) may potentially be influ- enced by the numbers of trophic interactions involved. Depending on these different trophic inter- actions, the potential biomagnification of metals is influenced by metal assimilation or metal efflux rate or both. Metals may potentially be biomagnified in benthic food chains; principally, in those with gastropods as top predators, as they can efficiently sequester dietary metals. The biodiminution of metals (concentration decreases at a higher trophic level) in the classical marine planktonic food chain (phytoplankton to copepods to fish) is largely the result of the effective efflux of metals by the copepods and the very low assimilation of metals by marine fish. Although many past studies have considered the physico-chemical properties of metals in prey in controlling food chain transfer, it is suggested that the physiological and biochemical processes in the herbivore or predator should be emphasized in future studies. Also, the complicated metal-handling strategies found in different marine organisms clearly need to be appreciated.

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Interactions of trace metals and different marine food chains

Mercury accumulation in fish is a global public health concern, because fish are the primary source of toxic methylmercury to humans. Fish from all lakes do not pose the same level of risk to consumers. One of the most intriguing patterns is that potentially dangerous mercury concentrations can be found in fish from clear, oligotrophic lakes whereas fish from greener, eutrophic lakes often carry less mercury. In this study, we experimentally tested the hypothesis that increasing algal biomass reduces mercury accumulation at higher trophic levels through the dilution of mercury in consumed algal cells. Under bloom dilution, as algal biomass increases, the concentration of mercury per cell decreases, resulting in a lower dietary input to grazers and reduced bioaccumulation in algal-rich eutrophic systems. To test this hypothesis, we added enriched stable isotopes of Hg to experimental mesocosms and measured the uptake of toxic methylmercury (CH3200Hg+) and inorganic 201Hg2+ by biota at several algal concentrations. We reduced absolute spike detection limits by 50–100 times compared with previous techniques, which allowed us to conduct experiments at the extremely low aqueous Hg concentrations that are typical of natural systems. We found that increasing algae reduced CH3Hg+ concentrations in zooplankton 2–3-fold. Bloom dilution may provide a mechanistic explanation for lower CH3Hg+ accumulation by zooplankton and fish in algal-rich relative to algal-poor systems.

https://www.pnas.org/content/pnas/99/7/4419.full.pdf

Algal blooms reduce the uptake of toxic methylmercury in freshwater food webs

Copper and zinc in four freshwater fish species from Lake Pamvotis (Greece)

Biological mercury (Hg) hotspots were identified in the northeastern United States and southeastern Canada using a data set of biotic Hg concentrations. Eight layers representing three major taxa and more than 7300 observations were used to locate five biological Hg hotspots and nine areas of concern. The yellow perch and common loon were chosen as indicator species for the human and ecological effects of Hg, respectively. Biological Hg hotspots receive elevated atmospheric Hg deposition, have high landscape sensitivity, and/or experience large reservoir fluctuations. In the Merrimack River watershed, local Hg emissions are linked to elevated local deposition and high Hg concentrations in biota. Time series data for this region suggest that reductions in Hg emissions from local sources can lead to rapid reductions of Hg in biota. An enhanced Hg monitoring network is needed to further document areas of high deposition, biological hotspots, and the response to emissions reductions and other mitigation strategies.

https://www.niehs.nih.gov/research/supported/assets/docs/a_c/bioscience_508.pdf

Biological Mercury Hotspots in the Northeastern United States and Southeastern Canada

We present the cloning and characterization of an Arabidopsis gene, FRD3, involved in iron homeostasis. Plants carrying any of the three alleles of frd3 constitutively express three strategy I iron deficiency responses and misexpress a number of iron deficiency-regulated genes. Mutant plants also accumulate approximately twofold excess iron, fourfold excess manganese, and twofold excess zinc in their shoots. frd3-3 was first identified as man1. The FRD3 gene is expressed at detectable levels in roots but not in shoots and is predicted to encode a membrane protein belonging to the multidrug and toxin efflux family. Other members of this family have been implicated in a variety of processes and are likely to transport small organic molecules. The phenotypes of frd3 mutant plants, which are consistent with a defect in either iron deficiency signaling or iron distribution, indicate that FRD3 is an important component of iron homeostasis in Arabidopsis.

FRD3, a Member of the Multidrug and Toxin Efflux Family, Controls Iron Deficiency Responses in Arabidopsis

Recent studies have emphasized the need for understanding the accumulation and fate of metal contaminants at different trophic levels and across a broad spectrum of lake types. To address both issues, metal concentrations (Hg, Zn, Cd, As, and Pb) were measured in the water, two size fractions of zooplankton, and fish from 20 lakes in contaminated to pristine watersheds in the northeastern United States. Our goals were to examine links between watershed characteristics and aqueous metal levels in lakes and relationships between aqueous concentrations, metal burdens in different plankton groups and in fish. Two pairs of metals, (1) Hg and Zn and (2) As and Pb, exhibited strong similarities both in the factors that predict their concentrations in water and in the patterns of accumulation in particular trophic levels. Aqueous concentrations of Hg and Zn were highest in cool water lakes, whereas As and Pb were highest in more eutrophic lakes in agricultural areas. Aqueous Cd concentrations were closely correlated with the land-use variables, percentage of agricultural land, and road densities. Similarly, Hg and Zn both biomagnified from small plankton (45‐202mm) to macrozooplankton (.202 mm) and from macrozooplankton to fish. In contrast, bioaccumulation of both As and Pb diminished with increasing trophic level. Although aqueous metal and zooplankton metal levels were not significant predictors of As and Pb levels in fish, metal levels in zooplankton were predictive of Hg and Zn in fish, suggesting that sources of bioaccumulation differ for different metals. Our findings demonstrate the importance of investigating upper and lower trophic levels separately, to fully understand metal transfer pathways in aquatic food webs.

/pdf/accumulation-of-heavy-metals-in-food-web-components-across-a-3rz2vhetve.pdf

Bjorn Klaue

Papers

Accumulation of heavy metals in food web components across a gradient of lakes