Roxanne Karimi

Stoichiometric controls of mercury dilution by growth.

Rapid growth could significantly reduce methylmercury (MeHg) concentrations in aquatic organisms by causing a greater than proportional gain in biomass relative to MeHg (somatic growth dilution). We hypothesized that rapid growth from the consumption of high-quality algae, defined by algal nutrient stoichiometry, reduces MeHg concentrations in zooplankton, a major source of MeHg for lake fish. Using a MeHg radiotracer, we measured changes in MeHg concentrations, growth and ingestion rates in juvenile Daphnia pulex fed either high (C:P = 139) or low-quality (C:P = 1317) algae (Ankistrodesmus falcatus) for 5 d. We estimated Daphnia steady-state MeHg concentrations, using a biokinetic model parameterized with experimental rates. Daphnia MeHg assimilation efficiencies (≈95%) and release rates (0.04 d−1) were unaffected by algal nutrient quality. However, Daphnia growth rate was 3.5 times greater when fed high-quality algae, resulting in pronounced somatic growth dilution. Steady-state MeHg concentrations in Daphnia that consumed high-quality algae were one-third those of Daphnia that consumed low-quality algae due to higher growth and slightly lower ingestion rates. Our findings show that rapid growth from high-quality food consumption can significantly reduce the accumulation and trophic transfer of MeHg in freshwater food webs.

A quantitative synthesis of mercury in commercial seafood and implications for exposure in the United States.

Background: Mercury (Hg) is a toxic metal that presents public health risks through fish consumption. A major source of uncertainty in evaluating harmful exposure is inadequate knowledge of Hg concentrations in commercially important seafood. Objectives: We examined patterns, variability, and knowledge gaps of Hg in common commercial seafood items in the United States and compared seafood Hg concentrations from our database to those used for exposure estimates and consumption advice. Methods: We developed a database of Hg concentrations in fish and shellfish common to the U.S. market by aggregating available data from government monitoring programs and the scientific literature. We calculated a grand mean for individual seafood items, based on reported means from individual studies, weighted by sample size. We also compared database results to those of federal programs and human health criteria [U.S. Food and Drug Administration Hg Monitoring Program (FDA-MP), U.S. Environmental Protection Agency (EPA)]. Results: Mean Hg concentrations for each seafood item were highly variable among studies, spanning 0.3–2.4 orders of magnitude. Farmed fish generally had lower grand mean Hg concentrations than their wild counterparts, with wild seafood having 2- to12-fold higher concentrations, depending on the seafood item. However, farmed fish are relatively understudied, as are specific seafood items and seafood imports from Asia and South America. Finally, we found large discrepancies between mean Hg concentrations estimated from our database and FDA-MP estimates for most seafood items examined. Conclusions: The high variability in Hg in common seafood items has considerable ramifications for public health and the formulation of consumption guidelines. Exposure and risk analyses derived from smaller data sets do not reflect our collective, available information on seafood Hg concentrations.

Mercury, selenium and fish oils in marine food webs and implications for human health

Humans who eat fish are exposed to mixtures of healthful nutrients and harmful contaminants that are influenced by environmental and ecological factors. Marine fisheries are composed of a multitude of species with varying life histories, and harvested in oceans, coastal waters and estuaries where environmental and ecological conditions determine fish exposure to both nutrients and contaminants. Many of these nutrients and contaminants are thought to influence similar health outcomes (i.e., neurological, cardiovascular, immunological systems). Therefore, our understanding of the risks and benefits of consuming seafood require balanced assessments of contaminants and nutrients found in fish and shellfish. In this paper, we review some of the reported benefits of fish consumption with a focus on the potential hazards of mercury exposure, and compare the environmental variability of fish oils, selenium and mercury in fish. A major scientific gap identified is that fish tissue concentrations are rarely measured for both contaminants and nutrients across a range of species and geographic regions. Interpreting the implications of seafood for human health will require a better understanding of these multiple exposures, particularly as environmental conditions in the oceans change.

Multielement Stoichiometry in Aquatic Invertebrates: When Growth Dilution Matters

Element concentrations in organisms can be variable, often causing deviations from otherwise consistent, taxon‐specific multielement stoichiometries. Such variation can have considerable ecological consequences, yet physiological mechanisms remain unclear. We tested the influence of somatic growth dilution (SGD) on multiple element concentrations under different bioenergetic conditions. SGD occurs when rapid individual growth causes a disproportional gain in biomass relative to gain of a specific element. SGD can strongly affect elements in various organisms, but we lack a general framework to unify results across studies and assess its overall importance. We derived the general conditions that trigger SGD from an element accumulation model. We parameterized the model with bioenergetic and element‐specific rates summarized from the literature to compare SGD effects on 15 elements (nonessential metals, essential trace elements, macronutrients) in three aquatic invertebrate taxa. For all taxa, we found that SGD (1) occurs to some degree for all 15 elements over realistic ranges of growth and ingestion rates and (2) has the greatest effect on elements with low efflux (excretion) rates, including certain nonessential metals (e.g., MeHg, Po), essential trace elements, and macronutrients (e.g., N, Fe). Thus, SGD can strongly affect concentrations of a spectrum of elements under natural conditions. These results provide a framework for predicting variation in the elemental composition of animals.

Elevated blood Hg at recommended seafood consumption rates in adult seafood consumers

Mercury (Hg) exposure from seafood continues to be a public health concern due to health effects from elevated exposure, increasing worldwide seafood consumption, and continued Hg inputs into the environment. Elevated Hg exposure can occur in populations with specialized diets of sport-caught freshwater fish. However, we need a better understanding of Hg exposure from seafood, the most common exposure source, and from specific seafood types. We examined Hg exposure in avid seafood consumers, and the seafood items and consumption frequency that confer the largest Hg exposure. Adult, avid seafood consumers, in Long Island, NY, USA, with blood total Hg concentrations predicted to exceed the USEPA reference concentration that is considered safe (5.8 μg L(-1)), were eligible for the study; 75% of self-reported avid seafood consumers were eligible to participate. We measured blood total Hg concentrations and seafood consumption in 285 participants. We examined relationships between Hg and seafood consumption using multiple linear regression. Seafood consumption rate for our population (14.4 kg yr(-1)) was >2 times that estimated for the U.S. (6.8 kg yr(-1)), and lower than the worldwide estimate (18.4 kg yr(-1)). Mean blood Hg concentration was 4.4 times the national average, and 42% of participants had Hg concentrations exceeding 5.8 μg L(-1). Elevated Hg exposures occurred at all seafood consumption frequencies, including the recommended frequency of 2 meals per week. Blood Hg concentrations were positively associated with weekly tuna steak or sushi intake (β=6.30 change in blood Hg, μg L(-1)) and monthly (β=2.54) or weekly (β=9.47) swordfish, shark or marlin intake. Our findings show that seafood consumers in this population have elevated Hg exposures even at relatively low seafood consumption rates that are at or below current dietary recommendations. Further study should examine health risks and benefits of avid seafood consumption, and consider modifying guidelines to include recommendations for specific seafood types.

Contrasting Food Web Factor and Body Size Relationships with Hg and Se Concentrations in Marine Biota

Marine fish and shellfish are primary sources of human exposure to mercury, a potentially toxic metal, and selenium, an essential element that may protect against mercury bioaccumulation and toxicity. Yet we lack a thorough understanding of Hg and Se patterns in common marine taxa, particularly those that are commercially important, and how food web and body size factors differ in their influence on Hg and Se patterns. We compared Hg and Se content among marine fish and invertebrate taxa collected from Long Island, NY, and examined associations between Hg, Se, body length, trophic level (measured by δ15N) and degree of pelagic feeding (measured by δ13C). Finfish, particularly shark, had high Hg content whereas bivalves generally had high Se content. Both taxonomic differences and variability were larger for Hg than Se, and Hg content explained most of the variation in Hg:Se molar ratios among taxa. Finally, Hg was more strongly associated with length and trophic level across taxa than Se, consistent with a greater degree of Hg bioaccumulation in the body over time, and biomagnification through the food web, respectively. Overall, our findings indicate distinct taxonomic and ecological Hg and Se patterns in commercially important marine biota, and these patterns have nutritional and toxicological implications for seafood-consuming wildlife and humans.

Sustaining seafood for public health

Concern about the collapse of overexploited fish populations and the safety of consuming seafood can complicate determining what types of fish are best to eat. In recent years, public attention has become increasingly focused on oceanic environmental contaminants, which may be toxic to seafood consumers in sufficient doses. Laudable education campaigns have been established to inform consumers about seafood choices that are sustainable, and to provide information on which fish are deemed safe for human consumption. We found that unsustainable seafood items also present higher health risks (as indexed by mercury concentrations) and do not necessarily provide greater health benefits (as indexed by omega-3 fatty acid concentrations) as compared with sustainable seafood items. Our results have broad implications for identifying effective approaches for informing consumers about the health risks and benefits of different seafood choices, while simultaneously addressing the ecological consequences of fishing an...

Comparing nearshore benthic and pelagic prey as mercury sources to lake fish: the importance of prey quality and mercury content

Mercury (Hg) bioaccumulation in fish poses well-known health risks to wildlife and humans through fish consumption. Yet fish Hg concentrations are highly variable, and key factors driving this variability remain unclear. One little studied source of variation is the influence of habitat-specific feeding on Hg accumulation in lake fish. However, this is likely important because most lake fish feed in multiple habitats during their lives, and the Hg and caloric content of prey from different habitats can differ. This study used a three-pronged approach to investigate the extent to which habitat-specific prey determine differences in Hg bioaccumulation in fish. This study first compared Hg concentrations in common nearshore benthic invertebrates and pelagic zooplankton across five lakes and over the summer season in one lake, and found that pelagic zooplankton generally had higher Hg concentrations than most benthic taxa across lakes, and over a season in one lake. Second, using a bioenergetics model, the effects of prey caloric content from habitat-specific diets on fish growth and Hg accumulation were calculated. This model predicted that the consumption of benthic prey results in lower fish Hg concentrations due to higher prey caloric content and growth dilution (high weight gain relative to Hg from food), in addition to lower prey Hg levels. Third, using data from the literature, links between fish Hg content and the degree of benthivory, were examined, and showed that benthivory was associated with reduced Hg concentrations in lake fish. Taken together, these findings support the hypothesis that higher Hg content and lower caloric content make pelagic zooplankton prey greater sources of Hg for fish than nearshore benthic prey in lakes. Hence, habitat-specific foraging is likely to be a strong driver of variation in Hg levels within and between fish species.

Mercury–nutrient signatures in seafood and in the blood of avid seafood consumers☆

Dietary recommendations for seafood are confusing due to the desire to balance both benefits from nutrients and risks from contaminants. The overall health value of different fish and shellfish items depends on concentrations of multiple nutrients (e.g., selenium (Se), omega-3 fatty acids) and contaminants (e.g., mercury (Hg)). However, few studies have examined the connections between human exposure to multiple nutrients and contaminants and the consumption of specific types of seafood. Our goals were to compare 1) Hg, Se and omega-3 fatty acid concentrations (Hg-nutrient signatures) among common fish and shellfish items and 2) Hg-nutrient signatures in the blood of avid seafood consumers, based on seafood consumption habits. We compiled nutrient and Hg concentration data for common fish and shellfish items from the literature. We also measured blood concentrations of Hg and seafood nutrients collected from adult, avid seafood consumers on Long Island, NY. Canonical discriminant analyses revealed distinct Hg-nutrient signatures among seafood items, and these signatures were reflected in the blood of consumers based on different consumption habits. For example, consumers with a salmon-dominated seafood diet had relatively high percentage of omega-3 fatty acids in blood, and consumers who tend to eat top predator seafood have higher Hg, but similar blood nutrient concentrations compared to consumers who tend to eat low trophic level seafood. These results provide direct evidence of links between the ecological characteristics of the type of seafood consumed and Hg-nutrient exposure. This approach helps assess the overall human health value of specific seafood types, leads to specific diet recommendations, and can be used to characterize risk:benefit status among seafood consumers.

Mercury exposure and a shift toward oxidative stress in avid seafood consumers.

Mechanisms of mercury (Hg) toxicity at low doses from seafood consumption, the most common exposure route, are not well understood. We tested the hypothesis that seafood Hg exposure is related to a shift in redox status, indicated by a decrease in the ratio of reduced to oxidized glutathione (GSH:GSSG) in blood, or increase in redox potential (Eh). We also examined whether key seafood nutrients (selenium (Se), omega-3 fatty acids) confound or modify this shift. We measured blood concentrations of total Hg, Se, GSH, GSSG, and the Omega-3 Index (% omega-3s of total fatty acids in red blood cell membranes) in seafood consumers in Long Island, NY. We examined relationships between Hg, GSH:GSSG ratio and Eh. Elevated blood Hg (>5.8µgL(-1)) was associated with lower GSH:GSSG (β=-116.73, p=0.01), with no evidence of confounding by Se or Omega-3 Index. However, in models stratified by Omega-3 Index levels, Hg-GSH:GSSG associations were weakened among those with high Omega-3 Index levels (>6% of fatty acids, β=-63.46, p=0.28), and heightened among those with low Omega-3 Index (β=-182.53, p<0.01). We observed comparable patterns for Eh in relation to Hg. These results support the hypothesis that Hg exposure from seafood is linked to a shift in redox status toward oxidative stress, modified by omega-3 fatty acids in this population. Further work should examine the role of different seafood nutrients and Hg-induced shifts in redox status in the diverse health effects associated with elevated Hg exposure.