Sarah E. Rothenberg

Rice methylmercury exposure and mitigation: a comprehensive review.

Rice cultivation practices from field preparation to post-harvest transform rice paddies into hot spots for microbial mercury methylation, converting less-toxic inorganic mercury to more-toxic methylmercury, which is likely translocated to rice grain. This review includes 51 studies reporting rice total mercury and/or methylmercury concentrations, based on rice (Orzya sativa) cultivated or purchased in 15 countries. Not surprisingly, both rice total mercury and methylmercury levels were significantly higher in polluted sites compared to non-polluted sites (Wilcoxon rank sum, p<0.001). However, rice percent methylmercury (of total mercury) did not differ statistically between polluted and non-polluted sites (Wilcoxon rank sum, p=0.35), suggesting comparable mercury methylation rates in paddy soil across these sites and/or similar accumulation of mercury species for these rice cultivars. Studies characterizing the effects of rice cultivation under more aerobic conditions were reviewed to determine the mitigation potential of this practice. Rice management practices utilizing alternating wetting and drying (instead of continuous flooding) caused soil methylmercury levels to spike, resulting in a strong methylmercury pulse after fields were dried and reflooded; however, it is uncertain whether this led to increased translocation of methylmercury from paddy soil to rice grain. Due to the potential health risks, it is advisable to investigate this issue further, and to develop separate water management strategies for mercury polluted and non-polluted sites, in order to minimize methylmercury exposure through rice ingestion.

Prenatal methylmercury exposure through maternal rice ingestion: insights from a feasibility pilot in Guizhou Province, China.

Maternal hair and blood were investigated as biomarkers for prenatal methylmercury (MeHg) exposure among seventeen mothers recruited at parturition in Wanshan, Guizhou Province, China, where rice ingestion was the primary MeHg exposure pathway and atmospheric mercury (Hg) levels were elevated. For all three trimesters (nxa0=xa051), hair total Hg (THg) and MeHg concentrations ranged from 0.27 to 4.9xa0μg/g (median: 0.96xa0μg/g) and 0.077 to 2.3xa0μg/g (median: 0.43xa0μg/g), respectively, while blood THg levels ranged from 1.7 to 11xa0μg/L (median: 3.0xa0μg/L, nxa0=xa017). Despite adequate hair washing procedures, median %MeHg (of THg) was 37% (range: 14-89%, nxa0=xa051), indicating exogenous inorganic Hg(II) contamination or incorporation of elemental Hg (Hg(o)) into the hair shaft were important. Rice MeHg levels (nxa0=xa017) were highly correlated with blood THg (r(2)xa0=xa00.66) compared to hair MeHg (r(2)xa0=xa00.31) (when variables were log10-transformed), suggesting blood THg was a more preferable biomarker for prenatal MeHg exposure within this population.

Spatiotemporal patterns of mercury accumulation in lake sediments of western North America

For the Western North America Mercury Synthesis, we compiled mercury records from 165 dated sediment cores from 138 natural lakes across western North America. Lake sediments are accepted as faithful recorders of historical mercury accumulation rates, and regional and sub-regional temporal and spatial trends were analyzed with descriptive and inferential statistics. Mercury accumulation rates in sediments have increased, on average, four times (4×) from 1850 to 2000 and continue to increase by approximately 0.2μg/m(2) per year. Lakes with the greatest increases were influenced by the Flin Flon smelter, followed by lakes directly affected by mining and wastewater discharges. Of lakes not directly affected by point sources, there is a clear separation in mercury accumulation rates between lakes with no/little watershed development and lakes with extensive watershed development for agricultural and/or residential purposes. Lakes in the latter group exhibited a sharp increase in mercury accumulation rates with human settlement, stabilizing after 1950 at five times (5×) 1850 rates. Mercury accumulation rates in lakes with no/little watershed development were controlled primarily by relative watershed size prior to 1850, and since have exhibited modest increases (in absolute terms and compared to that described above) associated with (regional and global) industrialization. A sub-regional analysis highlighted that in the ecoregion Northwestern Forest Mountains, <1% of mercury deposited to watersheds is delivered to lakes. Research is warranted to understand whether mountainous watersheds act as permanent sinks for mercury or if export of legacy mercury (deposited in years past) will delay recovery when/if emissions reductions are achieved.

Low-level methylmercury exposure through rice ingestion in a cohort of pregnant mothers in rural China

BACKGROUNDnRice ingestion is an important dietary exposure pathway for methylmercury. There are few studies concerning prenatal methylmercury exposure through rice ingestion, yet the health risks are greatest to the developing fetus, and thus should be investigated.nnnOBJECTIVESnOur main objective was to quantify dietary methylmercury intake through rice and fish/shellfish ingestion among pregnant mothers living in southern China, where rice was a staple food and mercury contamination was considered minimal.nnnMETHODSnA total of 398 mothers were recruited at parturition, who donated scalp hair and blood samples. Total mercury and/or methylmercury concentrations were measured in biomarkers, in rice samples from each participants home, and in fish tissue purchased from local markets. Additional fish/shellfish mercury concentrations were obtained from a literature search. Dietary methylmercury intake during the third trimester was equivalent to the ingestion rate for rice (or fish/shellfish)×the respective methylmercury concentration.nnnRESULTSnDietary methylmercury intake from both rice and fish/shellfish ingestion averaged 1.2±1.8µg/day (median=0.79µg/day, range=0-22µg/day), including on average 71% from rice ingestion (median: 87%, range: 0-100%), and 29% from fish/shellfish consumption (median 13%, range: 0-100%). Median concentrations of hair total mercury, hair methylmercury, and blood total mercury were 0.40µg/g (range: 0.08-1.7µg/g), 0.28µg/g (range: 0.01-1.4µg/g), and 1.2µg/L (range: 0.29-8.6µg/L), respectively, and all three biomarkers were positively correlated with dietary methylmercury intake through rice ingestion (Spearmans rho=0.18-0.21, p≤0.0005), although the correlations were weak. In contrast, biomarkers were not correlated with fish/shellfish methylmercury intake (Spearmans rho=0.04-0.08, p=0.11-0.46).nnnCONCLUSIONSnAmong pregnant mothers living in rural inland China, rice ingestion contributed to prenatal methylmercury exposure, more so than fish/shellfish ingestion.

The influence of obesity on blood mercury levels for U.S. non-pregnant adults and children: NHANES 2007-2010.

BACKGROUNDnIn animal studies obesity is associated with higher blood and tissue mercury concentrations; however human studies are lacking. Although the mechanism underlying this association is uncertain, obesity may alter the metabolism and distribution of methylmercury.nnnOBJECTIVESnWe determined whether obesity influenced blood mercury levels, the majority of which was methylmercury, for U.S. non-pregnant adults (≥20 years) and children (2-19 years) after controlling for methylmercury intake through fish and shellfish consumption, and other confounders.nnnMETHODSnWe completed secondary data analysis of the National Health and Nutrition Examination Survey (NHANES) (2007-2010) for participants who consumed fish/shellfish within 24h of blood collection for mercury analysis. Weighted least squares regression models related blood mercury levels (the dependent variable) to methylmercury exposure (μg) from fish consumed in the previous 24h, body mass index (BMI) (for adults), BMI z-scores (for children), and other confounders.nnnRESULTSnIn adjusted models, blood mercury levels were inversely correlated with BMI for adults [β, 95% confidence interval (CI)=-0.54 (-0.90, -0.18)]. For children, blood mercury levels were inversely correlated with BMI z-scores but the trend was not significant [β (95% CI)=-0.016 (-0.066, 0.035)]. When obese adults or children were compared with those who were overweight/normal weight, blood mercury averaged 22% lower for obese adults (95% CI: -33%, -8.2%), while blood mercury did not differ significantly for obese children [β (95% CI)=-1.7% (-31%, +39%)].nnnCONCLUSIONSnAfter adjusting for the main, if not exclusive, exogenous source of methylmercury exposure (through fish/shellfish intake) and other confounders, our results support potential changes in the metabolism, distribution or excretion of methylmercury with increasing BMI (for adults).

The role of gut microbiota in fetal methylmercury exposure: Insights from a pilot study

PURPOSEnThe mechanisms by which gut microbiota contribute to methylmercury metabolism remain unclear. Among a cohort of pregnant mothers, the objectives of our pilot study were to determine (1) associations between gut microbiota and mercury concentrations in biomarkers (stool, hair and cord blood) and (2) the contributions of gut microbial mercury methylation/demethylation to stool methylmercury.nnnMETHODSnPregnant women (36-39 weeks gestation, n=17) donated hair and stool specimens, and cord blood was collected for a subset (n=7). The diversity of gut microbiota was determined using 16S rRNA gene profiling (n=17). For 6 stool samples with highest/lowest methylmercury concentrations, metagenomic whole genome shotgun sequencing was employed to search for the mercury methylation gene (hgcA), and two mer operon genes involved in methylmercury detoxification (merA and merB).nnnRESULTSnSeventeen bacterial genera were significantly correlated (increasing or decreasing) with stool methylmercury, stool inorganic mercury, or hair total mercury; however, aside from one genus, there was no overlap between biomarkers. There were no definitive matches for hgcA or merB, while merA was detected at low concentrations in all six samples.nnnMAJOR CONCLUSIONSnProportional differences in stool methylmercury were not likely attributed to gut microbiota through methylation/demethylation. Gut microbiota potentially altered methylmercury metabolism using indirect pathways.

Water management impacts rice methylmercury and the soil microbiome.

Rice farmers are pressured to grow rice using less water. The impacts of water-saving rice cultivation methods on rice methylmercury concentrations are uncertain. Rice (Oryza sativa L. cv. Nipponbare) was cultivated in fields using four water management treatments, including flooded (no dry-downs), alternating wetting and drying (AWD) (with one or three dry-downs), and furrow-irrigated fields (nine dry-downs) (n=16 fields). Anoxic bulk soil was collected from rice roots during the rice maturation phase, and rice grain was harvested after fields were dried. Total mercury and methylmercury concentrations were determined in soil and polished rice samples, and the soil microbiome was analyzed using 16S (v4) rRNA gene profiling. Soil total mercury did not differ between fields. However, compared to continuously flooded fields, soil and rice methylmercury concentrations averaged 51% and 38% lower in the AWD fields, respectively, and 95% and 96% lower in the furrow-irrigated fields, respectively. Compared to flooded fields, grain yield was reduced on average by <1% in the AWD fields and 34% in the furrow-irrigated fields. Additionally, using 16S (v4) rRNA gene profiling, the relative abundance of genera (i.e., highest resolution via this method) known to contain mercury methylators averaged 2.9-fold higher in flooded and AWD fields compared to furrow-irrigated fields. These results reinforce the benefits of AWD in reducing rice methylmercury concentrations with minimal changes in rice production yields. In the furrow-irrigated fields, a lower relative abundance of genera known to contain mercury methylators suggests an association between lower concentrations of soil and rice methylmercury and specific soil microbiomes.

Maternal methylmercury exposure through rice ingestion and offspring neurodevelopment: A prospective cohort study.

BACKGROUNDnDietary methylmercury intake can occur not only through fish ingestion but also through rice ingestion; however, rice does not contain the same beneficial micronutrients as fish.nnnOBJECTIVESnIn rural China, where rice is a staple food, associations between prenatal methylmercury exposure (assessed using maternal hair mercury) and impacts on offspring neurodevelopment were investigated.nnnMETHODSnA total of 398 mothers were recruited at parturition at which time a sample of scalp hair was collected. Offspring (n=270, 68%) were assessed at 12 months using the Bayley Scales of Infant Development-II, yielding age-adjusted scores for the Mental Developmental Index (MDI) and Psychomotor Developmental Index (PDI).nnnRESULTSnAmong 270 mothers, 85% ingested rice daily, 41% never or rarely ingested fish/shellfish and 11% ingested fish/shellfish at least twice/weekly. Maternal hair mercury averaged 0.41μg/g (median: 0.39μg/g, range: 0.079-1.7μg/g). In unadjusted models, offspring neurodevelopment (both MDI and PDI) was inversely correlated with hair mercury. Associations were strengthened after adjustment for fish/shellfish ingestion, rice ingestion, total energy intake (kcal), and maternal/offspring characteristics for both the MDI [Beta: -4.9, 95% Confidence Interval (CI): -9.7, -0.12] and the PDI (Beta: -2.7, 95% CI: -8.3, 2.9), although confidence intervals remained wide for the latter.nnnCONCLUSIONSnFor 12-month old offspring living in rural China, prenatal methylmercury exposure was associated with statistically significant decrements in offspring cognition, but not psychomotor development. Results expose potential new vulnerabilities for communities depending on rice as a staple food.

Co-exposure to methylmercury and inorganic arsenic in baby rice cereals and rice-containing teething biscuits

Background: Rice is an important dietary source for methylmercury (MeHg), a potent neurotoxin, and inorganic arsenic (As), a human carcinogen. Rice baby cereals are a dietary source of inorganic As; however, less is known concerning MeHg concentrations in rice baby cereals and rice teething biscuits. Methods: MeHg concentrations were measured in 36 rice baby cereals, eight rice teething biscuits, and four baby cereals manufactured with oats/wheat (n = 48 total). Arsenic (As) species, including inorganic As, were determined in rice baby cereals and rice teething biscuits (n = 44/48), while total As was determined in all products (n = 48). Results: Rice baby cereals and rice teething biscuits were on average 61 and 92 times higher in MeHg, respectively, and 9.4 and 4.7 times higher in total As, respectively, compared to wheat/oat baby cereals. For a 15‐g serving of rice baby cereal, average MeHg intake was 0.0092 &mgr;g day−1 (range: 0.0013–0.034 &mgr;g day−1), while average inorganic As intake was 1.3 &mgr;g day−1 (range: 0.37–2.3 &mgr;g day−1). Inorganic As concentrations in two brands of rice baby cereal (n = 12/36 boxes of rice cereal) exceeded 100 ng/g, the proposed action level from the U.S. Food and Drug Administration. Log10 MeHg and inorganic As concentrations in rice baby cereals were strongly, positively correlated (Pearsons rho = 0.60, p < 0.001, n = 36). Conclusions: Rice‐containing baby cereals and teething biscuits were a dietary source of both MeHg and inorganic As. Studies concerning the cumulative impacts of MeHg and inorganic As on offspring development are warranted. HighlightsMethylmercury (MeHg) and inorganic arsenic (As) were measured in rice baby foods.MeHg was 61–92 times higher in rice‐based baby foods compared to non‐rice foods.Total As was 4.7–9.4 times higher in rice‐based foods compared to non‐rice foods.Inorganic As in 2 brands of rice cereals exceeded 100 ng/g, the proposed action level.MeHg (log) and inorganic As in rice baby cereals were strongly, positively correlated.

Blood total mercury and methylmercury among pregnant mothers in Charleston, South Carolina, USA

BackgroundMaternal blood total mercury (THg) is a biomarker for prenatal methylmercury (MeHg) exposure. Few studies have quantified both blood THg and MeHg during pregnancy, and few studies have reported longitudinal trends.ObjectivesWe analyzed blood THg and MeHg in a cohort of pregnant mothers in Charleston, South Carolina (nu2009=u200983), and investigated whether blood THg or MeHg changed between early and late gestation.MethodsTHg and MeHg were analyzed in blood samples from early (12u2009±u20091.7 weeks) and late (35u2009±u20092.2 weeks) gestation.ResultsBlood %MeHg (of THg) averaged 63% (range: 10–114%) and 61% (range: 12–117%) during early and late gestation, respectively. In unadjusted analyses, blood MeHg decreased from early to late pregnancy (paired t-test, pu2009=u20090.04), while THg did not change (paired t-test, pu2009=u20090.34). When blood MeHg was normalized by the hematocrit, this decrease was no longer statistically significant (paired t-test, pu2009=u20090.09).ConclusionsIn unadjusted analyses, blood MeHg, but not THg, decreased significantly between early and late gestation; this decrease was due in part to hemodilution. Percent MeHg (of THg) varied by up to one order of magnitude. Results highlight the importance of Hg speciation in maternal blood samples to assess prenatal MeHg exposure.