Mercury and Selenium in Stranded Indo-Pacific Humpback Dolphins and Implications for Their Trophic Transfer in Food Chains

<div><p>As top predators in the Pearl River Estuary (PRE) of China, Indo-Pacific humpback dolphins (<i>Sousa chinensis</i>) are bioindicators for examining regional trends of environmental contaminants in the PRE. We examined samples from stranded <i>S. chinensis</i> in the PRE, collected since 2004, to study the distribution and fate of total mercury (THg), methylmercury (MeHg) and selenium (Se) in the major tissues, in individuals at different ages and their prey fishes from the PRE. This study also investigated the potential protective effects of Se against the toxicities of accumulated THg. Dolphin livers contained the highest concentrations of THg (32.34±58.98 µg g⁻¹ dw) and Se (15.16±3.66 µg g⁻¹ dw), which were significantly different from those found in kidneys and muscles, whereas the highest residue of MeHg (1.02±1.11 µg g⁻¹ dw) was found in dolphin muscles. Concentrations of both THg and MeHg in the liver, kidney and muscle of dolphins showed a significantly positive correlation with age. The biomagnification factors (BMFs) of inorganic mercury (Hg_inorg) in dolphin livers (350×) and MeHg in muscles (18.7×) through the prey fishes were the highest among all three dolphin tissues, whereas the BMFs of Se were much lower in all dolphin tissues. The lower proportion of MeHg in THg and higher Se/THg ratios in tissues were demonstrated. Our studies suggested that <i>S. chinensis</i> might have the potential to detoxify Hg via the demethylation of MeHg and the formation of tiemannite (HgSe) in the liver and kidney. The lower threshold of hepatic THg concentrations for the equimolar accumulation of Se and Hg in <i>S. chinensis</i> suggests that this species has a greater sensitivity to THg concentrations than is found in striped dolphins and Dall’s porpoises.</p></div

The mean concentrations and standard deviations (SD), in µg g⁻¹ dry weight, of total mercury (THg), selenium (Se) and methyl mercury (MeHg); the molar ratio (%) of Se to THg; and the percentage (%) of MeHg/THg in liver, kidney and muscle tissue of Indo-Pacific humpback dolphins from the Pearl River Estuary (PRE), China.

<p>JM: juvenile male (<12 years); JF: juvenile female (<9 years); AM: adult male (>12 years); AF: adult female (>9 years).</p><p>n.s.: not significant.</p><p>The mean concentrations and standard deviations (SD), in µg g⁻¹ dry weight, of total mercury (THg), selenium (Se) and methyl mercury (MeHg); the molar ratio (%) of Se to THg; and the percentage (%) of MeHg/THg in liver, kidney and muscle tissue of Indo-Pacific humpback dolphins from the Pearl River Estuary (PRE), China.</p

The mean concentrations and standard deviations (SD), in µg g⁻¹ dry weight, of total mercury (THg), inorganic mercury (Hg_inorg), methylmercury (MeHg), selenium (Se), and the percentage (%) of MeHg/THg in the prey fishes (whole body) for Indo-Pacific humpback dolphins and their average biomagnification factors (BMFs) in the dolphin tissues collected from the Pearl River Estuary (PRE), China.

<p>The mean concentrations and standard deviations (SD), in µg g⁻¹ dry weight, of total mercury (THg), inorganic mercury (Hg_inorg), methylmercury (MeHg), selenium (Se), and the percentage (%) of MeHg/THg in the prey fishes (whole body) for Indo-Pacific humpback dolphins and their average biomagnification factors (BMFs) in the dolphin tissues collected from the Pearl River Estuary (PRE), China.</p

Relationships of dolphin age with the concentrations of THg, MeHg, Se and MeHg/THg ratio in the liver, kidney and muscle tissue, respectively, in the Indo-Pacific humpback dolphins stranded in the Pearl River Estuary (PRE) region.

<p>Relationships of dolphin age with the concentrations of THg, MeHg, Se and MeHg/THg ratio in the liver, kidney and muscle tissue, respectively, in the Indo-Pacific humpback dolphins stranded in the Pearl River Estuary (PRE) region.</p

Contribution of Coexisting Sulfate and Iron Reducing Bacteria to Methylmercury Production in Freshwater River Sediments

We investigated microbial methylmercury (CH₃Hg) production in sediments from the South River (SR), VA, an ecosystem contaminated with industrial mercury (Hg). Potential Hg methylation rates in samples collected at nine sites were low in late spring and significantly higher in late summer. Demethylation of ¹⁴CH₃Hg was dominated by ¹⁴CH₄ production in spring, but switched to producing mostly ¹⁴CO₂ in the summer. Fine-grained sediments originating from the erosion of river banks had the highest CH₃Hg concentrations and were potential hot spots for both methylation and demethylation activities. Sequencing of 16S rRNA genes of cDNA recovered from sediment RNA extracts indicated that at least three groups of sulfate-reducing bacteria (SRB) and one group of iron-reducing bacteria (IRB), potential Hg methylators, were active in SR sediments. SRB were confirmed as a methylating guild by amendment experiments showing significant sulfate stimulation and molybdate inhibition of methylation in SR sediments. The addition of low levels of amorphous iron­(III) oxyhydroxide significantly stimulated methylation rates, suggesting a role for IRB in CH₃Hg synthesis. Overall, our studies suggest that coexisting SRB and IRB populations in river sediments contribute to Hg methylation, possibly by temporally and spatially separated processes

Regression analysis of Se with THg, and of log₁₀ (Se/Hg) with log₁₀ MeHg in the liver and kidney of Indo-Pacific humpback dolphins (n = 28) stranded in the Pearl River Estuary (PRE) region.

<p>Regression analysis of Se with THg, and of log₁₀ (Se/Hg) with log₁₀ MeHg in the liver and kidney of Indo-Pacific humpback dolphins (n = 28) stranded in the Pearl River Estuary (PRE) region.</p

MeHg concentration (µg g⁻¹ dw) (A) and concentration ratio of MeHg to THg (B) against THg concentration (µg g⁻¹ dw) in the liver samples of the stranded Indo-Pacific humpback dolphins in the Pearl River Estuary region (n = 28).

<p>MeHg concentration (µg g⁻¹ dw) (A) and concentration ratio of MeHg to THg (B) against THg concentration (µg g⁻¹ dw) in the liver samples of the stranded Indo-Pacific humpback dolphins in the Pearl River Estuary region (n = 28).</p

Sampling sites in the Pearl River Estuary (PRE) where the stranded Indo-Pacific humpback dolphins (n = 28) were collected from 2004 to 2012.

<p>Sampling sites in the Pearl River Estuary (PRE) where the stranded Indo-Pacific humpback dolphins (n = 28) were collected from 2004 to 2012.</p

Spatiotemporal Trends of Heavy Metals in Indo-Pacific Humpback Dolphins (<i>Sousa chinensis</i>) from the Western Pearl River Estuary, China

We assessed the spatiotemporal trends of the concentrations of 11 heavy metals (HMs) in the liver and kidney of Indo-Pacific humpback dolphins (<i>Sousa chinensis</i>) from western Pearl River Estuary (PRE) during 2004–2015. The hepatic levels of Cr, As, and Cu in these dolphins were among the highest reported for cetaceans globally, and the levels of Zn, Cu, and Hg were sufficiently high to cause toxicological effects in some of the animals. Between same age-sex groups, dolphins from Lingdingyang were significantly more contaminated with Hg, Se, and V than those from the West-four region, while the opposite was true for Cd. Generalized additive mixed models showed that most metals had significant but dissimilar temporal trends over a 10-year period. The concentrations of Cu and Zn increased significantly in recent years, corresponding to the high input of these metals in the region. Body-length-adjusted Cd levels peaked in 2012, accompanied by the highest annual number of dolphin stranding events. In contrast to the significant decrease in HM levels in the dolphins in Hong Kong waters (the eastern reaches of the PRE), the elevated metal exposure in the western PRE raises serious concerns