Temporal correspondence of selenium and mercury, among brine shrimp and water in Great Salt Lake, Utah, USA

The relationship between mercury and selenium in the eggshell and egg content of the Whiskered tern (Chlidonias hybridus)

Mercury and its compounds do not have beneficial biological roles in living organisms. In contrast, selenium, within permissible limits, is an essential micronutrient and antioxidant. There is an antagonistic relationship between mercury and selenium. Therefore, the molar ratio of selenium to mercury is used as an appropriate index. Measuring the mercury in the environment without considering the protective effects of selenium does not accurately reflect the risks posed by mercury. This study aimed to measure the mercury, selenium, and the molar ratio Se:Hg in the shell and content of the eggs of the Whiskered Tern (Chlidonias hybridus) in northern Iran. After the onset of egg laying, one egg was collected from each nest (38 eggs in total). The results showed that the concentration of mercury in the eggshell was higher, while the concentration of selenium and the molar ratio Se:Hg were higher in the egg content. A molar ratio Se:Hg in both the eggshell and the egg content indicates a positive effect of selenium in reducing the risks associated with mercury toxicity. The parameters of egg shape index, egg weight, and clutch size did not have a significant effect on the concentrations of mercury, selenium, and the molar ratio Se:Hg in the eggshell and egg content. There was a significant negative correlation between the concentration of mercury in the eggshell and the molar ratio Se:Hg in both the eggshell and egg content. The lack of a positive correlation between the concentrations of mercury or selenium in the eggshell and egg content suggests that the transferability of heavy metals from the egg to the embryo in the egg content cannot be confirmed.

Variable toxicity of inorganic mercury compounds to Artemia elicited by coexposure with dissolved organic matter

The chemical behavior of mercury (Hg) and its interactions with naturally occurring ligands shape its environmental fate and impact. The neurotoxic properties of Hg are widely known and studied both in vitro and in vivo. However, there continues to be limited information on the influence of chelation with large organic ligands on the toxicity to marine macro-organisms. This work examined the effect of Hg complexed with various types of dissolved organic matter (DOM) on the mortality and hatching success of Artemia sp. nauplii under varying marine media conditions. The results confirmed both, an alleviating as well as additive, DOM-specific, effect on mortality. DOM coexposure resulted in a compound specific decreased or increased toxicity in comparison with single exposure in artificial seawater, with LC50 values ranging from 2.11 to 62.89 µM. Hatching success under conditions of Hg exposure was almost two orders of magnitude more sensitive than toxicity in hatched individuals. Elevated DOM concentrations had no statistically significant impact on hatching success with computed EC50 values ranging from 196 to 324 nM.

Recent advance of microbial mercury methylation in the environment

Methylmercury formation is mainly driven by microbial-mediated process. The mechanism of microbial mercury methylation has become a crucial research topic for understanding methylation in the environment. Pioneering studies of microbial mercury methylation are focusing on functional strain isolation, microbial community composition characterization, and mechanism elucidation in various environments. Therefore, the functional genes of microbial mercury methylation, global isolations of Hg methylation strains, and their methylation potential were systematically analyzed, and methylators in typical environments were extensively reviewed. The main drivers (key physicochemical factors and microbiota) of microbial mercury methylation were summarized and discussed. Though significant progress on the mechanism of the Hg microbial methylation has been explored in recent decade, it is still limited in several aspects, including (1) molecular biology techniques for identifying methylators; (2) characterization methods for mercury methylation potential; and (3) complex environmental properties (environmental factors, complex communities, etc.). Accordingly, strategies for studying the Hg microbial methylation mechanism were proposed. These strategies include the following: (1) the development of new molecular biology methods to characterize methylation potential; (2) treating the environment as a micro-ecosystem and studying them from a holistic perspective to clearly understand mercury methylation; (3) a more reasonable and sensitive inhibition test needs to be considered. KEY POINTS: • Global Hg microbial methylation is phylogenetically and functionally discussed. • The main drivers of microbial methylation are compared in various condition. • Future study of Hg microbial methylation is proposed.

Algal Density Controls the Spatial Variations in Hg Bioconcentration and Bioaccumulation at the Base of the Pelagic Food Web of Lake Taihu, China

Algal density can significantly impact mercury (Hg) bioaccumulation and biomagnification in aquatic food webs, but the underlying mechanisms remain controversial especially in subtropical and tropical regions. We conducted a comprehensive field study on Hg bioconcentration in phytoplankton and bioaccumulation in size-fractionated zooplankton across 17 sampling sites in Lake Taihu, a large shallow lake in eastern China with large spatial differences in algal density. The higher algal density in the northern sites is highly associated with the lower THg bioconcentration factor (BCF) in phytoplankton and lower THg bioaccumulation factor (BAF) in zooplankton. The low Hg BCFs or BAFs at productive sites could not be explained by algal bloom dilution but attributed to the low Hg bioavailability, which is highly associated with the elevated pH levels at productive sites. The smaller body size of the dominant zooplankton species at higher algal density sites also contributed to their lower Hg bioaccumulation. Importantly, we provide evidence that high algal density is associated with a low proportion of methylmercury (MeHg) in total Hg (% MeHg) in phytoplankton, which is further transferred to zooplankton. Such a low THg BCF or BAF and low % MeHg in plankton at high algal density sites hamper the entry of Hg into the pelagic food webs, which are important but yet underestimated driving forces for the low Hg contents in pelagic fish that are commonly observed in anthropogenic-impacted eutrophic lakes in subtropical regions.

Trace element bioaccumulation in hypersaline ecosystems and implications of a global invasion

Hypersaline ecosystems are under increasing threat due to anthropogenic pressures such as environmental pollution and biological invasions. Here we address the ecotoxicological implications of the Artemia franciscana (Crustacea) invasion in saltpans of southern Spain. This North American species is causing the extinction of native Artemia populations in many parts of the globe. The bioaccumulation of trace elements (As, Cd, Cu, Co, Cr, Mn, Ni, Pb and Zn) in native populations (A. parthenogenetica) from Cabo de Gata and Odiel saltpans and invasive Artemia from Cádiz saltpan was studied at different salinities. Furthermore, in Odiel, the most polluted study site, we also analysed the bioaccumulation of trace elements by Chironomus salinarius larvae (Diptera) and Ochthebius notabilis adults (Coleoptera). High levels of trace elements were detected in the studied saltpans, many of them exceeding the recommended threshold guidelines for aquatic life. Bioaccumulation of trace elements by Artemia was lowest at the highest salinity. The invasive A. franciscana showed higher potential to bioaccumulate trace elements than its native counterpart (in particular for As, Cd, Ni and Cr). In Odiel, O. notabilis stood out as showing the highest potential to bioaccumulate As and Cu. Results showed that the shift from a native to an alien Artemia species with a higher bioaccumulation capacity may increase the transfer of trace elements in hypersaline food webs, especially for waterbirds that depend on Artemia as food. Thus, our study identifies an indirect impact of the Artemia franciscana invasion that had not previously been recognised.

Geochemical partitioning and possible heavy metal(loid) bioaccumulation within aquaculture shrimp ponds

Limited work has been conducted on trace metal(loid) exchange between sediment, water, feed, and shrimp, particularly in estuarine aquaculture environments. To identify metal(loid) sources and the processes controlling bioaccumulation in shrimp, we analyzed paired aquaculture water, sediment, and shrimp samples collected in Southwest Bangladesh in the late dry season (May), as well as several common artificial feeds. Additionally, we analyzed sediment extract samples from 24-hour laboratory batch experiments as an analogue for aquaculture pond sediment porewater to examine element mobilization from pond sediment. Weak correlations between element concentrations in extracts, pond water, and bulk sediment indicate equilibrium with sediment was not achieved in extract experiments or ponds, and that sediment composition has little influence on pond water or shrimp composition. Aquaculture shrimp metal(loid) concentrations were similar to artificial feed but not pond sediment or pond water, suggesting that shrimp composition is mostly influenced by diet. Although arsenic (As) and selenium (Se) were present at high concentrations in shrimp pond waters and sediment, they were often below detection in shrimp. However, the highly toxic metal thallium (Tl) was detected in shrimp at levels that pose significant human health risks based on calculated target hazard quotient (THQ) values. Thus, further research into potential Tl contamination in Bangladesh aquaculture environments is warranted.