Mercury and methylmercury uptake and trophic transfer from marine diatoms to copepods and field collected zooplankton

Bioconcentration of Inorganic and Methyl Mercury by Algae Revealed Using Dual-Mass Single-Cell ICP-MS with Double Isotope Tracers

Algae are an entry point for mercury (Hg) into the food web. Bioconcentration of Hg by algae is crucial for its biogeochemical cycling and environmental risk. Herein, considering the cell heterogeneity, we investigated the bioconcentration of coexisting isotope-labeled inorganic (199IHg) and methyl Hg (201MeHg) by six typical freshwater and marine algae using dual-mass single-cell inductively coupled plasma mass spectrometry (scICP-MS). First, a universal pretreatment procedure for the scICP-MS analysis of algae was developed. Using the proposed method, the intra- and interspecies heterogeneities and the kinetics of Hg bioconcentration by algae were revealed at the single-cell level. The heterogeneity in the cellular Hg contents is largely related to cell size. The bioconcentration process reached a dynamic equilibrium involving influx/adsorption and efflux/desorption within hours. Algal density is a key factor affecting the distribution of Hg between algae and ambient water. Cellular Hg contents were negatively correlated with algal density, whereas the volume concentration factors almost remained constant. Accordingly, we developed a model based on single-cell analysis that well describes the density-driven effects of Hg bioconcentration by algae. From a novel single-cell perspective, the findings improve our understanding of algal bioconcentration governed by various biological and environmental factors.

Mercury dynamics at the base of the pelagic food web of the Gulf of Gdańsk, southern Baltic Sea

Planktonic organisms, which have direct contact with water, serve as the entry point for mercury (Hg), into the marine food web, impacting its levels in higher organisms, including fish, mammals, and humans who consume seafood. This study provides insights into the distribution and behavior of Hg within the Baltic Sea, specifically the Gulf of Gdańsk, focusing on pelagic primary producers and consumers. Phytoplankton Hg levels were primarily influenced by its concentrations in water, while Hg concentrations in zooplankton resulted from dietary exposure through suspended particulate matter and phytoplankton consumption. Hg uptake by planktonic organisms, particularly phytoplankton, was highly efficient, with Hg concentrations four orders of magnitude higher than those in the surrounding water. However, unlike biomagnification of Hg between SPM and zooplankton, biomagnification between zooplankton and phytoplankton was not apparent, likely due to the low trophic position and small size of primary consumers, high Hg elimination rates, and limited absorption.

Multistress Interplay: Time and Duration of Ocean Acidification Modulate the Toxicity of Mercury and Other Metals

The current understanding of multistress interplay assumes stresses occur in perfect synchrony, but this assumption is rarely met in the natural marine ecosystem. To understand the interplay between nonperfectly overlapped stresses in the ocean, we manipulated a multigenerational experiment (F0-F3) to explore how different temporal scenarios of ocean acidification will affect mercury toxicity in a marine copepod Pseudodiaptomus annandalei. We found that the scenario of past acidification aggravated mercury toxicity but current and persistent acidification mitigated its toxicity. We specifically performed a proteomics analysis for the copepods of F3. The results indicated that current and persistent acidification initiated the energy compensation for development and mercury efflux, whereas past acidification lacked the barrier of H+ and had dysfunction in the detoxification and efflux system, providing a mechanistic understanding of mercury toxicity under different acidification scenarios. Furthermore, we conducted a meta-analysis on marine animals, demonstrating that different acidification scenarios could alter the toxicity of several other metals, despite evidence from nonsynchronous scenarios remaining limited. Our study thus demonstrates that time and duration of ocean acidification modulate mercury toxicity in marine copepods and suggests that future studies should move beyond the oversimplified scenario of perfect synchrony in understanding multistress interaction.

Properties influencing flux and diatom uptake of mercury and methylmercury from estuarine sediments

Mercury (Hg) is a conspicuous and persistent global pollutant. Ionic Hg can be methylated into noxious methylmercury (CH3Hg), which biomagnifies in marine tropic webs and poses a health risk to humans and organisms. Sediment Hg methylation rates are variable, and the output flux of created CH3Hg are dependent on sediment characteristics and environmental factors. Thus, uncertainties remain about the formation and flux of CH3Hg from sediment, and how this could contribute to the bioaccumulative burden for coastal organisms in shallow ecosystems. Cores were collected from 3 estuarine locations along the Eastern USA to examine how sediments characteristics influence the introduction of Hg and CH3Hg into the base of the food chain. Stable isotopes of inorganic 200Hg and CH3199Hg were injected into sediments of individual cores, with cultured diatoms constrained to overlying waters. Five different treatments were done on duplicate cores, spiked with: (1) no Hg isotopes (control); (2) inorganic 200Hg; (3) CH3199Hg; (4) both 200Hg and CH3199Hg isotopes, (5) both 200Hg and CH3199Hg into overlying waters (not sediment). Experimental cores were incubated for 3 days under temperature and light controlled conditions. These results demonstrate that upper sediments characteristics lead to high variability in Hg cycling. Notably, sediments which contained abundant and peaty organic material (∼28 %LOI), had the highest pore water DOC (3206 μM) and displayed bands of sulfur reducing bacteria yielded the greatest methylation rate (1.97 % day-1) and subsequent diatom uptake of CH3200Hg (cell quota 0.18 amol/cell) in the overlying water.

Exploring the subcellular distribution of mercury in green alga Chlamydomonas reinhardtii and diatom Cyclotella meneghiniana: A comparative study

Mercury (Hg) is a priority pollutant of global concern because of its toxicity, its ability to bioaccumulate throughout the food web and reach significant concentrations in top predators. Phytoplankton bioconcentrate large amounts of Hg and play a key role in the entry of Hg into the aquatic food web. However, the subcellular distribution of Hg in freshwater phytoplankton, known to affect it toxicity and trophic transfer is understudied. The present study aimed at investigating the accumulation of inorganic Hg (iHg) and its subcellular distribution in freshwater phytoplankton species. To this end green alga Chlamydomonas reinhardtii and diatom Cyclotella meneghiniana were exposed to 10 and 100 nM of iHg for 2 h. The concentrations of Hg in the adsorbed, intracellular and subcellular (granules, debris, organelles, heat-stable peptides (HSP) and heat-denaturable proteins (HDP)) fractions were determined. The results showed that C. meneghiniana accumulated more Hg compared to C. reinhardtii at both iHg exposure concentrations (10 nM: 4.41 ± 0.74 vs. 1.10 ± 0.25 amol cell-1; 100 nM: 79.35 ± 10.78 vs. 38.31 ± 4.15 amol cell-1). The evaluation of the subcellular distribution of Hg, revealed that the majority of Hg was concentrated in the organelles fraction (59.7 % and 74.6 %) in the green algae. In the diatom, Hg was mainly found in the organelles (40.9 % and 33.3%) and in the HSP fractions (26.8 % and 40.1 %). The proportion of Hg in HDP fraction decreased in favor of the organelles fraction in C. reinhardtii when the exposure concentration increased, whereas the proportions in the debris and organelles fractions decreased in favor of HSP fraction in C. meneghiniana. This study provides pioneering information on the subcellular distribution of Hg within in freshwater phytoplankton, a knowledge that is essential to understand the toxicity and trophic transfer of Hg in contaminated aquatic environment.

Linking seasonal plankton succession and cellular trace metal dynamics in marine assemblages

Factors affecting trace metal dynamics in marine plankton still need to be fully understood. Underlying mechanisms affecting cellular metal distribution, seasonal changes, and the influence of plankton community structure are poorly explored. This study comprehensively analyzed the seasonal changes in environmental factors, plankton community structure, and their impact on plankton cellular metal dynamics. Plankton samples were isolated, and trace metals (Cr, Mn, Fe, Co, Ni, Cu, As, Cd, Hg, and Pb) were analyzed with an inductively coupled plasma mass spectrometer (ICP-MS). Plankton community structure significantly changed with seasons (p < 0.05), which were mainly driven by temperature (seasonal change) and nutrients (eutrophication). Mean plankton cellular trace metals did not significantly change (p > 0.05) in the study area but were higher along estuaries likely due to differences in metal influx from rivers. However, their distribution patterns significantly differ between the wet and dry seasons, likely influenced by the changes in community structure and anthropogenic influx. Cellular trace metals, particularly in phytoplankton, strongly correlated with selected species suggesting the impacts of community structure in trace metal distribution. Hence, the influence of environmental factors in driving plankton succession may have caused a ripple effect on cellular trace metal distribution, especially in phytoplankton. However, both blooming species Skeletonema and Chaetoceros (diatoms) showed a contrasting relationship with cellular metals, suggesting the cooccurrence of bioaccumulation or biodilution mechanisms. This study shows the potential influence of community structure in cellular trace metal dynamics for marine plankton assemblages. However, more than plankton abundance and functional diversity, i.e., species diversity, might be needed to assess the community-level impacts on cellular metals.

Dissolved organic matter thiol concentrations determine methylmercury bioavailability across the terrestrial-marine aquatic continuum

The most critical step for methylmercury (MeHg) bioaccumulation in aquatic food webs is phytoplankton uptake of dissolved MeHg. Dissolved organic matter (DOM) has been known to influence MeHg uptake, but the mechanisms have remained unclear. Here we show that the concentration of DOM-associated thiol functional groups (DOM-RSH) varies substantially across contrasting aquatic systems and dictates MeHg speciation and bioavailability to phytoplankton. Across our 20 study sites, DOM-RSH concentrations decrease 40-fold from terrestrial to marine environments whereas dissolved organic carbon (DOC), the typical proxy for MeHg binding sites in DOM, only has a 5-fold decrease. MeHg accumulation into phytoplankton is shown to be directly linked to the concentration of specific MeHg binding sites (DOM-RSH), rather than DOC. Therefore, MeHg bioavailability increases systematically across the terrestrial-marine aquatic continuum as the DOM-RSH concentration decreases. Our results strongly suggest that measuring DOM-RSH concentrations will improve empirical models in phytoplankton uptake studies and will form a refined basis for modeling MeHg incorporation in aquatic food webs under various environmental conditions.

Mercury fractionation - Problems in method application

Mercury (Hg) is a global pollutant with a negative effect on human and ecosystem health. Mercury is toxic in all forms. The toxicity, however, varies depending on the form of mercury, determining its physical and chemical properties. Therefore, knowledge on the chemical speciation of mercury is key for the understanding of its transport and transformations in the environment. Analysis of mercury speciation, however, is time-consuming and involves high risk of contamination. The mercury thermodesorption method offers many new possibilities. The main advantages of this method are identifying which groups of compounds are being transformed in the atmosphere, sediment and soil, suspended matter and plankton, and in organisms from different trophic levels. A great advantage of the method is also its application in mercury analyzers, where it is possible to control the heating and cooling temperatures of. The standardisation of fractionation nomenclature for all matrices (both biotic and abiotic) will be helpful in application of this mercury fractionation method too. It has also disadvantages, mostly in the technical preparation of the analyzer. The analyzer must be prepared for fractionation: setting the ventilator and adjusting the PID parameters so that the pre-set heating (t1) and combustion (t2) times reach the set value in the method program. Also, any modification of the heater forces a re-optimisation of the method with mercury standards, as certified reference materials for Hg fractionation in environmental matrices are not available. The HgF2 fraction cannot be used as the methylmercury concentration, which is undoubtedly the biggest drawback of this method.

Non-proportional distribution and bioaccumulation of metals between phytoplankton and zooplankton in coastal waters

Metal concentrations were concurrently quantified in seawater, phytoplankton, and zooplankton from a heavily impacted coast of southern Taiwan. Combined size and density fractionation were used to accurately quantify metal concentrations in phytoplankton. Cr, Co, Ni, Cu, As, and Pb were analyzed using an inductively coupled plasma mass spectrometer (ICP-MS). As expected, metals significantly increased with an order of seawater < phytoplankton < zooplankton (p < 0.05); but did not differ between estuarine, nearshore, and offshore sites (p > 0.05). Metals were higher along Kaohsiung Harbor and marine outfall diffusion sites, highlighting their major impacts on plankton metal contamination. Notably, phytoplankton (Cr BCF > 100; half of the sites) significantly accumulated more metals contrary to zooplankton (BAF < 10). Metal concentrations and bioaccumulation factors between phytoplankton and zooplankton showed significant negative correlations. This demonstrates a non-proportional distribution and bioaccumulation of metals in phytoplankton and zooplankton-corroborating laboratory findings on zooplankton ability to control metals, irrespective of significantly high bioaccumulation in phytoplankton.

Plankton population dynamics and methylmercury bioaccumulation in the pelagic food web of mine-impacted surface water reservoirs

Thermal stratification of reservoirs can lead to anaerobic conditions that facilitate the microbial conversion of mercury (Hg) to neurotoxic and bioaccumulative methylmercury (MeHg). But MeHg production is just the first step in a complex set of processes that affect MeHg in fish. Of particular relevance is uptake into suspended particulate matter (SPM) and zooplankton at the base of the pelagic food web. We assessed plankton dynamics and Hg uptake into the pelagic food web of four Hg-impaired California water reservoirs. Combining water chemistry, plankton taxonomy, and stable carbon (C) and nitrogen (N) isotope values of SPM and zooplankton samples, we investigated differences among the reservoirs that may contribute to differing patterns in MeHg bioaccumulation. Methylmercury accumulated in SPM during the spring and summer seasons. Percent MeHg (MeHg/Hg*100%) in SPM was negatively associated with δ¹⁵N values, suggesting that "fresh" algal biomass could support the production and bioaccumulation of MeHg. Zooplankton δ¹³C values were correlated with SPM δ¹³C values in the epilimnion, suggesting that zooplankton primarily feed in surface waters. However, zooplankton MeHg was poorly associated with MeHg in SPM. Our results demonstrate seasonal patterns in biological MeHg uptake and how multiple data sources can help constrain the drivers of MeHg bioaccumulation.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10750-022-05018-0.

Dual role of titanium dioxide nanoparticles in the accumulation of inorganic and methyl mercury by crustacean Daphnia magna through waterborne and dietary exposure

Titanium dioxide nanoparticles (nTiO₂) are widely used in numerous products, yet their role in the accumulation and transfer of other contaminants in the aquatic food webs is not well understood. The influence of nTiO₂ on inorganic (IHg) and monomethyl mercury (MeHg) accumulation in invertebrate Daphnia magna through waterborne and dietary exposure was thus thoroughly investigated. The results showed that nTiO₂ led to a substantial decrease of the total mercury body burden (THg) in D. magna in direct waterborne exposure to IHg/MeHg. However, exposure to nTiO₂ pre-treated with IHg/MeHg resulted in an increase of the THg body burden in daphnids. The presence of nTiO₂ led to a substantial decrease of the THg in D. magna when exposed to IHg/MeHg via algal food. These effects were more pronounced for IHg than that for MeHg due to the higher adsorption capabilities of nTiO₂ for IHg. In addition, high concentrations of nTiO₂ favored the trophic transfer of IHg/MeHg through feeding on nTiO₂ pre-treated with Hg, however lessened it when D. magna were fed on alga pre-treated with IHg/MeHg. Comparable assimilation efficiency (AE), determined as Hg retained in daphnids after depuration, was observed in D. magna when exposed to IHg/MeHg via algal food regardless the absence or presence of 20 mgL^-1 nTiO₂. By contrast, an increase of the AE of MeHg through feeding on nTiO₂ and alga was found in the presence of higher concentration of 200 mgL^-1 nTiO₂. The present results will help to better understand the role of nTiO₂ on bioavailability and trophic transfer of global contaminants, such as mercury, known to bioaccumulate and biomagnify in the aquatic environment.