Mercury and selenium concentrations in the oysters Saccostrea palmula and Crassostrea corteziensis from four coastal lagoons of NW Mexico: health risk assessment

Mercury (Hg) and selenium (Se) concentrations in the soft tissue of the oysters Saccostrea palmula and Crassostrea corteziensis from four coastal lagoons (Altata, Macapule, Navachiste, El Colorado) of Northwest Mexico were determined. A total of 2520 specimens of S. palmula and 3780 of C. corteziensis (72.15 ± 4.95 and 73.57 ± 5.31 mm, respectively) were sampled seasonally from summer 2019 to spring 2020. The higher Hg concentration (wet weight) for S. palmula (0.13 ± 0.03–0.11 ± 0.02 µg g −1 ) and C. corteziensis (0.12 ± 0.02–0.11 ± 0.03 µg g −1 ) were obtained in summer−autumn 2019; Hg level was similar among the coastal lagoons and did not exceed the limit established by Mexican legislation and by the Food and Drug Agency. The higher Se concentration occurred in spring 2020 for both oyster species in El Colorado lagoon (4.55 ± 0.02 and 4.08 ± 0.05 µg g −1 , respectively). The Se/Hg molar ratio of S. palmula and C. corteziensis ranged from 48.76–149.59 and 31.72–155.37, respectively; while the Se health benet value was 19.23–42.28 and 17.82–35.30, respectively. The hazard quotient for Hg estimated as methylmercury- and Se in both species of oyster was below 1. The high molar ratio obtained (Se/Hg > 1) indicates that the concentration of Se was sucient to neutralize the possible toxicity of the Hg, therefore, the consumption of S. palmula and C. corteziensis from the four coastal lagoons studied does not represent a risk.


Introduction
Metals and metalloids are pollutants that represent a serious environmental global concern; these elements are characterized by their toxicity, persistence, bioaccumulation, and biomagni cation in the ecosystem ( (Apeti et al. 2012), and, therefore, many organisms are exposed to these elements to some extent (Azad et al. 2019).
In the environment, Hg is a highly toxic metal without any known biological function; in fact, it is the element responsible of various environmental and human health problems, even at low concentrations Selenium is an essential metalloid that plays an important role in the formation of selenocysteine-rich proteins within the cells of most organisms (Stroud et al. 2010; Vega-Sánchez et al. 2020); it also intervenes in brain functions, growth, thyroid hormone metabolism, calcium regulation, and prevents/reverses oxidative stress in humans ( Diet (food and water) is considered the main absorption route of essential and toxic elements for humans (Mok et al. 2015). Due to their physicochemical properties, some metals and metalloids can accumulate to high levels through the consumption of certain food, such as shell sh (including oysters) (Zhang and consequently, it leads to a situation of potential risk to human health (Goyer 1997). On the other hand, knowledge on the Se dynamic in both oyster species is scarce. There is only one report that evaluates the presence of Se in the oysters C. gigas and C. virginica from the San Francisco Bay (California, USA) (Okazaki and Panietz 1981), and no information on the Se/Hg molar ratio in oysters (Scopus 2021) is available. Thus, research related to Se in S. palmula and C. corteziensis is necessary to determine its protective effect against mercury toxicity and to establish reference information for comparative studies.
The aim of this study was to determine the total concentrations of mercury and selenium of S. palmula and C. corteziensis in four coastal lagoons (Altata, Macapule, Navachiste and El Colorado) of Northwest Mexico, to estimate the human health bene t and risk throught the Se/Hg molar ratios, the selenium health bene t value (HBV Se ), and the hazard quotient (HQ). In addition, the Hg contamination status of oysters from other lagoons in NW Mexico and other regions of the world was compared. It is hypothesized that the Hg and Se concentrations of the oysters in this study do not represent a risk for the environment or for human health.

Field sampling
Oyster samplings were carried out seasonally (summer 2019 to spring 2020) in ten sites distributed within four coastal lagoons of NW Mexico: Altata (AL, three sites), Macapule (ML, two sites), Navachiste (NL, three sites) and El Colorado (ECL, two sites) (Fig. 1). However, oyster species did not coexist at all sampling sites; therefore, S. palmula was collected in six sites and C. corteziensis in nine. From each site and by species, 105 oysters were selected (75 for Hg and Se analysis and 30 for condition index determination, CI; Muñoz-Sevilla et al. 2017) with a length interval of 60 to 90 mm, to avoid variability in metal content due to differences in specimen size ).
The oysters were detached from the mangrove roots by hand, placed in metal-free polyethylene bags and transported on ice to the laboratory, where they were cleaned, measured (mm), weighed (g), and chipped (Frías-Espericueta et al. 2018). The CI was determined using the equation (1) proposed by Walne and Mann (1975).

Chemical analysis
Oyster soft tissue samples were lyophilized (Labconco, Kansas City, MO, USA) at low temperature (-85 °C) and high vacuum (0.035 mBar) for 96 h; then, they were ground in a Te on mortar, homogenized by quartering, and the water content (%) was determined. Two ~ 1.0 g aliquots of each sample were predigested (~ 12 h) at room temperature with 10 mL of concentrated nitric acid (HNO 3 , trace metal grade) in sealed Te on vials (Savillex, 60 mL). Digestion was performed on a heating plate (Barnstead) for 4 h at 120 °C; subsequently, each sample was diluted with deionized water to a nal volume of 50 mL Quality assurance/quality control The precision of the analytical method and the results were validated by using certi ed reference material (DOLT-5 ® ; National Research Council Canada, NRCC, USA), with recovery percentages of 116.76 ± 4.46 and 90.29 ± 1.90% for Hg and Se (n = 10 for both elements), respectively. Analytical blanks were used to check samples for possible contamination, and all materials used in the sampling and in the laboratory were acid washed (Moody and Lindstrom 1977). The detection limits were 0.002 and 0.013 µg g -1 for Hg and Se, respectively. The coe cient of variation for duplicate samples was < 7.67%. Mercury and Se concentrations are expressed as µg g -1 wet weight (ww, 80.74 ± 1.60 % moisture content was used for all samples).
Se/Hg molar ratio and health bene t The Se/Hg molar ratio in the oysters was calculated according to Burger and Gochfeld (2013), dividing the concentration of the element by its molecular weight (78.96 and 200.59 g mol -1 for Se and Hg, respectively). The selenium health bene t value (HBV Se ) was calculated using the equation (2) proposed by Ralston et al. (2016).
For the estimation of HBV Se , the element concentration is presented as µmol kg −1 ww. A positive HBV Se value is considered healthy, while a negative value indicates health risks associated with Hg exposure; the magnitude of the value means the degree of surplus or de cit of selenium, related to the consumption of oysters.

Health risk assessment
The hazard quotient (HQ) was used to estimate the potential risk to human health due to the level of exposure and/or ingestion of any contaminant. It was calculated for Hg and Se with the equation (3) proposed by Newman and Unger (2002).

Statistical analyses
Normality and homoscedasticity of data were analyzed with the Kolmogorov-Smirnov and Bartlett tests, respectively. Comparisons of the biometric data of oysters and average concentrations of Hg and Se among the coastal lagoons, sampling sites, and seasons of the year, were assessed by ANOVA's test. Correlations between the concentrations of mercury, selenium, and the molar ratios (Se/Hg) with the size, total weight, and condition index of oysters were estimated through a Pearson correlation analysis (r p ).
The differences between the content of Hg and Se in S. palmula and C. corteziensis were detected with the Student's t test for independent variables. The level of signi cance was a = 0.05 for all statistical analyzes (Zar 2010), which were performed using the STATISTICA 7 software package (StatSoft, Tulsa, OK, USA).

Results
Size, total weight, and condition index of oysters Size, total weight, and condition index (CI) of S. palmula (n = 2520) and C. corteziensis (n = 3780) collected in the four coastal lagoons (NW Mexico), are presented in Table 1. The annual mean value of the shell size in both species did not present signi cant differences (p > 0.05) among the sampling sites. Total weight of S. palmula (35.19 ± 7.64 g) and C. corteziensis (49.11 ± 7.97 g) were signi cantly higher at the ECL9 and NL7 sites, respectively; while the CI in both species of oysters was higher in Macapule lagoon (50.23 ± 17.05 and 51.04 ± 16.61 for S. palmula and C. corteziensis, respectively) (p < 0.05, Table  1).

Mercury And Selenium In The Soft Tissue Of Oysters
The total Hg concentration in S. palmula and C. corteziensis presented a range between 0.03-0.16 and 0.02-0.17 µg g −1 , respectively. In most of the sampling sites, the higher and lower seasonal mean concentrations of Hg were recorded in summer−autumn 2019 and winter−spring 2020 (p < 0.05, Table 2), respectively. In both oyster species, the level of Se presented an order of magnitude higher than those values determined for Hg. The Se content in S. palmula and C. corteziensis showed a seasonal variation, with intervals of 0.99-4.55 and 0.81-4.08 µg g −1 , respectively. In both species, the Se showed a tendency to increase seasonally; the lowest concentrations were recorded in summer 2019 and increased in autumn 2019; subsequently, the level of this metaliod increased in winter 2020, and its maximum level was reached in spring 2020 (p < 0.05, Table 2). No relationship (p 0.05) was observed between Hg and Se with the shell height and total weight of S. palmula and C. corteziensis. CI showed correlation with Hg (r p = -0.29, p < 0.05), but no correlation was found between Se and CI (p 0.05).   (Table 3). No correlations of Se/Hg were observed between length and total weight in none of the ostreid species (p > 0.05). The Se/Hg molar ratio of S. palmula was correlated with CI (r p = 0.30, p 0.05). The HBV Se obtained for S. palmula and C. corteziensis indicates that Se concentration found in the two oyster species represents a bene t for human health (Table 3).

Health Risk Assessment
The HQ value in oysters did not exceed the safe level (HQ = 1) for people with mean body weights of 70 kg (adult men), 60 kg (adult women) and 16 kg ( ve year-old children) (Table 4).

Mercury
The total Hg in the soft tissue of S. palmula and C. corteziensis ranged from 0.03-0.16 and 0.02-0.17 µg g −1 (wet weight), respectively. In relation to other studies in Mexico, the Hg concentrations are similar to those obtained in this study (Table 5), but lower than those found for C. virginica in the Términos lagoon (Gulf of Mexico), mainly affected by oil, agricultural and urban activities (Aguilar et al. 2012). Table 5 Total mercury range concentrations (µg g −1 ww) in different oyster species from some coastal lagoons of NW Mexico and other worldwide regions   (Eisler 1987) and in some coastal and marine regions of Italy where the Japanese oyster is cultivated (Burioli et al. 2017) ( Table 5). These differences could be due to the abundance of Hg in the continental crust (on mean it is higher than 0.056 µg g −1 ), since the level of Hg in the world is not homogeneous and some regions are more enriched than others (Wedepohl 1995;Rytuba 2003). Also, Hg concentration can vary among oyster species due to biological aspects, related to their metabolic activity, The annual mean concentration of Hg in both oyster species did not show signi cant differences (p 0.05) among the sampling sites (Table 3)

Selenium
Information about the effect of Se on oysters is scarce. Okazaki and Panietz (1981) reported that the level of this metalloid was higher than that of Hg, in different tissues (mantle, gill and digestive gland) of C. gigas and C. virginica, from San Francisco Bay (California, USA), which agrees with the present study. However, the concentration of Se reported by these authors is lower than those determined in S. palmula and C. corteziensis (Table 2). This could be due to factors such as the bioavailability of this element in coastal lagoons, as well as the sizes and stages of the organisms (Jara- In the case of S. palmula and C. corteziensis collected from the same mangrove roots in sites AL2, ML5, NL6, NL8 and ECL9 (Fig. 1), we hypothesized that the total Hg and Se concentrations were similar between both species, probably because the oysters were under the same water conditions and exposure levels. According to Trudel and Rasmussen (2006), there is a direct correlation between the Hg content and the age or body size of aquatic organisms, due to the greater exposure of the metal, which is excreted more slowly as it is ingested. Acosta and Lodeiros (2004)

Health Risk Assessment
Recent studies highlight the importance of evaluating the potential risk to human health from the ingestion of metals and metalloids contained in commercially important shery products (including oysters). This is done to identify populations at risk, especially in cities and coastal communities dedicated to the capture and production of seafood, where speci cally, the annual consumption of oysters is double or more (Delgado-Alvarez 2015) than established as national apparent consumption

Conclusions
Obtained results indicate that Hg concentrations in S. palmula and C. corteziensis were similar among the four coastal lagoons (NW of Mexico) and the highest concentrations of this metal occurred in summer−autumn 2019, without exceeding the maximum permissible limit of 1.0 µg g −1 established by national and international regulations. On the other hand, the total concentrations of Se in both species of oysters were signi cantly higher in the El Colorado lagoon during spring 2020. No signi cant differences (p 0.05) were detected in the levels of Hg and Se between S. palmula and C. corteziensis, which is explained by sharing the same habitat, implying being exposed to the same water conditions, exposure levels of the elements and feeding on the same resources. The negative correlation observed between Hg and CI in S. palmula, is probably related to its reproductive cycle. The Se/Hg molar ratio is reported for the rst time in S. palmula and C. corteziensis; Hg toxicity was neutralized by Se, indicating that consumption of raw oysters does not represent a risk.

Declarations
Ethics approval and consent to participate "Not applicable" in this section.
Consent for publication "Not applicable" in this section.
Availability of data and materials The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.
Competing interests The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper.  Figure 1