Abstract

Increases in mercury concentrations in biota over the past decades have sparked an increasing amount of studies to determine the effects of mercury contamination on the environment and human health. According to WHO, food is the main source of mercury presence in non-occupationally exposed populations; the mean dietary intake of mercury in various countries ranges from 2 to 20 µg/day per person. In this study, total mercury (THg) concentration was determined in biota species collected in the Adriatic and Ionian Sea fishing regions. Obtained results revealed that species collected in the Ionian Sea (Saranda region) exhibited higher concentration of THg compared to species of the Adriatic Sea. The highest concentration was found in Mullus surmuletus, (0.102 mg/kg ww) while the mean values of THg ranged between 0.003 – 0.102 mg/kg ww. The same trend of THg abundance in biota was followed even for the bioconcentration factors, BCFs. Total concentration of Hg in all selected species did not exceed the recommended values, based on EU, WHO and FDA regulations. Estimation of average daily intake, EDI and Total Hazard Quotient, THQ values revealed that that consumption of sea food is safe to regional population.

1. Introduction

Mercury and its chemical forms are amongst the most dangerous substances that can be present in the environment ^{1, 2}. It is one of the most enduring pollutants, arising from both natural and human activity ². Natural atmospheric sources of mercury include volcanoes eruption, weathering of rocks, forest fires, ocean water evaporation, etc., while incineration of medical and urban wastes as well as burning of fossil fuels contribute to the anthropogenic sources of mercury ³. In the atmosphere, it can reside for a period of approximately one year, while can be transported for more than thousands kilometres from the emission point sources before it is deposited on terrestrial or aquatic media.

It is estimated that the deposition time of Hg from the moment of its emission varies from 5-14 days up to 1 year, during which it can be transported around the globe ^{3, 4, 5}.

Mercury persists in the environment for prolonged periods by fluctuating between the atmosphere and soil, while concurrently transforming its chemical forms ^{3, 4, 5}. The atmospheric lifespan of inorganic elemental mercury is anticipated to be two years, whereas organic methyl mercury may endure in soils for decades. Mercury is not eradicated from the environment; it is simply transferred to other locations and ultimately deposited beneath soils and sediments ^{4, 5}.

Over the years, emissions of Hg to the environment tend to rise ⁵. For example, the quantity of anthropogenic Hg released to the environment was around 1.5 million tons in 2010, of which 500 thousand tons had been emitted to the atmosphere and about 1.2 million tons were released to land and water systems ^{5, 6}.

Mercury (Hg), included among heavy elements, lacks any recognized beneficiary biological role ⁷. Even minimal amounts of mercury in the human body can induce critical disruptions in metabolic processes, resulting in irreversible damages of the neurological system and cognitive function ^{7, 8}. Beside, cardiovascular disorders are caused due to mutagenic and hepatotoxic properties of this element ^{7, 9, 10}.

Among the species of mercury, methylmercury (MeHg) is known as the most bioavailable and toxic, formed through the methylation of inorganic mercury by microorganisms ¹¹. Usually, methylation of Hg (II) in water media occurs in the interface between the oxic and anoxic zones, meaning the upper part of the sediments where the oxygen concentration is depleted ^{11, 12}. These conditions favour at a large scale the methylation of inorganic mercury by the sulphate reducing bacteria ¹³. Among the factors that increase the rate of mercury methylation include temprerature, dissolved organic matter (when it is used as energy source by microorganisms), pH (being higher in lower pH), concentration of bioavailable Hg²⁺, etc., ^{14, 15}. Higher concentration of complex ligands and organic matter that binds Hg²⁺ may decrease the degree of the mercury methylation ¹⁵.

Consumption of big fishes or their special parts of the body, like eggs, etc., by humans can induce an increased risk of dietary mercury exposure ^{14, 15}. The adverse health consequences of MeHg exposure have heightened global awareness of the consumption of marine and freshwater species that may have elevated MeHg levels, particularly among numerous vulnerable communities ¹⁶. The Minamata incident in 1953 led to an increased attention from scientists regarding the devastating health effects of mercury. In 1973, the 2^nd Minamata Disease Research Group proposed that mercury exposure from eating fish could have long-term consequences ^{17, 18}. As a testament to the significance of tackling the environmental problem of mercury, the United Nations Environment Program (UNEP) facilitated the adoption of the Minamata Convention on Mercury in 2013, which became enforceable on August 16, 2017, following 74 countries' ratification ^{18, 19, 20, 21}. The Convention forbids the manufacturing of various mercury-emitting processes and urges the establishment and enhancement of environmental mercury monitoring initiatives ^{18, 20}.

According to the European Commission, mercury is included in the list of the priority substances (European Commission 2013), advising monitoring to be conducted in the biota matrix under the EU Water Framework Directive (European Commission 2000), ²². A threshold value for mercury is established just for the major matrices of fish muscle and mussel soft tissue, set at 0.02 mg/kg. The World Health Organization indicates that food is the primary source of mercury for non-occupationally exposed populations, with the average dietary intake of mercury across different nations varying from 2 to 20 µg/day per individual, ²³.

In 2004, The European Food Safety Authority (EFSA) issued a scientific report on mercury and methyl mercury in food, but mainly focusing in methylmercury ²⁴. But, European Commission found this report not complete, considering that new developments in mercury toxicity have raised the awareness in the following years, ²⁴. Main concerning issues were the re-evaluation of the appropriateness of the provisional tolerable weekly intake (PTWIs) limits set by FAO/WHO Expert Committee on Food Additives (JECFA) 1.6 μg/kg body weight (b.w.) for methylmercury and of 4 μg/kg b.w. for inorganic mercury ²⁵. The Panel also concluded that the occurrence data available at that time did not allow reliable estimations of the intakes by high consumers in different populations ^{25, 26}.

In 2018, the CONTAM Panel set a tolerable weekly intake (TWI) for methylmercury at 1.3 μg/kg body weight, represented as mercury, and a TWI for inorganic mercury at 4 μg/kg body weight, also expressed as mercury.

Prior to the 1990s, Albania experienced a phase of industrial advancement encompassing the mineral, metal, and chemical sectors. The cessation of these activities without a decommissioning plan to mitigate pollution has resulted in significant environmental repercussions. The most evident example was the soda-PVC manufacturing facility where mercury was utilized to make caustic soda and PVC. Subsequent to the cessation of operations at this factory, about 60 tons of metallic mercury was released into the Vlora Bay area, directly contaminating the ecosystem.

In 2002, a UNEP/MAP identification mission (GEF Project GF/ME/6030-00-08) classified this site as a "hot spot" after a soil sample indicated mercury concentrations above 10,000 ppm, which is 1,000 times greater than the usual EU threshold ²⁷. Vlora Bay is a prominent fishing area, with its harvest distributed throughout all cities in Albania.

This paper presents results obtained regarding the exposure degree to total Hg of marine biota species in Adriatic and Ionian Seas fishing areas.

2. Materials and Methods

In total, 62 specimens were collected from January, 2023 to June 2023. Fresh fish samples were obtained randomly from the fisherman of the main coastal regions of Adriatic and Ionian Sea in Albania, respectively in Vlora, Durresi and Saranda cities ²⁷. Selected samples were packed in polyethylene ziplock bags, labelled, and transferred to the laboratory in cooler boxes.

Upon their arrival, samples were washed with deionized water, drained and representative parts of the body were stored in plastic bags at -40 °C freezer until the day of analysis. About 1.0 g of each biota sample (3 replicates) was weighed in Tefon digestion vessels where 10 ml of ultrapure HNO₃ (69 %) and 1 ml of H₂O₂ (30 %) was added for digestion. Samples were left in closed Teflon vessels for 24 hours in ambient temperature and for 3-5 hours in temperature 70^oC. After cooling, 5 ml of K₂Cr₂O₇ 5% was added aiming to preserve Hg²⁺ from reduction.

The same digestion procedure was applied also for blanks and for certified reference material, CRM.

The total concentration of mercury in biota samples was assessed by using the Cold Vapour Atomic Absorption Spectroscopy (CVAAS). The Analytic Jena 800F spectrometer was employed for this purpose, with a quartz cell positioned on the instrument's burner head, linked to the reaction cell with a Teflon tube. Mercury vapour was produced by the reaction of Hg²⁺ in samples and/or standards with 15% SnCl₂ in the presence of H₂SO₄. Atomic mercury generated in the reaction cell was sent to the quartz cell via an air current.

The entire volume of the digested sample was added into the reaction cell, together with 5 ml of 15% SnCl₂ and 2 ml of concentrated H₂SO₄. To minimize matrix interferences from the complex biota samples, the signals of standards were measured by directly incorporating them into the sample solutions. Given that various biota samples may demonstrate distinct matrix interferences, we established calibration curves for each sample. The procedure involved adding the sample to the reaction cell and measuring absorbance. Then, presuming no residual mercury remains, three spikes of the pure Hg standard (5, 10, and 20 µl of the 2.5 ppm Hg standard, corresponding to 0.0125, 0.0250, and 0.05 µg respectively) were introduced for the respective sample, followed by absorbance measurement. Concentration was then calculated by means of the calibration curve.

The calibration curves were linear within the range of mercury contents (regression coefficients R² ≥ 0.999). The detection limit (LOD) of the CVAAS technique was 0.005 mg/kg. The lyophilized certified material (IAEA 407 fish homogenate, provided by IAEA Environment Laboratories) was also analyzed along with the considered samples for the content of Hg. The recovery rates ranged between 92–108.0%. Statistical treatment of the obtained results was carried out by using MINITAB 22 statistical software. In cases of the results under the detection limit of the method, values were replaced with half of the LOD value. The contamination degree of biota samples with heavy metals was evaluated by comparing obtained results with the values recommended by WHO/FAO, 1997, EFSA and European Commission, 2022. ^{25, 26, 29}.

High concentrations of metals in saltwater can build to hazardous levels in marine organisms. Bioconcentration denotes the direct transfer of a chemical from the surrounding environmental medium into an organism, excluding uptake through food ³⁰. Fish can acquire harmful components from contaminated water, absorption of suspended particulates, consumption of food, adsorption through tissues or skin, and lipophilic tissues such as gills ^{31, 32}.

Numerous factors can influence metal intake and accumulation, including sex, age, size, reproductive cycle, swimming activity, feeding habits, and geographical location ^{31, 32}. The differing affinities of metals for fish tissues, along with variations in absorption, deposition, and excretion rates, result in disparities in bioaccumulation inside fish bodies ³³.

The bio-concentration factor (BCF) was computed in this investigation to assess the accumulation levels of each metal using the equation shown below ³⁴:

(1)

where: C_m is the metal concentration in marine organism (mg/kg) and C_sw is the same metal concentration in the seawater (mg/L).

The estimated daily intake (EDI) is contingent upon the metal concentration in species, fish consumption, and body weight. The FAO (2024) projected the per capita seafood intake in Albania at 8.68 kg per year, equating to around 23.8 g per day ³⁵. The estimated daily intake (EDI) was computed as:

(2)

where: C is metal concentration (mg/kg) in biota species; Cons. Is the average daily consumption of sea food; Bw is the average body weight of adult people in Albania (70 kg). The acquired results were juxtaposed with dietary reference intake (DRI) values for necessary and benign components, as well as with risk reference values for toxic elements. ^{36, 37, 38}.

The non-carcinogenic effect, quantified as the target hazard quotient (THQ), is defined as the ratio of prospective exposure to a chemical and the threshold level at which no adverse effects are anticipated, derived using the equation:

(3)

where: EDI is the estimated daily intake and RfD reference dose which for Hg is 0.0001 mg/kg/day. A THQ value below 1 indicates no adverse effect for human health; if THQ is greater than 1, then adverse health effects are possible ^{39, 40, 41, 42}.

3. Results and Discussion

In total, 62 biota species were selected, belonging to marine and fresh water environments. Respectively, 26 species were collected in Saranda city; 23 species in Vlora region; 13 species in Durresi region. Among the biota samples, eggs, gills and liver tissues were also analyzed for some of the species.

A summary of the obtained results is presented in Table 1 while in Figure 1 it is presented the distribution of THg concentration in each selected sample. Results are expressed in mg/kg (ww).

THg concentration ranged between 0.034 to 0.059 mg/kg ww in species collected in the Adriatic Sea and from 0.007 to 0.102 mg/kg in Ionia Sea species.

The highest concentration of THg was found in Mullus surmuletus, (0.102 mg/kg) and Pagellus acarne (0.080 mg/kg), both collected in Saranda region while the THg content of the rest of the other species varied from 0.003 to 0.074 mg/kg.

Based on our results, it was concluded that concentration of THg in selected species did not exceed the values recommended by EC (0.3-05 mg/kg), EFSA (0.28 mg/kg) and WHO/FAO (0.50 mg/kg).

By integrating the quantified mercury concentrations in seafood (averaging from 0.003 to 0.102 mg/kg across various species), the average seafood consumption rate, and the current US EPA mercury reference dose (0.0001 mg/kg/d), it was determined the number of meals per month for adults only for species collected in the Ionian sea, where the highest concentrations were observed. Results are presented in Figures 1-4. Obtained results revealed that the number of meals per month varied between 8-16.

Mullus Surmulletus was identified as the species which should be consumed in a moderate quantity, while Sardinella aurita; Diplodus vulgaris; Trigla lyra; Pagellus acarne; eggs of Merlucius merlucius and Sepia officinalis species are classified as per 12 meals per month.

The rest of the species, including the ones collected in the Adriatic Sea can be consumed for up to 16 meals per month.

Bioconcentration factors for selected species implies calculation of the ratio of Hg content in a certain species (mg/kg) divided by the concentration of Hg in respective water media.

Concentration of Hg in water of the regions where the selected species were collected was taken from the existing literature. In 2007 and 2015, Ogrinic et.al., 2015, ⁴⁴, have studied mercury speciation in Ionian and Adriatic seas, respectively and have determined the average concentration of total mercury in areas including Albania regions. Based on their results, the average concentration of Hg in the Adriatik sea was found to be 3.3 pM (corresponding to 0.66 ng/L while for Ionian Sea the average concentration was about 1.3 pM (corresponding to 0.26 ng/L). This value was used to calculate the BCF for biota species selected for this study, considering that regions like Vlora and Durresi belong to the Adriatic sea while Saranda to the Ionian sea. Results are presented in Figure 5.

Obtained results presented show that biota species collected in Saranda region have exhibited higher BCFs compare to Durresi and Vlora region. Maximum BCF values in species like Mullus surmuletus; Pagellus acarne; Sepia officinalis (big one); Sardina pilchardus and Trigla lyra ranged between 280 to 390 L/kg while in species like Dicentrarchus labrax; Trigla lyra; Lithognatus mormyrus; Solea vulgaris; Pagellus erythrinus; Mytilus galloprovincialis of Vlora and Durresi region varied between 70 to 90 L/kg. These results comply with the values of Hg content in selected species of Saranda region.

Numerous studies support the assumption that waters of northern Adriatic Sea are characterized by higher levels of THg than the southern part (where Durresi and Vlora region belong), due to industries of Hg emissions, ^{44, 45, 46}. On the other hand, even in the Ionian Sea, concentration of Hg can vary in a wide range of values, ^{46, 47}.

Factors that affect the higher THg content in biota of the Ionian sea biochemical cycling of Hg, type of biota species, the way they are feeding, etc.

Usually, fish that live near to sediment bottom, (demersal fish), exhibit higher THg concentration compared to the ones that live in water column, (pelagic fish), ⁴⁶. Furthermore, mercury is accumulated to a higher degree in fish that feed on benthic organisms ⁴⁷. Another possibility is active Hg-release mechanism from sediments to the water column and its transfer to benthic biota ^{48, 49}.

Depth is also an important factor influencing Hg concentrations in fish. Deep-sea species usually have higher THg values than fish inhabiting shallow waters ^{49, 50, 51}. Values above 1.0 µg g-1 have been reported for various fish species from the western Mediterranean waters deeper than 1000 m, even for the small planktivorous Mediterranean spiderfish (Bathypterois mediterraneus) ⁵⁰. To explain these observations, Cresson et al. (2014) have recently proposed that food quality and quantity decreases with depth, affecting organisms by slowing their growth. Deep-water fish would appear older and would be able to accumulate Hg during a longer time, ⁴⁵.

The estimated daily intake, (EDI) was calculated according to equation 2. Obtained results are presented in table below. The FDA's reference dose (RfD) for mercury, specifically methylmercury, is 0.1 micrograms per kilogram of body weight per day (µg/kg/day) while for inorganic mercury is 0.57 μg/kg/day (inorganic Hg).

According to WHO, Food is the main source of mercury in non-occupationally exposed populations; the mean dietary intake of mercury in various countries ranges from 2 to 20 µg/day per person (IPCS (2003) Elemental mercury and inorganic mercury compounds). Calculated EDI of THg in selected species showed that obtained values did not exceed the limit value, 0.57 µg/kg/day. Total Hazard Quotient, THQ was calculated based on the EDI values.

Results are presented in Figures 6 and 7, respectively. Obtained results showed that species collected at the Ionian region exhibited higher THQ values compared to the Adriatic region. But, all selected species exhibited values of THQ<1, meaning that sea food is safe to be consumed by population in Albania.

4. Conclusions

In this study, a screening of the THg concentration in 62 marine biota species, belonging to Adriatic and Ionian seas fishing areas was carried out. Obtained results revealed that the total concentration of THg in all selected species fall below the values recommended by WHO/FAO, EFSA and EC. Species selected in the Ionian Sea region exhibited higher concentration of THg compared to ones selected in the Adriatic Sea. For some species, the number of meals per month should be limited to 8-12.

Estimation of average daily intake, EDI and Total Hazard Quotient, THQ revealed that that consumption of sea food is safe to regional population.

References