3.1 Trophic transfer of metal(loid) concentrations in the food web
The TMF values (Fig. S1) show that the concentrations of As, Cd, Pb, Co, Cu, Fe, Mn and Zn in seafood products decreased significantly with increasing TL values (p < 0.05), suggesting bio-dilution processes occur along the marine food web. This finding was in accordance with previous studies indicating that most heavy metals showed bio-dilution instead of biomagnification (Borrell et al. 2016; Cheng et al. 2013; Cui et al. 2011). The bio-dilution of Ni along the food web was not significant (p = 0.325). No significant trend of biomagnification of Hg was observed in the entire aquatic food web (p = 0.603, TMF > 1), similar to the previous conclusion (Borrell et al. 2016). However, Hg typically showed biomagnification behavior (Dominik et al. 2014; Ikemoto et al. 2008; Kehrig et al. 2013; Nguyen et al. 2012). The differences in Hg bioaccumulation were correlated with broad biodiversity and complicated food webs (Al-Reasi et al. 2007). Generally, higher trophic level predators with omnivorous habits, such as cephalopods and fish, are more bio-accumulative Hg (Dou 1995). The biomagnification behavior of Hg including organic Hg was mainly affected by the habitats, growth rate and age of the organisms.
Biomagnification and bio-dilution effects in the aquatic food web involved various factors, including the length of the food web, the breadth of biological species, habitat characteristics and seasonal factors. In addition, they were related to the physiological structure and growth status of the organisms themselves (Griboff et al. 2018; Nfon et al. 2009).
3.2 Metal(loid) elements in aquatic products and health risk
The concentrations of As, Cd, Pb, Hg, Co, Ni, Mn, Fe, Zn and Cu were detected in freshwater fish and the five seafood categories (Table 1 and Table S1). Generally, there is no apparent difference between the concentrations of metal(loid)s determined in freshwater fish and saltwater fish. Seaweeds exhibited a higher accumulation capability on As, Pb and Fe, while cephalopods showed a more robust accumulation capability on Cd, Cu and Zn.
Table 1
The content of metal(loid)s (mg kg− 1 dry weight) in aquatic products
Samples (amount) | | As | Cd | Pb | Hg | Co | Cu | Fe | Mn | Ni | Zn |
Seaweeds (49) | mean ± SDa | 40.18 ± 13.31a | 1.94 ± 1.46bc | 1.15 ± 1.07a | 0.013 ± 0.016b | 0.30 ± 0.21ab | 11.1 ± 8.4b | 445.6 ± 269.2a | 45 ± 62.7a | 1.61 ± 1.83a | 33.7 ± 18b |
| Range | 15.20–74.58 | 0.15–5.11 | 0.11–4.63 | <LOD–0.064 | 0.03–1.44 | 0.65–26.7 | 35.9–1264.3 | 3.5–412.2a | 0.10–12.9 | 5.1–124.6 |
Freshwater fish (6) | mean ± SD | 0.05 ± 0.03b | <LODb | 0.02 ± 0.003a | 0.030 ± 0.010b | 0.01 ± 0.002b | 1.1 ± 0.4b | 12.3 ± 1.1b | 1.8 ± 0.90a | 0.08 ± 0.03a | 28.0 ± 4.9b |
| Range | 0.02–0.07 | <LOD | 0.01–0.02 | 0.020–0.04 | 0.01–0.01 | 0.70–1.5 | 11.2–13.5 | 0.80–2.7a | 0.06–0.11 | 23.2–32.9 |
Saltwater fish (48) | mean ± SD | 2.59 ± 1.60bc | 0.02 ± 0.03b | 0.02 ± 0.02a | 0.12 ± 0.10a | 0.02 ± 0.03b | 1.1 ± 0.3b | 10.7 ± 5.9b | 1.2 ± 1.2a | 0.43 ± 0.27a | 30.4 ± 11.6b |
| Range | 0.42–6.22 | <LOD–0.11 | <LOD–0.05 | 0.008–0.46 | <LOD–0.10 | 0.60–1.7 | <LOD–20.8 | 0.20–5.4a | 0.09–1.16 | 0.07–54.0 |
Crustaceans (20) | mean ± SD | 14.85 ± 6.42bc | 0.96 ± 0.95bc | 0.02 ± 0.02a | 0.12 ± 0.05a | 0.06 ± 0.06b | 21.4 ± 17b | 0.90 ± 1.0b | 37.1 ± 51.8a | 0.06 ± 0.07a | 73.1 ± 17.0ab |
| Range | 3.79–19.47 | <LOD–1.99 | <LOD–0.04 | 0.060–0.18 | <LOD–0.15 | 5.5–47.2 | <LOD–2.2 | 0.60–125.6a | <LOD–0.17 | 53.7–100.2 |
Shellfish (70) | mean ± SD | 13.92 ± 6.61bc | 4.94 ± 5.59ac | 0.51 ± 0.50a | 0.075 ± 0.045ab | 0.37 ± 0.27a | 73.4 ± 147.7ab | 198.7 ± 138.1ab | 17.3 ± 20.1a | 0.97 ± 1.10a | 264.2 ± 455.7a |
| Range | 5.11–21.47 | 0.12–13.49 | <LOD–1.24 | 0.019–0.13 | 0.10–0.91 | 3.8–434.7 | 13.8–373.6 | 1.5–61.4a | 0.12–2.99 | 1.5–1374.8 |
Cephalopods (6) | mean ± SD | 16.76 ± 8.44c | 8.49 ± 7.95a | 0.12 ± 0.12a | 0.14 ± 0.02a | 0.23 ± 0.23ab | 108.0 ± 100.4a | 8.6 ± 8.6b | 2.9 ± 2.5a | 0.59 ± 0.59a | 144.9 ± 130.2ab |
| Range | 8.32–25.20 | 0.54–16.45 | <LOD–0.24 | 0.13–0.16 | <LOD–0.46 | 7.6–208.3 | <LOD–17.1 | 0.50–5.4a | <LOD–1.19 | 14.7–275.1 |
aSD, standard deviation |
bLOD, limit of detection. Different letters indicate statistical significance |
3.2.1 Arsenic content in aquatic products and health risk
Table 1 and S1 show that freshwater fish contains the lowest concentration of As (0.02–0.07 mg kg− 1 d.w.), and the total As concentrations in seafood follow the trend: seaweeds > cephalopods > shellfish ≈ crustaceans > saltwater fish. Organoarsenic are the major As species in marine organisms, in which more than 300 species of naturally occurring organoarsenic have been identified (Xue et al. 2021). In this study, As species of seafood were separated into water-soluble arsenic, lipid-soluble arsenic, and residual arsenic. The average As concentration in freshwater fish was below the iAs limit (0.1 mg kg− 1 w.w.) given by the national food safety standard of China (GB 2017).
Water-soluble arsenic detected in seafood was comprised of AsB, AsC, Oxo-Gly, Oxo-PO4, Oxo-SO3, DMAs(V), MAs(V), As(V) and As(III) (Fig. 1, Table S2). In general, AsB was the highest concentration of As species and accounted for more than 50% of the total As content in most of marine animal products. AsB concentrations in Loligo oshimai Sasaki were up to 21.60 mg kg− 1 d.w. (86% of the total As content). AsB concentrations in crustaceans ranged from 2.58–17.11 mg kg− 1 d.w. and accounted for 67%-92% of the total As content. AsB accounted for more than 50% of the total As content in most of shellfish, although AsB was found at relatively low concentration in shellfish Sinonovacula constricta and Mytilus edulis, accounting for 1% and 2% of the total As content, respectively. Other small-molecular organoarsenic compounds, including AsC, MAs(V), and DMAs(V) were detected in a trace amount or absent in the seafood (Table S2). Moreover, the total concentrations of iAs, As(V) and As(III) in all seafood products were below the iAs limit (0.5 mg kg− 1 w.w. for aquatic animals and its products or 0.1 mg kg− 1 w.w. for fish and its products) given by GB 2761 − 2017 (GB 2017). Weekly exposure to iAs in the surveyed population was far below the provisional tolerable weekly intake (PTWI) (Table S4).
However, arsenosugars were the major As species in nori (Oxo-Gly and Oxo-PO4 accounting for 39% of total As) and kombu (Oxo-PO4 and Oxo-SO3 accounting for 58% of total As) (Fig. 2b, Table S2), consistent with previous studies (Huang 2015; Li et al. 2006; Zhao et al. 2020). Most in vitro tests of arsenosugar toxicity showed that arsenosugars did not directly exerted cytotoxicity or genotoxicity even at very high concentrations (Kaise et al. 1996; Oya-Ohta et al. 1996; Sakurai et al. 1997), although trivalent Oxo-Gly was more cytotoxic than its pentavalent counterpart (Andrewes et al. 2004). However, arsenosugars are metabolized in humans into numerous other compounds (Francesconi et al. 2002; Van Hulle et al. 2004). The main arsenosugar metabolite, DMAs(V), has been reported to enhance lung carcinogenesis in a transplacental mouse (Fujioka et al. 2020), and induce bladder tumors in adult rats (Arnold et al. 2006). Its thio-analogue thio-DMAs(V) exerted much higher cytotoxicity and genotoxicity than As(III), likely due to the generation of DMAs(III) (Bartel et al. 2011; Leffers et al. 2013a). The metabolites dimethylarsinoyl-acetic acid (DMAA(V)), thio-DMAA(V) and dimethylarsinoylethanol showed lower bioavailability in a Caco-2 intestinal barrier model than MAs(V) and DMAs(V), whereas thio-DMAs(V) and thio- dimethylarsinoylethanol showed high intestinal bioavailability, similar to As(III) (Leffers et al. 2013b). Moreover, those metabolites might occur as more toxic trivalent species but have not yet been found.
ICP-MS analysis for DCM layer arsenic shows that concentrations of arsenolipids in seafood range from 0.11 to 5.17 mg kg− 1 d.w. (Table S2). The highest arsenolipid concentration was found in Charybdis feriata (5.17 mg kg− 1 d.w., 27% of total As), followed by Ostrea gigas Thunberg (5.06 mg kg− 1 d.w., 24% of total As) and Pectinidae (4.01 mg kg− 1 d.w., 23% of total As). The seafood with the largest proportion of arsenolipids was M. edulis, E. fuscoguttatus and S. constricta with 62%, 47% and 45% of total As content, respectively. More than 260 arsenolipids have been identified up to now, falling into six distinct classes (Xue et al. 2021). Among the arsenolipids, arsenic-containing hydrocarbons (AsHCs) were able to cross the in vitro Caco-2 intestinal barrier of humans (Meyer et al. 2015b) and porcine brain capillary endothelial cells (in vitro model for the blood–brain barrier) (Müller et al. 2018), and also can be transported into the milk of nursing mothers (Xiong et al. 2020). AsHCs directly exerted significant cytotoxic to in cultured human bladder (UROtsa) and liver (HepG2) cells (Meyer et al. 2014), showed pronounced neurodevelopmental effects on pre-differentiated human neurons (Witt et al. 2018), and massively disturbed the neuronal network of human differentiated neurons (Witt et al. 2017). Arsenic-containing fatty acids (AsFAs) or their minor water-soluble metabolites (Oxo/thio-dimethylarsenopropanoic acid) tested did not directly exhibit adverse effects on in vitro cultured human cells (Meyer et al. 2015a; Witt et al. 2017), but DMAs(V) was the major metabolites of AsFAs in some volunteers (Schmeisser et al. 2006).
Arsenolipids are the major As species present in fish liver and marine oils (Sele et al. 2012). Commercial fish oils are generally produced from tuna, sardines, salmon, mackerel, and herring (Peter et al. 2004). The fish oils are rich in long-chain omega-3 fatty acids and lipid-soluble vitamins, but also contain high concentration arsenolipids (Sele et al. 2014), ranging from 0.2 to 16 mg As kg− 1 oil (Table S3). As concentrations in crude fish oil varied considerably, likely due to differences in As concentrations among the deep-sea fish species used to make the fish oil (Sele et al. 2012). In order to assess the edible security of fish oils, total As concentrations in 9 kinds of commercial fish oil capsules were analyzed using ICP-MS (Table S3). As concentrations in commercially available fish oil capsules in the range of 0.02–0.05 As mg kg− 1 oil were lower than those previously reported (Sloth, et al., 2005). The results suggested that several clean-up procedures to enrich omega-3 fatty acids reduced clearly arsenolipid content during the complex process of fish oil capsule production. In addition, Hg (actually MeHg), Pb, Cd, and other heavy metals are water-soluble, not oil-soluble, so they stay behind in the fish meal when the fish oil is extracted. The data illustrate that crude fish oils with high levels of As, but low levels of As in commercial fish oil capsules do not pose a risk to human health.
Taken together, there are no risks of concern to human health in terms of the content of iAs and other small-molecular organoarsenic alone in all marine animal products studied in the work. However, the cytotoxicity of arsenosugar and arsenolipid metabolites may be comparable with iAs. Given this perspective, the maximum level for As in seafood should be re-evaluated and the toxicity and bio-availibility of arsenosugars and arsenolipids in seafood are also taken into account instead of iAs alone.
3.2.2 Cadmium content in aquatic products and health risk
Food is the major exposure source of Cd for the non-smoking general population. In this study, Cd concentration was higher in seaweeds, crustaceans, shellfish, and cephalopods than in fish (Table 1). The highest Cd concentrations were detected in cephalopod Octopus vulgaris, followed by shellfish Pectinidae and Neptunea cumingi Crosse. The Cd values in O. vulgaris (2.80 mg kg− 1 w.w.) and Pectinidae (2.29 mg kg− 1 w.w.) were slightly higher than the Cd limits set by GB 2762 − 2017 (2.0 mg kg− 1 w.w.) and the European Union (EC 2006). The Cd concentration in seaweeds (1.94 mg kg− 1 d.w.) was above the FAO/WHO limit for Cd (0.2 mg kg− 1 d.w.) (FAO/WHO 1995). The average Cd concentration of shellfish (0.84 mg kg− 1 w.w.) and crustaceans (0.16 mg kg− 1 w.w.) were higher than that of crustaceans in Taiwan (0.08 mg kg− 1 w.w.) (Lin and Lin 2019) and Europe (0.13 mg kg− 1 w.w.) (EFSA 2012) (Table 1). The Cd levels in all shellfish samples ranged from 0.12–13.49 mg kg− 1 d.w. These concentrations were considerably higher than those in mussels from the Albanian coast (0.27–0.77 mg kg− 1 d.w.) (Cullaj et al. 2007), Adriatic Sea (1.50–3.17 mg kg− 1 d.w.) (Jović et al. 2011) and Mediterranean Sea (0.25–1.7 mg kg− 1 d.w.) (Denkhaus and Salnikow 2002).
The total intake of Cd in average consumers (50th percentile of aquatic product consumption) was 1.35 µg/kg body weight (bw)/week, and that in excessive consumers (95th percentile of aquatic product consumption) was 4.16 µg/kg bw/week which was close to PTWICd (Table S4). The dominant contributions to the weekly Cd intake were seaweeds, cephalopods and shellfish. According to the standard, the maximum weekly tolerable amount was 0.18 kg d.w. of seaweeds, 0.41 kg w.w. of shellfish and 0.24 kg w.w. of cephalopods, given a single aquatic product foodstuff was consumed.
3.2.3 Lead content in aquatic products and health risk
The highest Pb concentration corresponded to seaweeds and shellfish, particularly Laminaria japonica, S. constricta and O. gigas Thunberg as shown in Table S1. Table 1 and S1 show that Pb concentrations of aquatic products analyzed in this work were below the maximum level set in GB 2762 − 2017 for Pb in different aquatic products and its products (0.5 mg kg− 1 w.w. for fish and crustaceans, 1.5 mg kg− 1 w.w. for bivalves), except that L. japonica (1.84 mg kg− 1 d.w.) was above the permissible limits set by (FAO/WHO 1995) and GB 2762 − 2017 (1.0 mg kg− 1 d.w.). Although the Pb concentrations in shellfish were 1 to 2 orders of magnitude higher than those in freshwater fish and other aquatic animals, the concentrations were far low than those in bivalve mollusk from African countries bordering the Indian Ocean and the Red Sea (0.24-147.55 mg kg− 1 w.w.) (Tamele and Vázquez Loureiro 2020).
The total intake of Pb in average consumers was 0.44 µg/kg bw/week, and that in excessive consumers was 1.00 µg/kg bw/week (Table S4). The mean and excessive weekly exposure to aquatic products were far below PTWIPb (Table S4). Therefore, the weekly exposure to Pb did not pose a risk to human health by consuming aquatic products.
3.2.4 Mercury content in aquatic products and health risk
Hg predominantly exists in the form of MeHg in aquatic products, the most toxic effect on humans (Stankovic et al. 2012). Low total Hg concentrations were detected in all aquatic products in this work (Table 1), and were far below the MeHg limit (0.5 mg kg− 1 d.w.) set in GB 2762 − 2017. The Hg concentrations in freshwater fish (0.002 mg kg− 1 w.w.) and saltwater fish (0.020 mg kg− 1 w.w.) were remarkably lower than those in Bosnia and Herzegovina (0.096 mg kg− 1 w.w. for marine fish and 0.063 mg kg− 1 w.w. for freshwater fish) (Hajrić et al. 2022) and Hong Kong (mean 0.091 mg kg− 1 w.w. for freshwater and saltwater fish) (Tang et al. 2009), but higher than those in Xiamen (not detected in saltwater fish) (Zhao et al. 2016).
The upper limits of average dietary exposure to total Hg from foods other than fish and shellfish for adults (1 µg/kg bw per week) and children (4 µg/kg bw per week) were at or below PTWIHg (Table S4). In this study, the calculated weekly intake of total Hg was 0.04 µg/kg bw/week for average consumers and 0.18 µg/kg bw/week for excessive consumers (Table S4). Moreover, the weekly exposure to MeHg of aquatic products was 0.03 and 0.11 µg/kg bw/week for average and excessive consumers. These values were much lower than the recommended PTWI of total Hg or MeHg (Table S4).
3.2.5 Essential metal content in aquatic products and health risk
Co concentrations in the edible part of the samples in this study were low. The highest mean concentration of 0.37 mg kg− 1 d.w. was found in shellfish and the lowest of 0.01 mg kg− 1 d.w. in freshwater fish. The concentrations were lower than those in molluscs from the Alexandria coast of the Mediterranean Sea (0.52–2.96 mg kg− 1 w.w.) (Abdallah 2013). No maximum limit was specified for Co in aquatic products.
Cu concentration was higher in cephalopods and shellfish than in fish (Table 1). The highest Cu concentrations were detected in O. gigas Thunberg, followed by cephalopods O. vulgaris (Table S1). Range concentrations of Cu in fish were 0.10 to 0.29 mg kg− 1 w.w. (Table S1). The mean concentration of Cu in fish, crustaceans, cephalopods and shellfish was higher than those from the Kedah and Selangor coastal regions of Malaysia (Salam et al. 2021), but lower than those found in mussel and clam from the Ebro River in Spain (Nadal et al. 2008). Based on the maximum Cu level in O. gigas Thunberg (73.90 mg kg− 1 w.w.) and PTWICu (Table S4), the maximum intake amount of O. gigas Thunberg per week is 2.8 kg w.w.
The concentrations of Fe in seaweeds varied from 35.9 to 1264.3 mg kg− 1 d.w. (Table 1) and were higher than those from the southeast coast of India (6.1-188.47 mg kg− 1 d.w.) (Thodhal Yoganandham et al. 2019) but similar to those from Hypnea sp. in Brazil (279–1163 mg kg− 1 d.w.) (Macedo et al. 2009). The concentrations of Fe in the aquatic animals (Table 1) were lower than in seaweeds. The highest Fe level corresponded to seaweeds, particularly Porphyra haitanensis (460.9 mg kg− 1 d.w.). The intake amount of P. haitanensis above PTWIFe needed to reach 0.7 kg d.w./week.
The Mn levels varied greatly in crustaceans (0.1-21.35 mg kg− 1 w.w.) and shellfish (1.96–10.44 mg kg− 1 w.w.), and were higher than those in clams from Australia (4.7 ± 2.2 mg kg− 1 w.w.) (O'Mara et al. 2019). The Mn concentrations found in this study were lower than the legal limit of 20 mg kg− 1 w.w. for fish species set by the Turkish guidelines (Dural et al. 2007). The weekly exposure to Mn did not pose a risk to human health by consuming aquatic products.
In this study, the highest Ni concentration was found to be in M. edulis, followed by Abalone (Table S1). The maximum Ni level found in M. edulis, limiting weekly intake of M. edulis without detrimental effects was 4.12 kg w.w. According to these results, Ni exposure through the consumption of other investigated aquatic products was not hazardous to human health.
The highest Zn concentration was found in O. gigas Thunberg (1374.8 mg kg− 1 d.w. or 233.72 mg kg− 1 w.w.), followed by O. vulgaris (275.1 mg kg− 1 d.w. or 46.77 mg kg− 1 w.w.) (Table S1). Range concentrations of Zn in other aquatic products tested in this study were 0.1 to 146.7 mg kg− 1 d.w. According to the PTWI of Zn (Table 4), the maximum weekly tolerable amount of O. gigas Thunberg and O. vulgaris was 0.03 and 0.15 kg w.w., respectively.
Moreover, the weekly intake of Co, Cu, Fe, Mn, Ni and Zn through the diet of aquatic products was measured according to a survey of aquatic product consumption among Chinese adult residents in 2015 (Liu 2016; Su et al. 2018) and the metal(loid) contents of aquatic products from this study. Table S4 shows that the estimated weekly exposure of those elements for average and excessive consumers is far below the recommended PTWI for adults weighing 60 kg, implying that regular consumption of aquatic products is within the safe limit.
3.3 Non-carcinogenic Risk
The health risks of aquatic products in average and excessive consumers were assessed using the THQ value (Table S5). THQ < 1 indicates an acceptable health risk to the exposed population, and THQ > 1 indicates a non-carcinogenic risk (Wang et al. 2005).
THQ values of most metal(loid)s were below 1, and indicated acceptable non-carcinogenic effects for each metal(loid) from these aquatic products at the current consumption rate. In this work, combined effects of all metal(loid)s from the same type of aquatic product on human health were analyzed via calculating TTHQ values, which ranged from 1.50×10− 2 (for average consumers ingesting crustacean) to 5.27 (for excessive consumers ingesting seaweeds) (Fig. 3 and Table S5). THQ of Cd contributed the most to TTHQ values of crustaceans, shellfish, cephalopods, and seaweeds, accounting for 32%, 45.2%, 63.0% and 39.9%, respectively (Fig. 3a). The THQ of Hg was predominant in the TTHQ of freshwater fish (49.1%) and saltwater fish (68.9%) (Fig. 3a), consistent with the previous results (Djedjibegovic et al. 2020; Hajrić et al. 2022). For potentially toxic essential elements, THQ of Zn contributed 24.6% to TTHQ of freshwater, and THQ of Cu was 17.8%, 16.8% and 20.0% of TTHQ of crustaceans, shellfish and cephalopods, respectively (Fig. 3a). Moreover, HI, the combined effects of all metal(loid)s from aquatic products tested, was > 1 for excessive consumers (Fig. 3). It suggested that consuming aquatic products could lead to chronic non-carcinogenic health risks. The risks were mainly from seaweeds (TTHQ = 0.527 for excessive consumers) and cephalopods (TTHQ = 0.427 for excessive consumers), suggesting that reduced ingestion of lower trophic organisms could sharply decrease the health risks from aquatic products.
3.4 Target cancer risk
Metal(loid)s (iAs, Cd, and Pb) may increase cancer risks in human beings (EPA 2016). Prolonged exposure to low concentrations of metal(loid)s may result in various malignancies. The highest TR values of iAs, Cd and Pb were 4.41×10− 6, 7.99×10− 5, and 1.06×10− 6 for excessive consumers of aquatic products, respectively (Table S6). According to EFSA, the unacceptable TR value of metal(loid) was 1 × 10− 4, which means that 1 per 10,000 people will experience cancer risk from aquatic products containing a metal(loid) over 70 years. The risk boundary varies from 1 × 10− 4 to 1 × 10− 6 (USEPA 2010). TR values of iAs in seaweeds, saltwater fish, shellfish and cephalopods were higher than 10− 6 (Table S6), while the iAs carcinogenic risk of other aquatic products was negligible. TR values of Cd in cephalopods (1.02 × 10− 4 for high consumers) were above 10− 4. The total TR values of Cd from ingestion of all aquatic products were > 10− 4 for excessive consumers (Fig. S6), implying that the carcinogenic risk of Cd would be up to 2.26 people per 10,000. The carcinogenic risk of Pb in seaweeds was within the acceptable range (10− 4-10− 6) for excessive consumers. For species other than seaweeds, TR values were < 10− 6, suggesting that Pb in aquatic products has no cancer risk. The results showed that the excessive consumption of cephalopods posed potential carcinogenicity to humans because of Cd ingestion.