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SOURCES OF METALS IN THE MARINE ENVIRONMENT
Nowadays, aquatic pollution has become a major concern especially in coastal areas that exhibit high population growth and urbanization (Creel, 2003; Neumann et al., 2015). Various and numerous contaminants are present in the marine environment coming from natural and anthropogenic sources. Metals are naturally found in the earth’s crust, but their concentrations are increased due to anthropogenic activities as they reach the marine environment through water or atmospheric ways. Natural sources include geological weathering of underlying bedrocks and volcanic eruptions that have been reported to highly contribute to metal pollution (He et al., 2005; Mohammed et al., 2011). Environmental contamination can also occur through metal corrosion, soil erosion of metal ions and leaching of metals in addition to metal re-suspension and evaporation from water sources (Nriagu, 1989). However, most environmental contamination by metals results from anthropogenic sources including mining, smelting operations, industrial production and use, in addition to domestic and agricultural use of metal and metal-containing compounds (He et al., 2005). Industrial sources such as metal refineries, coal burning in power plants, petroleum combustion, plastics, textiles, microelectronics, wood preservation and paper processing plants are considered important sources of metals (Arruti et al., 2010). Contaminated effluents of wastewater play an important role in metal transfer into the aquatic environment. In wastewater, metals may be found under particulate (oxides and silicates) or dissolved forms (sulfurs and carbonates), but most of them are associated to suspended particulate matter (INERIS, 2004). Leachate associated to landfills has been proved to contain large amounts of toxic pollutants including metals (Toufexi et al., 2013). An example is elements like Pb and Cd coming from batteries or Hg from instruments such as thermometers and barometers. Table 1.1 provides a summary of the main sources of metals in the environment.
FATE OF METALS IN THE MARINE ENVIRONMENT
The aquatic environment is very sensitive to environmental disturbances. Metals released in the marine environment are subject to multiple physicochemical factors that influence their forms and distribution between particulate or dissolved phase (Chovanec et al., 2003b). In real environmental conditions, metals have low solubility in water and stay mostly associated to the solid phase that’s why metallic elements end up mostly in the sediments (INERIS, 2004). Once metals reach the marine environment, they exert different behaviors and get distributed between the sediments, the water column, suspended matter and the biota. This distribution varies in space and time and is promoted in spring and summer as metals adsorb on phytoplankton. Phytoplankton is considered one of the most important biotic factors influencing the behavior and distribution of metals in the water column as it shows high metal bioconcentration with uptake mainly occurring from dissolved fractions (Phillips, 1980). On one hand, this adsorption of metals on phytoplankton reduces the amount of dissolved metals in the water, and on the other hand, it leads to contamination along the trophic chain through phytoplankton ingestion. In deep cold waters, due to the absence of primary production and mineralization of suspended matter, the concentration of dissolved metals increases (Morel and Price, 2003). Adsorption of metals on the sediments or suspended particles, is a function of the solid particles in question (Lion et al., 1982). Phytoplankton are also efficient bioaccumulators of trace metals. The incoroporation of trace metals by phytoplankton can regulate the form and availability of trace metals. Consequently, the metal contents of phytoplankton reflect the environmental availability and influence the distribution of metals in the ocean (Sanders and Riedel, 1998; Twining and Baines, 2013). Metals can potentially be biomagnified as well as they can be biodiminished along the food chain. These two phenomena are closely related to the nature of the trace element itself as well as the assimilation efficiency of the organisms in question. Although there is no general rule that predicts whether or not a trace metal is biomagnified along a marine food chain, it is known that the concentration of Cd for example generally decreases at higher trophic levels in classic planktonic food webs (e.g. phytoplankton to zooplankton to fish), whereas Hg concentrations increase at higher trophic levels (Ali and Khan, 2019; Chouvelon et al., 2019; Wang, 2002).
The free ionic metal forms, undergo a solvation process leading to the formation of hydrates that may later on form complexes with organic and inorganic ligands. The complexation process is governed by a multitude of parameters of which pH, redox potential, temperature and eventually the quantity of ligand available (Zhao et al., 2016). Inorganic (major anions such as carbonates, chlorides, and hydroxides) and organic (e.g. dissolved organic matter (DOM), particulate organic matter (POM)) chemical compounds in aquatic environments constitute ligands capable of complexing metal ions (Figure 1-2). Organic matter forms complexes with metals with its principal binding sites being carboxylic, nitrogenous and sulfured ones. Biological effects of metals are mostly related to the concentration of the free metallic ions in the water, thus complexation with DOM and POM reduces bioavailability and toxicity of trace elements to aquatic organisms by decreasing the amount of free ionic metals in the marine environment (de Souza Machado et al., 2016; Guo et al., 2001; Luther et al., 2001). The environmental behavior, bioavailability, and toxicity of trace metals in different matrices (water column, sediments, biota, etc.), are not only correlated with the total concentration of an element, but also with speciation which is an important index of the toxic and ecological risk of trace metals in the environment (Kwon and Lee, 2001; Fan et al., 2008; Ahumada et al., 2011; Nemati et al., 2011). According to IUPAC definition, chemical speciation in defined as the specific form of an element in terms of isotopic composition, electronic or oxidation state, and complex or molecular structure of a certain element (IUPAC, 2000). Consequently, in order to determine the reactivity of trace metals, chemical speciation is essential.
METAL UPTAKE AND ACCUMULATION IN MARINE ORGANISMS
The major factor governing the uptake of metals by marine organisms is the biological availability of the metal which at its turn is dependent from the physicochemical conditions of the environment (Ali et al., 2019; Chovanec et al., 2003; Rainbow, 1985). Bioavailability of metals in the marine environment is controlled by parameters such as pH, temperature, oxygen concentration, water hardness and salinity. The influence of the physicochemical parameters of the water on metal speciation, toxicity and uptake is well known and has been widely documented (Ansari et al., 2004; Campbell et al., 2002; Chovanec et al., 2003; Jakimska et al., 2011; Rüdel et al., 2015; Tchounwou et al., 2012). It has been shown that water hardness reduces toxicity of metals and this is due to the competition that takes place between calcium and divalent metals for the binding sites, resulting in reduced uptake and toxicity of metals in hard water. Some studies reported the protective effect of Ca2+ ions against dietary and waterborne Cd uptake (Baldisserotto et al., 2004; Brzóska and Moniuszko-Jakoniuk, 1997; Li et al., 2016; Song et al., 2013). Rising temperature, oxygen depletion and metabolic stimulation during reproduction or stress in fish for example, leads to an accelerated gill ventilation and thus increased toxicants’ uptake (Chovanec et al., 2003). Similarly to hardness, salinity reduces uptake and accumulation of metals by fish. When salinity decreases, free metal ions and toxicity increase. It has been reported that water acidification also affects metals accumulation rates by marine organisms. The effects can be direct following a damage of tissues which makes them more permeable to metals but at the same time, a competitive uptake of H+ ions inhibiting metal absorption may occur. In algae, an increase of pH leads to the ionization of the membrane’s functional groups making them negatively charged which leads to the attraction of cations. Conversely, a decrease of pH leads to either fast or slow protonation. During a fast protonation, anionic sites are neutralized while in the event of a slow one, metals are released and replaced by protons (INERIS, 2004). In fish, a lower pH leads to a higher accumulation of Cd, Pb and Cu but not Zn (Çoǧun and Kargın, 2004; Jezierska and Witeska, 2006).
THE MARINE STRATEGY FRAMEWORK DIRECTIVE (MSFD)
The Marine Strategy Framework Directive (MSFD or DCSMM in french) was created and published on June 25th 2008. The directive establishes a community action plan in the field of marine environment policy and leads each Member State to build a strategy in order to reach a Good Ecological Status (GES). It was transposed into the Environment Code: articles L 219-9 to L 219-18 and R 219-2 to R 219-17 and applies to zones under sovereignty or French jurisdiction, divided into 4 marine sub-regions: the English Channel and the North Sea, the Celtic Seas, the Bay of Biscay and the western Mediterranean Sea. Under the scientific and technical coordination of the Ifremer and the Marine Protected Areas Agency, a set of public establishments (ANSES, BRGM, CNRS, Ifremer, MNHN, SHOM) provide a scientific technical work to define the Good Environmental Status. There are 11 descriptors of good environmental status: Biological diversity, non-indigenous species, commercially exploited species, marine food webs, eutrophication, sea-floor integrity, hydrographical conditions, contaminants, health issues, marine litter and marine energy (MSFD, 2018). The 8th and 9th descriptors are linked and deal with the contamination levels in the biota and the health risks they may engender. The descriptor 8 ensures that the “concentrations of contaminants are at levels that do not give rise to pollution effects” while descriptor 9 aims at ensuring that “contaminants in fish and other seafood for human consumption do not exceed levels established by Community legislation or other relevant standards” (EC, 2019a; MSFD, 2018). The MSFD program of measures coincides with that of the Water Framework Directive (WFD) that sets environmental targets and deadlines for improving the ecological and chemical status of surface water bodies as well as the quantitative and chemical status of underground water bodies (EC, 2019b). The main objectives of the MSFD are centered on the protection and the prevention of the deterioration of marine ecosystems as well as managing pressures from human activities in order to prevent and progressively eliminate pollution (Ifremer and AFD, 2018).
HEALTH RISKS RELATED TO THE CONSUMPTION OF CONTAMINATED SEAFOOD
Major routes of exposure to trace elements for humans are food, water and air. Individual metals have each different degrees of toxicity, that’s why food safety authorities around the world have established maximum allowable levels (MALs) and provisional tolerable weekly intakes for metals in foodstuffs. Marine organisms, especially those that can accumulate high levels of contaminants in their tissues, are a route of human exposure to dangerous compounds. That’s why the numerous health benefits provided by seafood may be compromised by the presence of toxic contaminants, of which metals and metalloids, that can be harmful to human health when consumed in high quantities (Pastorelli et al., 2012; Storelli and Barone, 2013). The toxicity of metals resides in their ability to alter normal biochemical functions by competing with some essential elements (Davidson et al., 2015). A non-essential metal may substitute for an essential one in a metabolic pathway which may block further reactions (Rainbow, 1985). All elements are toxic starting from a certain threshold; even essential elements when in excess, may become toxic and cause adverse effects. Figure 1-3 shows the relationship between essential and toxic elements in terms of health response.
Table of contents :
Chapter 1 – General Introduction
1.1. Sources of metals in the marine environment
1.2. fate of metals in the marine environment
1.3. Metal uptake and accumulation in marine organisms
1.4. Interspecific variation of metal accumulation: Bioindicator species
2.1. Regulated toxic metals in seafood
3.1. Toxic trace metals in seafood: Are seafood from the Mediterranean safe to eat?
4. Objectives of the study
5. Outline of the study
Chapter 2 – Strategies, protocols and methodologies
1 Presentation of the study area: Geographical and climatic context
2 Sampling sites
3 Biological models sampled
4 Samples collection and preparation
5 Analysis of trace elements by Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
6 The use of hyphenated techniques for elemental speciation: HPLC-ICP-MS
6.1. Methylmercury (MeHg)
Chapter 3 – Part 1: Seasonal and spatial variability of trace elements in livers and muscles of three fish species from the Eastern Mediterranean
Chapter 3 – Part 2: Assessment of trace element contamination and bioaccumulation in algae (Ulva lactuca), bivalves (Spondylus spinosus) and shrimps (Marsupenaeus japonicus) from the Eastern Mediterranean
Chapter 4 – Part 1: Levels of Pb, Cd, Hg and As in fishery products from the Eastern Mediterranean and health risk assessment
Chapter 4 – Part 2: Toward a routine methodology of speciation analysis of methylmercury in fishery products following the validation based on the accuracy profile approach
Chapter 5 – General discussion and perspectives
ANNEX
References
