# Effects of Diet on Mercury Bioaccumulation in Farmed Gilhead Seabream (_Sparus aurata_)
## 1 Introduction  
Fish is an essential source of food, nutrition, income and livelihoods [1, 2]. A worldwide rise in fish consumption, the overexploitation of fish stocks and the increasing marinepollution are gradually eroding marine resources and pushing us to consider aquaculture as the main future source of fish and seafood products [3; 4]. In the Mediterranean region, aquaculture is an active and growing sector that plays a critical role in food security, employment and economic development [1]. However, aquaculture facilities are often located in polluted marine coastal areas, raising concerns about the potential bioaccumulation of contaminants in the tissues of the farmed fish that could affect aquaculture quality and productivity [5]. To achieve a sustainable intensification, aquaculture must provide safe and uncontaminated food, preserving fishes from pollutants contamination and consequently the health of consumers. A major concern associated with fish consumption is human exposure to toxic mercury (Hg) [6; 7; 8; 9]. Mercury is internationally regarded as a priority environmental contaminant, due to its persistence in the environment, bioaccumulation and toxicity in living organisms [10]. In the aquatic environment, inorganic mercury (iHg) is the prevalent form of mercury released by anthropogenic sources [11; 12]. Fish can adsorb inorganic mercury directly from the water via respiration through the gills or by passive diffusion through the skin [13; 14]. Methylmercury (MeHg), the most toxic form, is produced from microbial methylation of the inorganic form in the water column and surface sediments and, upon uptake by fish, is rapidly assimilated and distributed to various body tissues [9; 10]. Neurotoxicological effects, hepatotoxicity, cardiorespiratory damages and toxicity on the reproductive system were previously reported in fishes exposed to sub-lethal concentrations of inorganic mercury [15; 16; 17]. Mercury bioaccumulation in fish tissues not only affects fish health and aquaculture productivity, but also poses a risk to the health of consumers [10]. It is well known that exposure to mercury can affect human health by altering the digestive, cardiovascular and/or central nervous systems [18] and the reproductive organs [19; 20]. One of the most promising strategies to control disease occurrence in aquaculture involves the strengthening of fish defense mechanisms through the administration of nutraceutical substances including the yeast _Saccharomyces cerevisiae_ and the bacteria _Bacillus subtilis_ and _Lactobacillus plantarum_[21]. These microorganisms can stimulate antioxidant defense mechanisms, non-immunity-related defense mechanisms, regulate the intestinal microbiota, promote growth performance and regulate immune homeostasis in fish, thus preventing toxic metal damages [22; 23]. Moreover, it has been reported that _B. subtilis_ has the potential to reduce toxic metals accumulation in fish tissues [24]. The yeast _S. cerevisiae_ is an advantageous biomaterial in metals biosorption for the following reasons: easy cultivation in a large scale, easy growth by non-fermentation methods, easy manipulation at the molecular level and also high biomass production [25]. Several studies have shown the ability of _S. cerevisiae_ to remove toxic metals, such as lead (Pb), cadmium (Cd), mercury (Hg), nickel (Ni), arsenic (As), chromium (Cr), gold (Au) and platinum (Pt) [26; 27; 28; 29; 30; 31; 32; 33]. Moreover, dead cells of _S. cerevisiae_ have shown comparable or even higher uptake capacity of metal ions, comparing with living cells [34]. Another promising bioremediation strategy involves the use of plant extracts rich in condensed tannins. These natural compounds have been reported to improve fish health by enhancing antioxidant capacity, immune function and altering intestinal microbiota composition [35]. Moreover, tannin-rich extracts from _Schinopsis balansae_ and _Castanea sativa_ have been reported as effective in removing toxic metals from water and wastewater [36; 37].