## 4 Discussion  
### Mercury Bioaccumulation and Elimination in Tissues with Climate Change  
The present study revealed that warming, acidification, and the co-occurrence of both climate change-related stressors influence Hg bioaccumulation and elimination in juvenile _Solea senegalensis_ in different ways, depending on tissue affinity to this element. The primary tissue for Hg uptake was the liver, which is consistent with the fact that this organ, in addition to it central role in basic physiology, is responsible for the storage, metabolization, and redistribution of xenobiotics via blood circulation to other tissues [64]. Other studies reported higher Hg accumulation in the brain than in other tissues, namely, in zebra fish [23; 65], Atlantic cod [66], and seabass [43] tissues. Such discrepancies may be due to species-specific factors related to Hg uptake [67], as well as to the slower metabolism and nutrient assimilation of _S. senegalensis_ (a benthic species) compared to pelagic fish species [68]. Figure 5: Acetylcholinesteraseerase (AchE) activity (nmol-min\({}^{-1}\cdot\)mg\({}^{-1}\) protein, mean \(\pm\) SD) in _S. senegalensis_ brain, after 28 days of Hg exposure. White bars represent non-contaminated treatments (non-CONT) and gray bars represent Hg-contaminated treatments (CONT). CTR—seawater temperature set at 19 \({}^{\circ}\)C and pH set at 8.0 units; A—seawater temperature set at 19 \({}^{\circ}\)C and pH set at 7.6 units; W—seawater temperature set at 23 \({}^{\circ}\)C and pH set at 8.0 units; A + W—seawater temperature set at 23 \({}^{\circ}\)C and pH set at 7.6 units. Different small letters denote significant differences between non-CONT treatments (ANOVA, \(p<0.05\)). Different capital letters denote significant differences between CONT treatments (ANOVA, \(p<0.05\)). Asterisks (*) represent differences between non-CONT and CONT for the same conditions (Student’s \(t\)-test: *\(p<0.05\)). In the present study, the consistent increase of THg in fish tissues under warmer temperatures (which translated into increased NARs and decreased EFs) is in accordance with previous studies in which juvenile seabass (_Dicentrachus labrax_; muscle, liver, and brain) [43], meagre (_Argyrosomus regius_; muscle, liver, and brain) [25], and even juvenile sole (_Solea senaglensi_; brain) [26] were used as biological models. The higher Hg bioaccumulation is due to the fact that fish experience a positive shift in metabolism and changes in the metabolic scope when subjected to increased temperatures, i.e., an enhancement [69], consequently translating into a higher intake of prey/feed [70] in order to meet the needs of enhanced animal metabolic rates, which was not followed by alterations in morphometric data or [PERSON]'s condition index. In line with this idea, significantly higher net accumulation rates were observed in the three tissues when fish were exposed to warmer temperatures, suggesting increased health risks for both marine species and seafood consumers in a global warming context. On the other hand, acidification alone acted as an antagonistic factor and hampered THg bioaccumulation (and, thus, facilitated its elimination) in the three studied tissues. This result was consistent with recent studies [26; 71]. Nonetheless, literature suggests that a decrease in seawater pH facilitates Hg accumulation, due to Hg+ competition with H+, limiting its sequestration and leaving more Hg for uptake [7].