## 4 Occurrence of Micro and Nano-Plastics in Fish Gills and Consequent Effects
Intestinal toxicity of different length of microplastic fibers (50 \(\pm\) 26 \(\upmu\)m and 200 \(\pm\) 90 \(\upmu\)m) has been studied in both larvae and adults of zebrafish proving that longer microfibers caused very serious intestinal damage compared to shorter microfibers causing a significant reduction in food intake [166]. Studies of metabolomics were also carried out to unravel the underlying mechanisms of microplastics toxicity through the study of chemical processes involving all the endogenous and exogenous metabolites of enteric cells that cause oxidative stress and inflammation [166]. The impact of polystyrene (PS) NPs was studied at the cellular, molecular, and systemic levels by [PERSON] and collegues [165]. It has been demonstrated by the same authors that the absorption of PS NPs takes place by endocytosis and phagocytosis respectively for 50-nm and 1-micron nanoparticles. Regardless of the pathway of internalization, the particles will be degraded by lysosomes resulting in alkalinization and increased of reactive oxygen species (ROS) that increase their susceptibility to pathogens [165; 167]. Oxidative damage and inflammation induced by co-exposure to MPs and cadmium have been studied in zebrafish by [PERSON] et al. [11; 31]demonstrating that MPs enhanced the toxicity of heavy metals in fish tissues, including the gut. Using zebrafish, the influence of different polymer types of MPs was evaluated to understand the toxicology of microplastics (MPs) in combination with other organic pollutants (biocidal compounds or polycyclic aromatic hydrocarbons) with accumulation and consequent increasing serious toxic effects even further leading to oxidative stress and lipid peroxidation also in the liver as well as enhanced neurotoxicity in the brain and metabolic disorders [144,168]. Uptake and tissue accumulation of polystyrene microplastics (PS-MPs) studies in zebrafish have been conducted. Both 5 um and 20 um diameter MPs were found in the intestine after seven days of exposure, associated with inflammation, lipid accumulation and alterations of metabolic profiles in fish liver [125]. Inflammation and oxidative stress were observed in the zebrafish gut exposed to polystyrene MPs 5-um beads in size, at different concentration (50 \(\upmu\)g/L and 500 \(\upmu\)g/L), for 21 days, associated with alterations in the gut microbiome and tissue metabolic profiles [154]. The shape-depended effects cannot be underestimated. The recent scientific studies have shown that microplastic fibers resulted in more severe intestinal toxicity than microplastic fragments and beads did. Their accumulation caused mucosal damage and increased permeability, inflammation, and metabolism disruption as well as gut microbiota dysbiosis and specific bacteria alterations interfere with the immune system deteriorating host health [154,169]. PS particles \(<\) 5 um in size, polyamides, polyethylene, polypropylene, and polyvinyl chloride cause intestinal damage of adult zebrafish gut [137] although the most serious effects were found in the larval stage [111,125]. In this regard polyethylene (PE) particles 25 nm and 50 nm in sizes were found in cells of the intestinal epithelium in embryos of 0, 24, and 72 hpf [141]. Changes in gut microbiota with disorders of the metabolome and microbiome, as well as changes in the expression of genes correlated with epithelium integrity, were observed in both larval [55] and adult zebrafish [154,165,170]. Upregulation of intestinal Cytochrome P450 gene (cyp1a) was shown by [PERSON] et al. [69] in adult zebrafish. In zebrafish, [PERSON] et al. [138] reported transcriptomic and histological alterations on the mucosal epithelium of zebrafish although it recognizes plastic particles as inedible materials but inges them when mixed with food or for accidental ingestion due to small particles (1-5 um) [23]. Fluorescent MPs were observed in the villi, in the apical surface of the enterocytes, in the lamina propria and very frequently inside the goblet cells.