Drenched in microplastic environment: Physiological and metabolic disruptions in fish

Introduction

Micro-plastics (MPs) are plastic particles sized between 1 μm and 5000 μm, while smaller particles are termed nano-plastics (NPs). MPs can be categorized into primary and secondary types. Primary MPs are intentionally manufactured small particles found in products like cosmetics and personal care items (Benson, N. U., et al., 2022) [3] while Secondary MPs arise from the degradation of larger plastic items, such as bags and bottles, due to environmental factors like sunlight, UV radiation, and abrasion. Bacteria also contribute to the breakdown of plastics into smaller particles (Ioni, Y. V., et al., 2025) [16] . The rate of MP formation is influenced by various factors, including the type of polymer, environmental conditions etc. (Ge, J., et al. 2023) [11] . Understanding the sources and impacts of MPs is essential for developing strategies to mitigate their environmental effects. Fish serve as excellent model organisms for studying microplastic impacts as it shares more common features with humans as compared to other invertebrates. Their physiological responses provide insights into the overall effects of microplastics on aquatic life (Prokić, M. D., et al., 2021; Foley, C. J., et al., 2018) [30,9] . Research has revealed that different fish species exhibit diverse responses (Hossain, M. A., & Olden, J. D. (2022) [15] ; At the cellular level, exposure results in inflammation, oxidative stress, and cell death, with damage severity depending on particle concentration and size (da Silva Brito, W. A., et al., 2023) [5] . Moreover, the potential entry of microplastics into the human food chain via food and other sources emphasizes the importance of assessing risks and establishing proper guidelines. This review aims to consolidate current knowledge about the specific physiological and metabolic disruptions in different fish species. It documents the most commonly affected organs, their histopathological changes, and variations in microplastic concentrations across diverse aquatic environments.

Methods

A comprehensive literature search was performed using databases such as PubMed, Web of Science, Scopus, ScienceDirect, and Google Scholar. The inclusion criteria focused on peerreviewed articles published between 2010 and 2025, centered on fish models and organ-

International Journal of Biology Sciences https://www.biologyjournal.net specific studies. Studies not peer-reviewed or lacking organ/tissue-level assessments were excluded. Keywords employed in the search included "microplastics," "fish," "organ damage," "metabolic disruption," "histopathology," "polymer type," "exposure duration," and "species-specific responses." This strategy captured a broad array of studies on the physiological and metabolic impacts of microplastics on fish, ranging from microplastic types to their specific effects on various fish species.

Effects of Microplastic Exposure on Freshwater Fish Survival Rates

Exposure to MPs can decrease the survival rate of fish by causing blockage or toxicity in their gastrointestinal tract. By inducing malnutrition or changing eating habits, it may also have an impact on the particular growth rate of fish. [6,29,28] . A Study conducted by Md., Moshiur, Rahman., et al., 2022 [24] revealed that males exposed to MPs had a considerably elevated death rate in comparison to the other groups. The impact of MP exposure on fish survival and growth rates might differ based on the specific characteristics of the MPs, such as their kind, size, and concentration in the surrounding environment (Mostafa, A. R., et al., 2022) [28] . [17,27,1] .

Effects of MP

Kidney

Kidneys are affected by MPs as they can accumulate in the renal system and may contribute to kidney damage or dysfunction. Exposure to MPs can induce oxidative stress in the kidneys, leading to inflammation and potential impairment of kidney function or other renal disorders. The presence of MPs in the kidneys can trigger renal toxicity, which can affect the normal filtration and excretion functions of the kidneys. (Mohamed, Hamed., et. al., 2021; Alaa, El-Din, H., 2022) [27,1] .

Gills

MPs can cause damage to gill tissues, reducing the surface area for oxygen exchange affecting the fish's ability to extract oxygen from water, leading to inflammation, impaired respiratory function and feeding issues (Cao, J., et al., 2023; Mohamed, Hamed., et al., 2021; Fritz-Peter, Hager., 2022) [4,27,10] .

Intestines

MPs can cause blockages and damage to the intestinal lining in fishes, leading to reduced nutrient absorption and overall health (Ricardo Almeida, 2023) [37] . [25,19,26,18] . [21,12,10,26,13,32,8,36,22] . [34,35,14] . Endocrine disruption caused by MP exposure may include interference with hormone regulation, potentially leading to reproductive, developmental or other detrimental impacts (Lin, W., et al., 2023, Solleiro-Villavicencio, H., et al., 2020) [23,33] . [27] lamellar fusions, lifting of epithelium, shortening and degeneration of secondary lamellae, hyperplasia, and deposition of MPs between primary lamellae. In spinal cord and notochord, the effects were degeneration and protrusion of meninges, deformation and deviation of notochord from its central axis, edema, degeneration of notochord (disappearance of vacuolar cells), deviation of spinal cord from the central axis, and loss of vacuolar cells in notochord. The intestinal tissue exhibited degeneration of basement membrane, inflammatory cells, goblet cells, atrophy of submucosa, pyknotic nuclei, hemorrhage, and vacuolization of mucosal cells. [12] 9.

Brain

Reproductive organs and endocrine disruption

Poecilia reticulata (Fish) and Gyrodactylus turnbulli (Parasite)

PP-MPs -host susceptibility to disease and mortality was assessed higher pathogen burdens, increased mortality events and reduced disease resistance Numair, Masud., et al., 2023 [29] 10. 2020) [13] .

Clupea harengus,

12. Cyprinus carpio mixture exposure of the PE-MPs and 4nonylphenol (4-NP)

Gills, brain, Liver, histopathological and histochemical biomarkers

Alterations included severe edema, lifting of the outer epithelium, interlamellar fusion and vacuolation, secondary lamellar shortening and complete fusion, increased mucous cell numbers in the gill tissue, enlargement of inner layer stratum periventricular, cell degeneration with pyknotic nuclei, increased blood capillaries, spongiosis in the brain tissue (optic tectum), central vein hemorrhage, shrunken and fatty degeneration of hepatocytes, rosette shapes around small Fritz-Peter, Hager., (2022) [10] congested blood sinusoids, vacuoles, necrosis, and severe glycogen reduction in the liver tissue.

13.

Coregonus peled Gmelin

Fluorescent PS-MPs Intestine

Activity of digestive enzymes (α-Amylase, non-specific esterases, pancreatic trypsine, bile saltactivated lipase, aminopeptidase N) Activity of antioxidant enzymes (catalase, glutathione S-transferase, glutathione reductase, glutathione peroxidase)

positive effect on the activity of pancreatic trypsine and bile salt-activated lipase. Intestinal membrane enzyme aminopeptidase stimulated. Increase in the specific activity of catalase.

Yulia, A., et al., 2023 [38] 14.

Zebrafish PS-MPs Intestine, brain

Growth inhibition and adverse effects on inflammatory responses and intestinal permeability Alteration of neurotransmitter metabolites and changes in microbial groups inhibited growth and affected inflammatory responses and intestinal permeability. Transgenerational deficits and disrupted the regulation of the brain-intestinemicrobiota axis.

Miao, Teng., et al., 2022 [26] 15. Villi height, width, and epithelial cell height. Indices of damage to the small intestine.

Villi height and width and epithelial cell height were affected. Indices of damage to the small intestine organ and reaction patterns of villi, epithelial, goblet and cryptic glandular cells, leucocytic infiltration and blood congestion revealed alteration. Induced small intestinal wall degeneration John, J., et al., 2022 [19] 17.

Chanos chanos,

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