The article was written as part of the Ministry of Science and Higher Education of the Republic of Kazakhstan’s project AP 23486220, titled “Study of the important organ systems reactivity in the main commercial fish species of the Ile-Balkhash basin under anthropogenic load.” (2024-2026). The article represents the study investigating the long-term effects of polyethylene terephthalate (PET) microplastics on zebrafish (Danio rerio). Zebrafish exposed to PET microplastics at concentrations of 1.9 mg/l and 5.64 mg/l exhibited significant kidney and gill damage. This included necrosis, circulatory disorders, and tissue damage. Histological analysis revealed specific pathological changes such as vacuolization and hyaline-droplet degeneration in kidneys, and destruction of epithelium and lamellae in gills. Notably, no compensatory-adaptive reactions in gills were observed, indicating a lack of adaptation to prolonged exposure. These findings suggest that microplastics may migrate through the bloodstream, disrupting normal kidney and gill structures. The study highlights the severe impact of microplastic pollution on fish organs, emphasizing the urgent need for further research to develop strategies for mitigating microplas-tic pollution in aquatic ecosystems. Additionally, the observed lack of adaptive responses underscores the potential for significant ecological consequences, urging immediate attention to address this critical environmental issue.

The ecotoxicological evaluation of Ile Alatau headwaters is extremely important for South-East Kazakhstan as mountain cryoshpere provide gross of water sources that supply Almaty agglomeration and surrounding industrial and agricultural areas of Almaty region. In the current study, the toxic potential of surface water of two gorges, namely Kuygensay gorge located directly above Almaty city and referenced Orta Turgen gorge, including moraine lakes, alpine lake, springs and main current at altitudes 2400 – 3500 m above sea level, was assessed by acute D.magna toxicity test according to OECD guideline. No significant toxicity was shown for water samples from Orta Turgen high-altitude tributaries and alpine lakes, whereas Titov moraine lake (the very source of Kuygensay river), main current and tributary spring of river Kuygensay demonstrated significant toxicity toward young daphnids in acute tests. The indexation based on D.magna immobilization/mortality data scoring, showed that headwaters of river Kuygensay are considered to meet “moderately toxic” to “toxic” class of toxicity, and Almaty agglomeration is probably the main source of pollution. These data provide opportunity to further investigation on pollution of glaciers and high-altitude water resources of Ile Alatau mountain range. Key words: high-altitude water sources, D.magna, bioassay, toxicity assessment, Ile Alatau

The gills of teleost fish are multifunctional organs involved in respiration, osmoregulation, and immune defense, and are highly sensitive to environmental stressors. This study evaluated the histological and ultrastructural condition of the gill epithelium of common carp (Cyprinus carpio Linnaeus, 1758) inhabiting the middle reaches of the Ile River (Kazakhstan) under chronic anthropogenic pressure. Although hydrochemical parameters remained within permissible limits for fishery waters, all examined specimens (n = 10) exhibited pronounced structural alterations. Histological analysis revealed edema, epithelial hyperplasia, lamellar fusion, necrosis, and desquamation, indicating substantial impairment of gill integrity. Ultrastructural examination demonstrated advanced developmental stages of rodlet cells, mast cell degranulation, and increased activity of mucous and chloride cells, accompanied by mitochondrial alterations and autophagosome formation. Alongside destructive changes, moderate compensatory reactions were observed, reflecting tissue-level adaptation to prolonged environmental stress. These findings demonstrate that gill histopathology provides a sensitive tool for detecting sublethal effects of chronic anthropogenic impact, even when conventional hydrochemical indicators suggest acceptable water quality. The results support the use of cellular and tissue-level biomarkers in freshwater environmental monitoring programs.

Bioassays are rarely used in ecotoxicological studies of high-altitude lakes located above 3000 m a.s.l., as ultra-low mineralization of water poses a major obstacle to toxicity assessment in biological objects. For the first time, we conducted a 96 h bioassay of surface water from supraglacial, proglacial and moraine-dammed lakes of two distinct mountain ranges of the Northern Tien Shan (average elevations of 3000–3600 m a.s.l.) using Danio rerio embryos. The study demonstrates the feasibility of using the D. rerio embryo toxicity test to survey the initially low-mineralized surface water of high-altitude lakes. For several Ile Alatau lakes, analysis of lethal and sublethal endpoints (including molecular biomarkers cyp1a, xpc, and dpolb gene expression) demonstrated a mild toxic effect, whereas physicochemical analysis failed to reveal contamination exceeding maximum permissible concentrations. In the Dzungarian Alatau, significant pollution with total petroleum hydrocarbons was detected in the lake cascade beneath the remote Tuyte Glacier. An altitude-dependent effect on zebrafish embryos was observed, ranging from moderate to no toxicity. The obtained results indicate that D. rerio embryo toxicity test might be applied to ecotoxicological studies of ultra-fresh water sources supplied by the melting mountain cryosphere, thus simplifying, reducing the cost of, and expanding the monitoring of remote high-mountain areas.