Assessing Low-Mineralized Surface Water via Danio rerio Embryo Toxicity Test: A Case Study of Glacial Lakes in the Northern Tien Shan.

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.