Pyraclostrobin disrupts ion regulation by suppressing mitochondrion-rich ionocyte differentiation in zebrafish embryos.

Pyraclostrobin (PCS) is a widely used strobilurin fungicide that often contaminates aquatic environments via surface runoff, posing risks to fish. Because ion regulation in fish embryos relies on ATP generated by mitochondrial respiration, and PCS inhibits mitochondrial complex III, we investigated the effects of PCS on ionic homeostasis in developing fish. We exposed zebrafish embryos to PCS (0-400 μg/L) for 96 h and assessed mortality, hatching rates, heart rates, whole-embryo ion contents (Na+, K+, and Ca2+), acid (H+) secretion, ionocyte densities, epidermal stem cell counts, and expressions of key ion transporter and differentiation genes. The 96-h median lethal concentration (LC50) was approximately 124 μg/L, and concentrations ≥75 μg/L significantly reduced hatching success. At 75 μg/L (a sublethal level), PCS caused decreases in Na+, K+, and Ca2+ contents, suppressed H+ secretion, and reduced the density of ionocytes. PCS exposure also diminished the number of tp63+ epidermal stem cells that differentiate into ionocytes. A quantitative PCR revealed significant changes in mRNA levels of ion regulation genes: upregulation of acid-secretion transporters (rhcg1 and atp6v1a) and differentiation factors (tp63 and gcm2), and downregulation of Na+/Ca2+ transporters (slc9a3 and trpv6). In summary, acute PCS exposure impaired the development and function of mitochondrion-rich ionocytes, leading to ionic imbalances (loss of Na+, K+, and Ca2+ and reduced H+ excretion) and physiological distress in zebrafish embryos. These findings highlight a potential mechanism by which PCS threatens early-life stages of fish and underscore the ecological risk of this contaminant.