# Acute styrene exposure induces hepatocellular injury and molecular stress responses in Oryzias celebensis: Evidence for a tropical sentinel species in ecotoxicological monitoring

## TL;DR

This study shows that styrene, a plastic chemical, harms the liver of a tropical fish species, making it a good indicator for monitoring pollution in tropical waters.

## Contribution

The study introduces Oryzias celebensis as a novel tropical sentinel species for ecotoxicological monitoring of styrene exposure.

## Key findings

- Styrene exposure caused concentration-dependent liver damage in Oryzias celebensis at ≥0.5 mg/L.
- CYP1A1 gene expression increased 36.9-fold at 0.75 mg/L, indicating AhR pathway activation.
- Strong correlations were found between gene expression and liver lesion severity, confirming molecular and histopathological links.

## Abstract

Plastic-derived monomers such as styrene are increasingly detected in tropical freshwater ecosystems at concentrations approaching 0.8 mg/L. These contaminants pose toxicological risks to aquatic organisms, particularly through oxidative stress, inflammation, and metabolic disruption. Oryzias celebensis (Celebes medaka), an endemic tropical freshwater species, offers high ecological relevance for assessing pollutant impacts in Southeast Asia. This study evaluated the hepatic histopathological and molecular responses of O. celebensis following acute styrene exposure and assessed its suitability as a tropical sentinel species.

Adult O. celebensis were exposed to 0.1, 0.25, 0.5, and 0.75 mg/L styrene for 96 h under semi-static conditions. Liver tissues were examined for cytoplasmic vacuolization, necrosis, inflammatory cell infiltration, and congestion using standard histopathological scoring. Expression of biomarker genes related to detoxification (cytochrome P450 1A1 [CYP1A1]), oxidative stress (catalase, superoxide dismutase [SOD]), and inflammation (tumor necrosis factor [TNF]) was quantified using reverse transcription quantitative polymerase chain reaction. Correlations between biomarker expression and tissue lesions were analyzed using Spearman’s coefficients.

Styrene exposure induced concentration-dependent hepatic injury. Histopathological lesions intensified markedly at ≥0.5 mg/L, with prominent vacuolization, necrosis, and inflammatory infiltration. CYP1A1 was strongly upregulated, showing a 36.9-fold increase at 0.75 mg/L (p < 0.01), indicating robust activation of the aryl hydrocarbon receptor (AhR) pathway. Antioxidant enzymes (catalase and SOD) and TNF expression also increased significantly, reflecting oxidative and inflammatory stress. Strong positive correlations (rs = 0.93–0.99) were observed between gene expression and lesion severity, confirming mechanistic links between molecular responses and tissue pathology.

Acute styrene exposure triggers coordinated hepatocellular injury and molecular stress responses in O. celebensis through activation of aryl hydrocarbon receptor-CYP1A1, nuclear factor erythroid 2-related factor 2, and nuclear factor kappa-light-chain-enhancer of activated B cells pathways. The strong correspondence between histopathological and transcriptional biomarkers demonstrates that this species is highly sensitive to styrene toxicity and suitable for ecotoxicological monitoring. Findings highlight the need for environmental surveillance of industrial pollutants in tropical freshwater systems.