# Toxicological effects of sublethal microcystin-LR exposure in Labeo rohita: histopathological, ultrastructural, immunological, and biochemical impairments

## TL;DR

This study shows how low levels of a toxic substance can harm fish over time, affecting their organs and health defenses.

## Contribution

The first detailed study on long-term sublethal effects of microcystin-LR in Labeo rohita, revealing new insights into fish health impacts.

## Key findings

- Sublethal microcystin-LR exposure caused histological and ultrastructural liver damage in Labeo rohita.
- MC-LR altered immune and biochemical parameters and upregulated detoxification-related gene expression.
- Glutathione-S-transferase (GST) showed significant upregulation, suggesting its role as a potential biomarker.

## Abstract

Microcystins (MCs) are alarming aquatic contaminants having extensive health implications in fish. Despite growing concern, comprehensive studies on long term effects of MCs remain unexplored in Labeo rohita (rohu). This study aims to bridge the gap by investigating the pathophysiological effects of long-term sublethal exposure to microcystin-LR (MC-LR), the most toxic congener.

Healthy rohu juveniles (mean weight 25 ± 2.1 g), sourced from institute farm were acclimatized for 2 weeks. The sublethal experimental study consisted of two treatments (control group: T0 and one-tenth 96 h-LD50 dose group: T1) in three replications (10 fish per tank). The toxic effects were examined after 90 days by analysing histomorphology, ultrastructure, oxidative stress level, serum biochemistry, and the gene expression levels of antioxidant enzyme [superoxide dismutase (SOD) and catalase], immune-related (lysozyme, and immunoglobulin M), pro-inflammatory cytokine (interleukin-1β), apoptosis (caspase 9) and detoxification enzyme [phase I: CYP1A and CYP3A; phase II: glutathione-S-transferase (GST)] genes following standard analytical methods. Statistical analysis was performed using SPSS v. 22.0, IBM software. Parameters were analysed using an unpaired t-test. The results were expressed as mean ± standard error (SE).

MC-LR induced significant histological and ultrastructural alterations including vacuolation, hepatocyte degeneration, disintegration of heterochromatin, loss of nucleolus and mitochondrial swelling. It significantly (p-value <0.05) altered the immune and serum biochemical indices. Interestingly, the modulation in the expression of SOD, catalase, GST, CYP1A and CYP3A genes in different organs indicated their involvement in the antioxidant and detoxification process. A significant upregulation of GST expression in all organs signifies its potential as a prominent biomarker other than phase I enzymes.

Based on these findings, it is deduced that even sublethal levels of MC-LR can disrupt intrinsic antioxidant defences, immune responses, and detoxification mechanisms in rohu, potentially compromising fish health in natural ecosystems. This is the first report to detail long-term impacts in rohu, elucidating the mechanism of damage induced by MC-LR and also providing valuable insights for environmental monitoring and toxin management.