Water pollution caused by ash from grassland fires alters the molecular, biochemical, and morphological biomarkers of non-biting midge larvae

The frequency and intensity of wildfires have been increasing in many parts of the world, which may result in biodiversity loss. Wildfires can devastate plant communities, generating toxic ash that pollutes watercourses through runoff. However, our understanding of the effects of ash exposure on aquatic biodiversity is still limited. Here, Chironomus sancticaroli larvae were exposed to various concentrations of grassland ash (ranging from 0.01 to 100 g/L) in acute and subchronic assays. The tested ash concentrations are environmentally relevant, as similar levels have been observed in nature after moderate runoff in post-fire conditions. We experimentally tested whether ash-contaminated waters increase mortality, induce molecular and biochemical biomarker changes, and reduce body size in chironomid larvae. Results showed that grassland ash contains micro and nanoparticles with high solubility and is rich in metals and polycyclic aromatic hydrocarbons. Adding ash to the water increased pH, conductivity, and total solids while decreasing dissolved oxygen levels. The lethal concentration (LC50) of grassland ash for C. sancticaroli was estimated at 31.43 g/L after acute exposure. However, exposure to sublethal ash concentrations led to higher metallothionein and hemoglobin gene expression related to metal sequestration and adaptation to lower oxygen levels. At the biochemical level, ash exposure increased the activity of enzymes associated with detoxification capacity, such as alpha and beta esterases. It also led to oxidative stress and damage to cell membranes, as indicated by the increased enzymatic activity of superoxide dismutase, glutathione S-transferase, and lipid peroxidation. Finally, larvae exposed to ash reduced body size by up to 55 % at exposure to higher concentrations. As wildland fires produce ashes that are later moved to the aquatic ecosystem when lixiviated, impacting the resident aquatic biota, environmental managers should adopt measures to reduce the likelihood of wildfires and the ash load to watercourses.