Acidification alters anxiety-like behaviour and brain gene expression in zebrafish

CO₂-driven acidification of freshwater ecosystems is an increasing problem that could impact aquatic life in the future. Despite their physiological tolerance to naturally fluctuating pH, freshwater fishes exhibit behavioural and neurological changes in response to acidification. To determine the molecular responses associated with these anticipated impairments for the near-future, we examined the behavioural and transcriptomic responses of zebrafish (Danio rerio) to acidification, focusing on the brain and gills, which mediate behaviour and acid-base regulation. Adult zebrafish were exposed to control (∼ 500 μatm) and elevated CO₂ (∼1000 μatm) for five days and submitted to Open Field and Novel Object Approach tests, revealing a decrease in anxiety-like behaviour under elevated CO₂. Acidification caused differential expression of genes involved in cytoskeletal organization, cellular transport, immunity, and the visual system in the brain, indicative of brain cell rearrangements. Conversely, there was no differential gene expression observed in the gills. However, the co-expression of genes involved in immune response and oxidoreduction, which are negatively correlated with elevated pCO₂, along with a reduction in anxiety-like behaviour indicate a lower level of oxidative stress. Our findings indicate that zebrafish can perform acid-base regulation despite acidity changes predicted for the end of the century, but reveal that physiological tolerance to acidification does not confer resistance to neurological and behavioural impairments caused by rapid climate change.