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

CO2-driven acidification of freshwater ecosystems is an accelerating process that can impact aquatic life in the future. Despite their importance in accurately predicting how future freshwater teleost fish will respond to changing environments, molecular mechanisms coordinating the response to elevated CO2 have not been extensively studied. Making use of the zebrafish (Danio rerio) considered 'tolerant' to acidified waters, we analyzed the behavioural and transcriptomic response to acidification to highlight potential response mechanisms in the brain and the gills which mediate behaviour and perform acid-base regulation. Adult zebrafish were exposed to control (500 uatm) and elevated CO2 (1000 uatm) for five days and submitted to Open Field and Novel Object Approach tests, revealing a decrease in anxiety-like behaviour in response to elevated CO2. In the brain, acidification caused differential expression of genes involved in cytoskeletal organization, cellular transport, immunity and the visual neural system. In the gills, an elevated expression of genes involved in immune response and oxidoreduction with increasing pCO2 in combination with decreased anxiety-like behaviour point to an increase of oxidative stress. However, the absence of differentially expressed genes in gills indicates that this species has adapted to regulating its acid-base balance in acidified waters, common in its natural habitat. Our study pinpoints the molecular response to elevated CO2 possibly triggering behavioural changes in zebrafish and suggest that not only marine, but also freshwater fishes will be behaviourally affected by acidification due to rapid climate change.