Aluminium exposure affects the morphology of gills and muscles and routine oxygen consumption of zebrafish

Aluminium (Al) is a metal highly diffuse in the Earth's crust. In the last decades, climate change and the associated phenomenon of acid rain are further increasing its environmental concentration. The acidification of water bodies facilitates the solubilization of the inorganic Al and increases its bioavailability for aquatic organisms. Studies carried out with different fish species have revealed that Al can produce toxic effects in fish by interfering with physiological activities and biochemical processes that, in turn, affect fertility, growth, and increase mortality. The neurobehavioral alterations link to the affections of swimming activity. However, until now, relatively scarce information is available about the effects of Al on the tissues relevant for the swimming performance of fish, such as gills and muscles. The gills of fish represent the first organ interacting with the metal in water bodies, and their alterations can affect the whole animal physiology. The impaired functionality of muscle is relevant for swimming behavior. Here we report the effects of the chronic exposure to 11 mg/L of Al for 10, 15, and 20 days on gills and muscle morphology, in vivo routine oxygen consumption (rMO2, a measure of the whole animal resting energy requirement), and routine activity parameters (tail beats) of zebrafish adults. This experimental model is an optimal bioindicator to evaluate environmental pollutants' ecotoxicological effects. The histological analysis highlights that the morphology of gills is altered by the Al treatment, although with different degrees depending on the exposure time. 10 days of Al exposure determines dramatic changes in branchial tissue which shows a disordered structure and a reduced lamellae number. After 15 days of Al exposure the branchial tissue shows signs of recovery, that is a more ordered structure and a higher number of lamellae. In 20 days- Al exposed animals there is a further morphological recovery. The morphological alterations of muscles parallel those of gills. A higher alteration of both white and red muscle tissues is evident in the organisms exposed for less time to Al, while a recovery of the normal morphological characteristics is observed after longer time of exposure. The recovery of muscle morphology is associated with its functionality, as demonstrated by the gradual increase of tail beats during the exposition. These changes reflect in the rMO2 consumption that is maximum after 10 days of exposure, suggesting a stress response for the animals. Then this parameter decreases and reaches the lowest levels after 20 days of treatment. This result suggests an adjustment of the resting metabolic pathways to face the reduced efficiency in the gaseous exchange of gill tissue.