Abstract
When mobilized from surrounding soils and binding to gills at moderately low pH, aluminum (Al) cations can adversely affect fish populations. Furthermore, acidification may lead to allostatic overload, a situation in which the costs of coping with chronic stress affects long-term survival and reproductive output and, ultimately, ecosystem health. The brain's serotonergic system plays a key role in neuroendocrine stress responses and allostatic processes. Here, we explored whether sublethal effects of Al in acidified water affects serotonergic neurochemistry and stress coping ability in a unique land-locked salmon population from Lake Bygelandsfjorden, in southern Norway. Fish were exposed to untreated water with pH 6.5 and 74 μg Al l−1 or acidified (pH 5.5) water with different aluminum concentrations ([Al]; 74–148 μg l−1) for 5–6 days. Afterward, effects on stress coping ability were investigated by analyzing plasma cortisol levels and telencephalic serotonergic neurochemistry before and after a standardized acute stress test. Before the stress test, positive dose-response relationships existed between [Al], serotonergic turnover rate and plasma cortisol. However, in acutely stressed fish, exposure to the highest [Al] resulted in reduced cortisol values compared with those exposed to lower concentrations, while the positive dose-response relationship between Al concentrations and serotonergic turnover rate persisted in baseline conditions. This suggests that fish exposed to the highest Al concentration were unable to mount a proper cortisol response to further acute stress, demonstrating that neuroendocrine indicators of allostatic load can be used to reveal sublethal effects of water acidification—and potentially, the environmental impacts of other factors.
Highlights
Exposure to environmental contaminants can affect the survival of organisms via direct toxicity
We demonstrated a positive dose dependency between brain 5-HT turnover rate and Al concentrations in moderately acidified water
This dose dependency was reflected in baseline cortisol values, it did not follow the same general pattern in the confinement-stressed fish
Summary
Exposure to environmental contaminants can affect the survival of organisms via direct toxicity. More often than not, effects are subtler, including chronic or intermittent activation of physiological stress responses, and changes in sexual behavior, prey-capture cap ability, and disease resilience. Such impacts have been suggested to mediate the effects on long-term organism survival, life history trajec tories, and reproductive output, compromising ecosystem health (Scholz et al, 2012). One or more intense perturbations resulting in more intense chronic stress may impose deficits in the way the brain and other coping systems respond to additional stressors (McEwen, 2000, 2007) This organismal state, often referred to as allostatic overload, is associated with more direct fitness-decreasing consequences
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