Intraspecific differences in tolerance to elevated pH and alkalinity in brook stickleback ( <i>Culaea inconstans</i> ) inhabiting neutral and alkaline lakes

This study aimed to characterize mechanisms of tolerance to elevated pH and carbonate alkalinity in brook stickleback (Culaea inconstans), a fish species with a broad pH habitat range, including highly alkaline waters such as Buffalo Lake (pH 9.2; [HCO3-] 20.0 mmol l-1) in Alberta, Canada. Stickleback from Buffalo Lake and a more pH-neutral, low-alkalinity reference lake (Buck Lake; pH 8.2; [HCO3-] 3.0 mmol l-1) were collected from the wild and acclimated to common conditions (pH 8.0; [HCO3-] 1.7 mmol l-1) for 2 months. Both populations were then exposed to alkaline conditions (pH 9.5; [HCO3-] 22.1 mmol l-1) in the laboratory, resulting in a significant decrease in survival (14% by 7 days of exposure) in Buck Lake fish, but no mortality in Buffalo Lake stickleback. In a 4 day alkaline exposure, no significant differences in nitrogen or ion regulation were observed between populations, but a trend towards reduced whole-body chloride concentration in Buck Lake stickleback suggested potential disruptions in ion and acid-base regulation in this population. In Buck Lake stickleback, genes involved in cellular stress and immune responses were differentially regulated in gill tissue in response to alkaline exposure, while this response was largely absent in Buffalo Lake stickleback. Overall, alkaline tolerance is higher in brook stickleback resident in an alkaline lake compared with those from a neutral lake, but tolerance does not appear to be a function of differences in nitrogen or ion/acid-base regulation. Rather, alkaline-tolerant stickleback mounted a relatively weaker transcriptomic stress response in the gill, which might represent transcriptional dampening that confers increased tolerance.