Exposure to alkaline water (pH > 9.0) is physiologically challenging for fish, yet our understanding of the physiology of alkaline tolerance in fishes is limited to a small number of ihighly specialized species. This study aimed to characterize mechanisms of alkaline tolerance 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) in Alberta, Canada. Stickleback from Buffalo Lake and a neutral reference lake (Buck Lake; pH = 8.2) were collected from the wild and acclimated to common conditions (pH = 8.0) for at least 2 months. Both populations were then exposed to alkaline conditions (pH = 9.5), resulting in a significant decrease in survival (14% by 7 d of exposure) in Buck Lake fish, but no mortality in Buffalo Lake stickleback. In a 4-d exposure to alkaline water, fish from both populations experienced characteristic inhibitions of ammonia excretion followed by subsequent recovery, in conjunction with an accumulation of ammonia within the body. However, no differences were observed between populations. Analysis of tissue Na+ and Cl- content showed a more pronounced decrease in Cl- in Buck Lake fish, suggesting that tighter regulation of Cl- homeostasis and/or acid-base balance may be an important feature of alkaline tolerance. RNA-sequencing analysis highlighted large differences in gene expression between the alkaline and neutral lake populations, and in response to alkaline exposure. Few of these changes in the expression involved genes known to be associated with nitrogen, ion, or acid-base balance. These data indicate that alkaline tolerance is higher in brook stickleback resident to an alkaline lake than those sourced from a neutral lake, a trait that may be related to differences in physiological and transcriptomic responses to alkaline exposure.
