Elevated CO increases energetic cost and ion movement in the marine fish intestine.

Energetic costs associated with ion and acid-base regulation in response to ocean acidification have been predicted to decrease the energy available to fish for basic life processes. However, the low cost of ion regulation (6-15% of standard metabolic rate) and inherent variation associated with whole-animal metabolic rate measurements have made it difficult to consistently demonstrate such a cost. Here we aimed to gain resolution in assessing the energetic demand associated with acid-base regulation by examining ion movement and O consumption rates of isolated intestinal tissue from Gulf toadfish acclimated to control or 1900 μatm CO (projected for year 2300). The active marine fish intestine absorbs ions from ingested seawater in exchange for HCO to maintain water balance. We demonstrate that CO exposure causes a 13% increase of intestinal HCO secretion that the animal does not appear to regulate. Isolated tissue from CO-exposed toadfish also exhibited an 8% higher O consumption rate than tissue from controls. These findings show that compensation for CO leads to a seemingly maladaptive persistent base (HCO) loss that incurs an energetic expense at the tissue level. Sustained increases to baseline metabolic rate could lead to energetic reallocations away from other life processes at the whole-animal level.