Defining the metabolic compensation pathways employed during low-level hypercapnia in red drum (Sciaenops ocellatus)

Since the pre-industrial era, anthropogenic CO₂ emissions have raised oceanic CO₂ by 40% and reduced ocean pH by 0.1 unit. This results in acid-base disturbances in marine organisms that are compensated through regulatory pathways. Many estuarine fishes, including red drum (Sciaenops ocellatus), regularly encounter periods of elevated CO₂, which may impart a level of species resilience to ocean acidification. Initial studies examined the time course of whole animal acid-base compensation in response to varying CO₂ concentrations. Under control conditions red drum showed net base excretion; however, the onset of CO₂ exposures resulted in a dose-dependent increase in acid excretion during the initial 2h time period. Notably, net acid excretion returned to baseline levels by 4h of exposure of up to 5,000 μatm, but remained elevated throughout 15,000 and 30,000 μatm exposures. Subsequent studies assessed the plasticity of branchial acid-base pathways after exposure to various CO₂ levels using qPCR. 1,000 μatm exposed fish were sampled at 1h, 4h, 24h, 72h, and 14d, while 6,000 and 30,000 μatm exposed fish were sampled after 1h, 4h, and 24h of exposure. Of a suite of acid-base relevant genes, only the Na⁺ HCO₃⁻ co-transporter (NBC) was upregulated in 1,000 and 6,000 μatm treatments. In contrast, the majority of relevant genes were up-regulated by 4h of exposure to 30,000 μatm, with the exception of the electrogenic anion exchanger slc26a3a, which was only upregulated by 24h of exposure to 30,000 μatm. Cytoplasmic carbonic anhydrase and Na⁺ H⁺ exchanger 1 exhibited no change in expression to 30,000 μatm. Localization studies examined the position of the V-type H⁺ ATPase (VHA) within gill ionocytes. Under control conditions, VHA is diffusely distributed throughout the cytoplasm of the cell, although oriented toward the apical pole; there was no evidence of basolateral localization. Exposure to 6,000 μatm CO₂ did not result in translocation of cytoplasmic VHA to the apical membrane. Overall, these results indicate that red drum can quickly compensate to a wide range of environmentally relevant acid-base disturbances using baseline cellular machinery, yet are capable of acid-base plasticity in response to extreme challenges.