Limits and patterns of acid-base regulation during elevated environmental CO₂ in fish

Area of Science:

• Physiological Ecology
• Comparative Physiology
• Environmental Toxicology

Background:

• Aquatic environments can experience hypercarbia (elevated CO2), posing acid-base challenges for fish.
• Fish typically compensate for hypercarbia by altering blood pH (pHe) and plasma bicarbonate (HCO3-) levels.
• A proposed 'apparent upper bicarbonate threshold' limits pHe recovery in fish during acute hypercarbia.

Purpose of the Study:

• To review pH regulation strategies in fish exposed to hypercarbia.
• To discuss the mechanisms and prevalence of preferential intracellular pH (pHi) regulation.
• To explore the evolutionary origins of pH regulatory strategies in vertebrates.

Main Methods:

• Modeling the capacity and theoretical limits of pHe compensation.
• Reviewing studies on fish exposed to severe acute hypercarbia.
• Examining the presence of preferential pHi regulation in embryonic reptiles.

Main Results:

• Some fish can fully protect pHi despite severe reductions in pHe during hypercarbia.
• Preferential pHi regulation is common in fish subjected to severe hypercarbia, exceeding pHe compensation limits.
• Preferential pHi regulation may be an ancestral trait in vertebrates, observed in embryonic reptiles.

Conclusions:

• Fish possess sophisticated mechanisms to regulate intracellular pH (pHi) under extreme hypercarbia.
• Preferential pHi regulation allows fish to tolerate high CO2 levels beyond previously understood limits.
• The evolution of pH regulation in vertebrates may involve the retention or loss of ancestral embryonic traits like preferential pHi regulation.