A COMPARISON OF CARBONIC ANHYDRASE AMONG INVERTEBRATES, WITH A FOCUS ON CEPHALOPODS

Abstract

Carbon dioxide (CO2) is both an unavoidable waste product of aerobic carbohydrate metabolism and a fuel source for autotrophic and chemoautotrophic organisms. At biological pH, CO2 rapidly reacts with interstitial and intracellular water to form carbonic acid (H2CO3), that then dissociates into protons (H+) and bicarbonate (HCO3-). While CO2 and H+ readily diffuse across biological membranes, the majority of the CO2 in living tissues is in the membrane-impermeable form HCO3-. The enzyme carbonic anhydrase (CA) catalyzes the reversible hydration of CO2 with water to maintain an instantaneous equilibrium between these chemical species. CA is not only central to the transport and excretion of CO2 in animals (or uptake in autotrophic organisms), but is also indirectly involved in important physiological processes, such as osmoregulation and acid-base balance. The multiple functions of CA are a result of multiple isoforms that are localized to specific subcellular compartments/fractions. Furthermore, the level of CA activity in an organism can be induced to change in response to conditions in the ambient environment, and may also reflect the metabolic rate of the organism. This enzyme has been studied in cell and tissue types from numerous organisms, but has never been systematically characterized in squids. This dissertation examined CA activity in gill and mantle muscle among several cephalopod species in terms of aerobic mass-specific metabolic rates (MR), evolutionary relationships, and environmental conditions. It also compared the protein-specific activity of CA in the respiratory tissue reported in the literature for a broad array of invertebrates. The CA activity in gill and mantle muscle tissue from three squid species was measured to examine whether differences in activity may be related to phylogenetic relationships or environmental adaptations. The three squid species, Dosidicus gigas, Lolliguncula brevis, and Doryteuthis pealeii have similar MR but endure different physiological demands due to their respective environmental conditions. The largest member of family Ommastrephidae, Dosidicus gigas, undergoes diel vertical migrations into a well-defined oxygen minimum zone in the eastern Pacific. The brief squid, Lolliguncula brevis, is the only squid species that inhabits the wide-ranging abiotic conditions of estuarine waters. This species is in the same family as Doryteuthis pealeii, yet the latter requires narrower environmental parameters. For all three species the total CA activity was greater in gill tissue than in mantle muscle, but the activity in each tissue was statistically the same between these species. The distribution of CA isozymes within the subcellular compartments, however, was