Secretory carbonic anhydrase II - Finding the evolutionary key to the symbiosis of animal hosts and their cellulose-fermenting bacteria

Grasses, leaves and wood contain large amounts of structural carbohydrates such as cellulose, which in animals are digested with the help of symbiotic, cellulolytic microorganisms living in fermentative chambers in the stomach or gut. Although we find the identical relationship in grass eaters (ruminants), in leave-eaters (Colobus monkeys) and in wood-eaters (termites), still little is known about the evolutionary pre-requisites that enabled the development of this very complex symbiosis. Two carbonic anhydrases (CA; CA-VI and the recently described CA-II) are prominently expressed in the saliva of ruminating animals, suggesting that the enzymatic production of bicarbonate is basic for the regulation of oral pH and for maintaining a consistent milieu in the alimentary tract allowing microbial digestion of cellulose. Interestingly, the fermentative chambers of termites (Arthropoda), Colobus monkeys and ruminants show a similar slightly acidic to basic pH. Therefore, the expression of high amounts of CA-II in saliva or mucus of ruminants, camelids, Colobus monkeys and termites is considered to be an essential basis for the development of cellulose fermentation with the help of symbiotic bacteria.