Glycoside hydrolases from the tunics of two Antarctic ascidians (Ascidia challengeri and Pyura bouvetensis) and the tropical species Phallusia nigra

Abstract

Tunicates are the only metazoans capable of synthesising cellulose, which is present in the tunic. Cellulases (endo and exoglucanases) that can degrade or remodel cellulose might be necessary during the growth phase and during scarring after injury to the tunic. However, the occurrence of these enzymes in the tunic has not been reported. Hence, the aim of the study was to investigate the presence of cellulases as well as other glycoside hydrolases, such as xyloglucanases and mannanases, in the tunic of two Antarctic ascidian species, Ascidia challengeri and Pyura bouvetensis, and in the tropical species Phallusia nigra. Endoglucanases and xyloglucanase were found in all three species. Galactomannanases were detected in P. nigra and P. bouvetensis but were not found in A. challengeri. Cellulases that can degrade Avicel were found in P. bouvetensis. To the best of our knowledge, this is the first report detailing the presence of these enzymes in the tunic of ascidians. In A. challengeri, the optimal temperature for the endoglucanase activity was 9 °C, higher than their habitat temperature, but within the temperature tolerance of the species, while that of P. nigra was at 25–30 °C, which is close to the maximum temperature of its habitat. The temperature at which the endoglucanase has optimal activity is discussed as a proxy to the limit of thermal tolerance of the species that express this enzyme.