Carbon Dioxide Fixation in Marine Invertebrates*

Carbon dioxide fixation has not been studied extensively in the invertebrate animal groups. In an early study Van Niel et al. (1) with Cl1 showed fixation of COZ into succinic acid in the protozoan Tetrahymena geleii. Hultin and Wessel (2) studied incorporation of C% into proteins of developing sea urchin eggs (Psammechinus miliaris) with attention on metabolic patterns of development rather than individual fixation reactions. The parasitic nematode worm Heterah%s gallinae was found by Fairbairn (3) to fix COZ into propionic acid, which is excreted, and into a nonvolatile acid, probably succinic. Malic enzyme has been extracted from the blood of the silkworm and several forest insects (4) and from muscle of the parasitic nematode Ascaris lumbricoides (5), indicating that these animals can also fix COZ. This report concerns processes of carbon dioxide fixation in the oyster, which has the special problems of converting COZ to shell carbonate and of secreting a matrix of conchiolin in which the CaC03 crystals are deposited when the shell is formed. The deposition of carbonate may well be related to decarboxylation reactions of the shell-forming mantle tissue. This is suggested by a high oxaloacetic decarboxylase activity in this tissue (6) and a marked increase in calcium deposition on the addition of oxaloacetate to a preparation in vitro consisting of mantle tissue and its attached shell (7). The present study has demonstrated that the oyster has the capacity to deposit bicarbonate of sea water as shell carbonate and to fix carbon dioxide into citric acid cycle intermediates and compounds which contribute to the organic matrix of shell. Details of the pathway have been followed by incubating mantle tissue with labeled compounds and identifying labeled intermediates.