PATHWAYS OF INORGANIC CARBON FIXATION IN THE ENDOSYMBIONT-BEARING LUCINID CLAM LUCINOMA-AEQUIZONATA .2. ANALYSIS OF THE INDIVIDUAL CONTRIBUTIONS OF HOST AND SYMBIONT CELLS TO INORGANIC CARBON ASSIMILATION

The incorporation of 14C from H14CO3- into acid-soluble metabolites was examined in the endosymbiont-containing gills and purified symbionts of the lucinid clam Lucinoma aequizonata. HPLC, paper chromatography, and enzymatic techniques were used to identify and quantify compounds into which label was incorporated in continuous and pulse-labeling experiments. Both symbiont and host cells fix carbon in considerable quantities; however, each incorporates carbon into a distinctly different subset of compounds. In intact gills the initial carbon fixation product is malate. Label subsequently accumulates in succinate, glucose phosphate, glutamate, alanine, and glycogen. In the symbiont cells carbon is incorporated into aspartate, 3-phosphoglycerate, malate, and citrate. Purified symbionts incorporate carbon into the same compounds in the same proportions as do symbionts that are exposed to labeled H14CO3- while still within the host tissues. Under the conditions tested, purified symbionts released no significant quantities of labeled compounds into the incubation medium.