INORGANIC CARBON AND TEMPERATURE REQUIREMENTS FOR AUTOTROPHIC CARBON FIXATION BY THE CHEMOAUTOTROPHIC SYMBIONTS OF THE GIANT HYDROTHERMAL VENT TUBE WORM, RIFTIA-PACHYPTILA

Riftia pachyptila, the giant hydrothermal vent tube worm, lives in a t hermally and chemically complex environment. It apparently derives the bulk of its nutrition from its chemoautotrophic symbionts, which are found at very high densities in a large internal organ, the trophosome . Although somewhat buffered from the environmental variation, the int ernal symbionts likely are exposed to significantly different temperat ures in different host individuals, in different positions in the same host individual, and at different times in the same host individual. Temperature optima for autotrophic carbon fixation varied from 25 degr ees to 35 degrees C in trophosome preparations from eight individuals. Because of a variety of factors, both the mode of supply of inorganic carbon to the symbionts in the trophosome and the adequacy of that su pply (with respect to saturation of demand) are not understood On the basis of experiments varying the concentrations of bicarbonate and dis solved carbon dioxide, an apparent K-m for dissolved carbon dioxide of between 0.021 and 0.035 mM is demonstrated, and dissolved carbon diox ide is determined to be the form of inorganic carbon taken up and used by the symbionts.