Autotrophic Carbon Fixation by the Chemoautotrophic Symbionts ofRiftia pachyptila

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Abstract
Preparations of trophosome tissue from Riftia pachyptila containing viable endosymbiotic bacteria were incubated with several substrates under a variety of conditions to characterize the symbionts physiologically. Of all the potential substrates tested, only sulfide stimulated carbon fixation by the trophosome preparations; neither hydrogen, ammonia, nor thiosulfate were effective. Trophosome preparations did not oxidize 14C-methane to either 14C-organic compounds of 14CO2, nor did they reduce acetylene under the conditions tested. Carbon fixation by the endosymbionts appears barotolerant. The symbionts require both sulfide and oxygen to fix carbon through autotrophic pathways, but are inhibited by free oxygen and by sulfide concentrations in the 300 .mu.M range. Maximal rates of carbon fixation were documented in incubations in dilute Riftia blood, which protects the symbionts from the inhibitory effects of free sulfide and oxygen while providing them with an abundant pool of both substrates, bound by the vestimentiferan hemoglobins.

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