SYMBIOTIC ASSIMILATION OF CO2IN TWO HYDROTHERMAL VENT ANIMALS, THE MUSSELBATHYMODIOLUS THERMOPHILUSAND THE TUBE WORMRIFTIA PACHYPTILA

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Abstract
Shipboard studies showed major differences in the incorporation of CO2 by the specific prokaryotic symbionts of two deep-sea vent invertebrates. The rate of CO2 fixation was optimal at approximately 22.degree. C in fresh trophosome material from the pogonophoran tube worm Riftia pachyptila. Sulfide, but not thiosulfate, served as the electron donor. Thirty-five percent of the aerobic rate remained in deoxygenated samples presumably due to traces of hemoglobin-bound oxygen. Gill preparations from the mytilid mussel Bathymodiolus thermophilus, however, assimilated CO2 with a maximum rate at 12.degree. to 15.degree. C. Thiosulfate, but not sulfide, served as the electron donor. The activity was completely inhibited in deoxygenated samples. These metabolic dissimilarities between the symbionts of the two hosts extend to DNA base ratios and cell sizes. Partial fractionation of the mussel gill preparation yielded a prokaryotic or "bacterial" fraction that showed a CO2 assimilation rate three-fold higher than that of the crude homogenate. This activity was not affected by the in situ hydrostatic pressure of 250 atm (100% barotolerance). The mussel gill symbiont represents the first sulfur-oxidizing vent prokaryote with psychrophilic growth characteristics. The natural distribution of R. pachyptila and B. thermophilus within the physico-chemical regime of the immediate vent vicinity corresponds well with the metabolic differences of their symbionts.