Spisula substruncata, a mactrid bivalve living in well-oxygenated sublittoral sandy bottoms, contains two types of intracellular bacteria in its gills. Chemoautotrophic metabolism of these bacteria was tested in different ways. Homogenates of gill tissues showed a slight ribulose -1.5 - bisphosphate carboxylase activity. Fixation of 14C- labelled bicarbonate in Spisula substruncata was higher than in Venus gallina, a clam without bacteria living in the same environment. Other experiments were performed with and without two reduced compounds (sodium thiosulphate and ammonium chloride) at different incubation times (45 to 300 min). With the two compounds, the levels of fixed carbon per g of gill tissue increased significantly after 60 min incubation. A possible ammonia utilization by gill bacteria is demonstrated and may be explained by the fact that the most abundant dissolved reduced compound available for chemoautotrophs is ammonia. The values obtained in this association were lower than those recorded for other bivalves with bacterial cells in their gills. These low values may be explained by the relatively small number of the two types of bacteria observed in the gills of S. substruncata.
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