testing this hypothesis are lacking due to the inability to culture
chemosymbionts. Even with cultured chemoautotrophs, sulfur
respiration has been difficult to show. To date, anaerobic sulfide
production has been shown to occur in the sulfur-oxidizers
Beggiatoa sp. (36) and Beggiatoa alba (42). Enhanced shortterm
growth was observed in Beggiatoa sp. cells with S0 granules
when the cells were exposed to acetate compared to what
was seen in cells without internal S0 stores, but the metabolic
pathway involved in this process is still unknown. Internal
sulfur stores are a possible source for the hydrogen sulfide
produced in the purified symbiont suspensions. Previous work
done on L. aequizonata showed that there was a relationship
between sulfide production and the S0 stores in the gill tissue
of the clam (3, 46). In our experiments, sulfide levels in the
medium appeared to be linked to an undetermined storage
compound in the symbiont cell; cell suspensions containing
2.56 108 cells/ml and 5.23 108 cells/ml yielded 600 and
1,200 M concentrations of sulfide, respectively, at the end of
the two time series. These data suggest that the number of cells
present in the suspension determines the final concentration of
sulfide found in the medium. Because S0 stores were not measured
before and after sulfide production, it is unclear whether
the sulfide originates from S0 stores or from the acidification of
internal sulfide in the symbiont. But initial release due to
acidification is unlikely because sulfide production occurred at
pH values of 7.3 in the cell suspension in two separate experiments.
When the pH began to approach 7, the sulfide levels
were 100 M in the external medium (Fig. 3 and 4).