Reduced Sulfur Compounds

The close timing between the discovery of sulfur-oxidizing bacteria in the gutless tube worms from hydrothermal vents and the symbiotic bacteria in gutless oligochaetes, led to the assumption that the oligochaete symbionts were also dependent on reduced sulfur compounds such as sulfide and thiosulfate from the environment. Most ecological studies have centered on a single species, Inanidrilus leukodermatus, from a single site in Bermuda (Giere et al. 1982). Sulfide concentrations at this site ranged between 2-32 |M in 5 and 10 cm sediment depth where the worms were most abundant (Giere et al. 1982). While these concentrations may seem low to microbiologists that often use much higher sulfide concentrations to isolate sulfur-oxidizing bacteria, they are comparable to those that hydrothermal vent fauna with sulfide-oxidizing bacteria are exposed to (Johnson et al. 1988; Le Bris et al. 2003). The concentration of other reduced sulfur compounds, such as thiosulfate, has not been determined in the habitat of gutless oligochaetes. Experiments on the uptake of CO2 in I. leukodermatus indicate that thiosulfate is preferred over sulfide, but the use of environmentally unrealistic concentrations of thiosulfate in the mM range and unusually high CO2 uptake rates in the absence of either sulfide or thiosulfate make the interpretation of these results difficult (Giere et al. 1988).

The presence of reduced sulfur compounds has been assumed to be one of the most important environmental factors for gutless oligochaetes, as the lack of these electron donors would lead to the starvation of the sulfur-oxidizing symbionts and in turn, their hosts. This perception has changed with the discovery of gutless oligochaetes from the island of Elba (Mediterranean) that live in sediments in which sulfide concentrations are usually in the nM range (Dubilier et al. 2001), and only occasionally in the low ^M range (Perner 2003). However, low sulfide concentrations need not be limiting if the flux of sulfide from production by free-living sulfate-reducing bacteria is high enough. Sulfate reduction rates in these sediments range between 100-300 nmol cm-3 per day, showing that sulfide is produced at rates comparable to those of other sandy sediments (Perner 2003). However, this sulfide is apparently oxidized very quickly, as free sulfide concentrations are extremely low in the pore waters. It is not known if the flux of free sulfide in these sediments is sufficient to support the growth of these worms. Since these worms also harbor sulfate-reducing symbionts that produce sulfide internally (see Sect. 12.6), they may be independent of reduced sulfur compounds from the environment.

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