Adaptations to Sulfide in Sulfide-System Meiofauna. Endproducts of Sulfide Detoxification in Three Turbellarians and a Gastrotrich

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Abstract
End-products of sulfide detoxification were compared in 3 turbellarians and a gastrotrich typical of lenitic beaches with a well-developed sulfide system. Oxidation products were correlated with the O2 concentration in the animal''s habitat indicating that adaptation to life in the sulfide system involves reduction of dependency on molecular O2 in metabolism. Archiloa wilsoni, a surface dwelling turbellarian, made sulfite and sulfate, which are rich in O2. Of the 3 sulfide-system species examined, the primary end-product in 2 (the turbellarian Pseudohaplogonaria sp. and the gastrotrich Dolichodasys carolinensis) was elemental S which lacks O. The 3rd species (the turbellarian Solenofilomorpha funilis) made thiosulfate, a compound with a high S:O ratio, plus sulfate and sulfite. The presence of an important pathway of sulfide oxidation with elemental S as the major end-product was heretofore unknown in the animal kingdom. Its presence in 2 phyla, Platyheminthes and Gastrotricha, indicates that it is widespread in the lower invertebrates. Similarly, the formation of thiosulfate as a major product of sulfide oxidation has not been reported previously in the invertebrates. The discoveries of 2 new pathways for sulfide detoxification and of major differences in sulfide metabolism within a single taxon, the turbellarian order Acoela, indicate that invertebrates have a variety of detoxification mechanisms based on sulfide oxidation and that a substantial degree of plasticity in the detoxification method can be expected even between closely related organisms. The possibility that the observed sulfide oxidation is by symbiotic prokaryotes, rather than the animals themselves, is discussed.