A Study On The Tolerating Mechanism For Sulfide In Urechis Unicinctus (Echiura: Urechidae)

As a creature that survives well in sulfide-rich habitats, echiuran gets more and more attention for its special sulfide tolerating mechanism. In this study, Urechis unicinctus is used as the type species of echiuran. The research purpose is to understand its sulfide tolerating mechanism from physiological, biochemical and molecular biological areas. A series of experiments show that U. unicinctus can respond to the sulfide of its living environment. When it is exposed to sulfide, its oxygen consumption changes (0.05~0.07mgO2h-1g-1) corresponding to various sulfide concentrations. The sulfide also has toxic effect on U. unicinctus. The time that experimental populations reach 50% mortality shortens as the sulfide concentration increasing. And the body color changes to dark. When U. unicinctus is exposed to sulfide for a long time, the relative amounts of hemoglobin, heme and hematin in the coelomic fluid change in regular patterns. The RNA extracted from the coelomic fluid show that some genes express during that process. The mitochondria isolated from four different tissues of U. unicinctus can oxidize sulfide and produce ATP at the same time. Sulfide-stimulated ATP production fluctuates as the sulfide concentration and the source of mitochondria change. When the inhibitors existing, the ways that the electrons enter the electron transport chain coupling with the sulfide-stimulate ATP production show significantly tissue-specific characters and corresponded with the tissues' physiological function. In addition, in its coelomic fluid cell, there are some unknown cellular organs that may relate to its sulfide tolerating mechanism. Under electron microscope, these cellular organs have very special structures. These results show that the function of coelomic fluid is not to oxidize sulfide but to store it.In a word, the U. unicinctus can tolerate high sulfide concentrations and low oxygen concentrations, and its tolerating mechanism is complex and versatile. But its tolerance is limited when the concentration of sulfide suddenly increases or the duration of exposure to high sulfide is too long.