Hydrogen sulfide was known as a toxic pollutant long before its physiological functions became apparent. The main effects of sulfide poisoning are the loss of central respiratory drive due to lesions in the brain stem, and inhibition of cytochrome oxidase, leading to impaired aerobic energy metabolism [1]. Sulfide-rich environments occur naturally in the sediments and grass marshes of the intertidal zone, in deep-sea hydrothermal vents, and in the hypolimneon of eutrophic lakes [2]. Moreover, mammalian as well as invertebrate tissues, such as the body wall of the lugworm Arenicola marina, enzymatically produce sulfide [3,4].In mammals, H 2 S acts as a gaseous transmitter and regulates several physiological processes [5]. Altered sulfide metabolism is associated with a number of disorders, such as Alzheimer's disease, Down's syndrome and ulcerative colitis [6][7][8]. Within a narrow concentration range, the effects of sulfide change from physiological to highly toxic, and therefore regulatory mechanisms are necessary to control endogenous sulfide levels within the physiological range.Sulfide is catabolized mainly by oxidation to nontoxic sulfur compounds. Mammals rapidly oxidize sublethal concentrations of sulfide to sulfate and excrete it in the urine [1,9]. The main sites of sulfide oxidation are liver and colon tissues, and the enzyme activity is present in the mitochondria [10,11]. Thiosulfate is produced as an obligate intermediate by a putative sulfide oxidase, which has not yet been identified [12].A mitochondrial pathway also catalyzes sulfide oxidation in many sulfide-adapted invertebrates including polychaetes, crustaceans and bivalves [2]. The lugworm A. marina is often used to study strategies of sulfide tolerance, because it is highly abundant in sandy to muddy intertidal flats, where high micromolar concentrations of sulfide regularly occur [2]. Hydrogen sulfide is a potent toxin of aerobic respiration, but also has physiological functions as a signalling molecule and as a substrate for ATP production. A mitochondrial pathway catalyzing sulfide oxidation to thiosulfate in three consecutive reactions has been identified in rat liver as well as in the body-wall tissue of the lugworm, Arenicola marina. A membranebound sulfide : quinone oxidoreductase converts sulfide to persulfides and transfers the electrons to the ubiquinone pool. Subsequently, a putative sulfur dioxygenase in the mitochondrial matrix oxidizes one persulfide molecule to sulfite, consuming molecular oxygen. The final reaction is catalyzed by a sulfur transferase, which adds a second persulfide from the sulfide : quinone oxidoreductase to sulfite, resulting in the final product thiosulfate. This role in sulfide oxidation is an additional physiological function of the mitochondrial sulfur transferase, rhodanese.Abbreviations GSH, glutathione; GSSH, glutathione persulfide; SQR, sulfide : quinone oxidoreductase.

No abstract

Many aquatic animal species can survive sulfide exposure to some extent through oxidation of the sulfide, which results mainly in thiosulfate. In several species, sulfide oxidation is localized in the mitochondria and is accompanied by ATP synthesis. In addition, blood-based and intracellular compounds can augment sulfide oxidation. The formation of thiosulfate requires oxygen, which results in an increase in oxygen consumption of some species. If not all sulfide is detoxified, cytochrome C oxidase is inhibited. Under these conditions, a sulfide-dependent anaerobic energy metabolism commences.

SUMMARYSince its invasion of Europe in the early 1980s, the Asian clam Corbicula fluminea has become very abundant in nearly all western river systems. Today this species is one of the most important biomass producers in the River Rhine. Monitoring the valve movements of C. fluminea over a period of 2 years revealed a circadian rhythm in summer,with extended periods (10-12 h) of valve closure, predominantly in the morning hours. Altogether valve movements were very scarce, frequently fewer than four movements per individual per day.Simultaneous measurements of heat dissipation and oxygen consumption(calorespirometry) revealed an intermittent metabolism in the clam. With the onset of valve closure, C. fluminea reduced its metabolic rate to 10%of the standard metabolic rate (SMR) measured when the valves were open. Nevertheless, this depressed metabolism remained aerobic for several hours,enabling the clam to save energy and substrates compared to the requirements of the tenfold higher SMR. Only during long-lasting periods of valve closure(more than 5-10 h) did the clams become anaerobic and accumulate succinate within their tissues (2 μmol g-1 fresh mass). Succinate is transported into the mantle cavity fluid, where it reaches concentrations of 4-6 mmol l-1. Because this succinate-enriched fluid must pass the gills when the valves open again, we suggest that this anaerobic end product is at least partly reabsorbed, thus reducing the loss of valuable substrates during anaerobiosis. Propionate was also produced, but only during experimental N2-incubation, under near-anoxic conditions.The intermittent metabolism of C. fluminea is discussed as an adaption to efficiently exploit the rare food supply, saving substrates by the pronounced metabolic depression during valve closure.