Aerobic Respiration during Sulfide Exposure in the Marine Echiuran WormUrechis caupo

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Abstract
Urechis caupo inhabit U-shaped burrows in mudflats where they are exposed to toxic hydrogen sulfide. During tidal exposure sulfide levels average 27μM and oxygen levels average 77 mmHg. The oxygen consumption rate ( ) of U. caupo was measured at varying partial pressures of oxygen and with and without sulfide to investigate whether aerobic respiration is maintained during tidal exposure. Animals exposed to PO₂'s as low as 32 mmHg and sulfide concentrations of less than 30 μM did not significantly change (mean mL O₂·g⁻¹·h⁻¹), although there was a general trend of increased with increased sulfide. We investigated whether this animal avoids sulfide toxicity by oxidation of sulfide catalyzed by coelomic fluid hematin. A positive correlation was found between and hematin concentration for animals exposed to 4-29 μM sulfide. The of coelomic fluid samples measured in vitro showed a peak and positive correlation with hematin concentration at 17 μM sulfide. Oxygen consumption rates of hematin isolated from the coelomic fluid showed a positive correlation between and sulfide concentration at 1.0 and 1.8 mM hematin, and was significantly greater at 1.8 mM hematin in the presence of sulfide. These results suggest that oxygen consumption in U. caupo is not inhibited at environmental oxygen and sulfide levels and that the elevation in observed in the presence of sulfide in vitro may be due to oxidation of sulfide by coelomic fluid hematin.