Metabolic Rates at Different Oxygen Levels Determined by Direct and Indirect Calorimetry in the Oxyconformer Sipunculus Nudus

Oxygen uptake and the mode of energy production in Sipunculus nudus L. were determined at different oxygen levels by means of direct and indirect calorimetry. Oxygen consumption declined linearly with decreasing ambient P_OO2 A similar decrease in heat production was observed down to a P_OO2 of 8.66 kPa. At lower oxygen tensions, a discrepancy between aerobic and total heat production indicated the onset of anaerobic metabolism. The occurrence of the critical P_OO2 between 8.66 and 2.66 kPa was confirmed by estimation of anaerobic end products in the body wall musculature. The contributions of aerobic and anaerobic metabolism to total ATP production were determined at a P_OO2 of 2.66 kPa and were found to be 48 and 52%, respectively. Measured heat dissipation under extreme hypoxia (P_OO2 nominally zero) (21.5±3.5 mJ h⁻¹ g⁻¹) agreed with the enthalpy changes calculated from the rates of formation of anaerobic end products (17.9±4.7 mJ h⁻¹ g⁻¹). For the sake of redox balance maintenance, saturation of fatty acids was assumed; this would be accompanied by an additional heat production of 3.3 mJ h⁻¹ g⁻¹, so that the total calculated enthalpy change amounted to 21.2 mJ h⁻¹ g⁻¹