Energy metabolism and ATP free-energy change of the intertidal worm Sipunculus nudus below a critical temperature

The intertidal worm Sipunculus nudus was exposed to various temperatures for an analysis of the integrated changes in energy and acid-base status. Animals were incubated in sea water or maintained for up to 8 days at 4 and 0 degrees C while dwelling in the sediment. Cannulation of the animals prior to experimentation allowed the analysis of blood gas parameters (P-O2, P-CO2 and pH). P-O2 fell to 0 torr within 8 days at 0 degrees C. A simultaneous reduction of ventilatory activity was derived from measurements of the pattern of coelomic fluid pressure changes associated with ventilatory movements. The increase in P-CO2 and an onset of anaerobic metabolism, indicated by the accumulation of end products like acetate and propionate both in the coelomic fluid and the body wall musculature, led to the development of a progressive acidosis and a deviation from the alphastat regulation of intracellular pH seen in unburied animals. The drop in intracellular pH together with the depletion of the adenylates and the phosphagen, phospho-L-arginine, reflect a significant decrease in the Gibb's free-energy change of ATP hydrolysis. These changes are interpreted to indicate lethal cold injuries, because recovery was not possible when the animals were returned to 12 degrees C after more than 2 days of exposure to 0 degrees C. A low critical temperature indicating the onset of cold-induced anaerobiosis is concluded to exist below 4 degrees C owing to the insufficient response of the ventilatory system to the developing hypoxia.