Adaptations to the Deep-Sea Oxygen Minimum Layer: Oxygen Binding by the Hemocyanin of the Bathypelagic Mysid, Gnathophausia ingens Dohrn

The bathypelagic mysid, Gnathophausia ingens Dohrn, lives aerobically at oxygen partial pressures as low as 6 torr in the oxygen minimum layer off southern California. This study is concerned with the O2 binding properties of this mysid's hemocyanin and the function of the pigment in O2 uptake at low P O2. The effect of temperature on in vivo hemolymph pH (ΔpH/ΔT = -0.018) was measured from 2.5 to 12.5°C. Hemocyanin concentration was estimated to be 24 mg/l, corresponding to an O2 binding capacity of about 0.3 mmol O2/1. Freezing of hemolymph samples significantly decreased the affinity and cooperativity of HcO2 binding, necessitating the use of fresh hemolymph. The HcO2 affinity was high ( P50 of 1.4 torr at 5.5°C, pH 7.87), allowing the loading of O2 even at 6 torr. The cooperativity of HcO2 binding was also high ( n50 = 3.5 at 5.5°C, pH 7.87); presumably allowing the pigment to function effectively as an O2 transporter within the small P O2 difference between the environment and the tissues. Temperature differences within the environmental range (2-10°C) had no significant effect on the oxygen affinity (ΔH = -6.7 kJ/mol, pH 7.7) or on the cooperativity of O2 binding. A large Bohr shift (Δ log P50/Δ pH = -0.80 to -0.81) was present at all temperatures. L-lactate produced moderate increases in HcO2 affinity (Δ log P50/Δ log [lactate] = -0.13 at pH 7.9) and in cooperativity. Regional and ontogenetic comparisons suggest that regional and ontogenetic differences in HcO2 affinity occur in this species. This mysid has a hemocyanin of unusually high O2 affinity and cooperativity of O2 binding for a crustacean living at low temperatures, and this appears to be an adaptation for oxygen loading and transport at the cold, low oxygen conditions in deep-sea oxygen minimum layers. The reduced temperature sensitivity of HcO2 affinity may also be an adaptation to low oxygen.