Mitochondrial oxyconformity and cold adaptation in the polychaete Nereis pelagica and the bivalve Arctica islandica from the Baltic and White Seas

The rates of oxygen uptake of the marine polychaete Nereis pelagica and the bivalve Arctica islandica depend on the availability of ambient oxygen. Th is is manifest both at the tissue level and in isolated mitochondria studie d between oxygen tensions (Po-2) of 6.3 and 47.6 kPa (47-357 mmHg), Oxyconf ormity was found in both Baltic Sea (Kiel Eight) and cold-adapted White Sea populations of the two species. However, mitochondria isolated from White Sea specimens of N, pelagica and A. islandica showed a two- to threefold hi gher aerobic capacity than mitochondria prepared from Baltic Sea specimens. We tested whether mitochondrial oxyconformity can be explained by an addit ional electron pathway that is directly controlled by Po,. Mitochondrial re spiration of both invertebrate species was inhibited by cyanide (KCN) and b y salicylhydroxamic acid (SHAM), The overall rate of mitochondrial oxygen c onsumption increased at high Po,. Phosphorylation efficiency (ADP/O ratio) decreased at elevated Po-2 (27.5-47.6 kPa, 206-357 mmHg), regardless of whe ther malate or succinate was used as a substrate. In contrast to the invert ebrate mitochondria studied, mitochondria isolated from bovine heart, as an oxyregulating control species, did not show an elevated rate of oxygen upt ake at high Po, in any respiratory state, with the exception of state 2 mal ate respiration. In addition, rates of ATP formation, respiratory control r atios (RCR) and ADP/O ratios remained virtually unchanged or even tended to decreased. In conclusion, the comparison between mitochondria from oxyregu lating and oxyconforming organisms supports the existence of an alternative oxidase in addition to the classical cytochrome c oxidase. In accordance w ith models discussed previously, oxidative phosphorylation does not explain the rate of mitochondrial oxygen consumption during progressive activation of the alternative electron transport system. We discuss the alternative s ystem, thought to be adaptive in confined, usually hypoxic environments, wh ere excess oxygen can be eliminated and oxygen levels can be kept low by an increase in the rate of oxygen consumption, thereby minimizing the risk of oxidative stress.