Synergistic effect of acidification and hypoxia: in vivo 31P-NMR and respirometric study in fishes.

We used 31P nuclear magnetic resonance (31P-NMR) spectroscopy to measure intracellular pH (pHi) and high-energy phosphate levels of white muscle of the fish tilapia (Oreochromis mossambicus) during exposure to the stressors hypoxia and water acidification separately and simultaneously. The protocol for the graded hypoxia load was 100, 40, 30, 20, 10, 5, and 3% air saturation for 1 h at each level. For environmental acidosis the pH of the water was lowered from 7.6 to 4.0 over 6 h, with time of exposure approximately 11 h. All protocols were followed by 6 h of reoxygenation at 20 degrees C. We also measured total oxygen consumption of the animal. The results of this in vivo study revealed that environmental acidification had no effect on oxygen consumption, pHi, or phosphocreatine depletion. Hypoxia caused moderate changes in these parameters and fast and complete recovery during reoxygenation. In contrast, the combination of environmental acidosis and hypoxia resulted in 50% mortality, increased depletion of the phosphocreatine pool, and a retarded recovery of the pHi during reoxygenation compared with the group with hypoxia as a single stressor. The combination of environmental acidification and hypoxia has a more profound effect and works synergistically compared with the conditions imposed separately. To investigate whether an adaptation response occurred during chronic exposure to environmental acidification, animals were exposed for 6 wk to pH 4.0 before experimentation. The pHi in the white muscle dropped from 7.2 (control group) to 6.9 during this period, whereas no effect was found in the phosphorylated compounds and oxygen consumption. Therefore, it is concluded that no adaptation response occurs in animals exposed to long-term environmental acidosis.