Aerobic and anaerobic enzyme activity in the hake Merluccius gayi gayi related to the Oxygen Minimum Zone off central-southern Chile

Abstract

The Humboldt Current System (HCS), off the coast of Chile, has an Oxygen Minimum Zone (OMZ) between 30 and 250 m depth, with oxygen concentrations < 0.5 ml L-1. Densities of Merluccius gayi gayi associated with this zone are highest between 100 and 300 m depth, suggesting metabolic adaptations allowing it to remain in the OMZ. The aerobic and anaerobic potential of this demersal fish was estimated through the measurement of key metabolic enzymes and the electron transport system (ETS) of different body tissues (white muscle, heart, brain and liver). The activities of the citrate synthase (CS) enzyme and the ETS were higher in the brain and lower in muscle, confirming the predominance of aerobic metabolism in vital organs, different of muscle. The typical activity of the anaerobic enzyme lactate dehydrogenase (LDH) fluctuated between 55 ± 17.5 and 263 ± 79 UI g-1 wet weight, with lower activities in heart and brain. The low LDH activity and high MDH/LDH ratio found in these vital organs indicate that they are able to cope with metabolic acidification during long periods of environmental hypoxia. Exceptionally high LDH activity was found in the liver which may be related to environmental factors such as pollution. Considering the low MDH/LDH ratio of muscle, the decreased activity of CS and ETS in this tissue and the high LDH activity in the liver may indicate metabolic profile of M. gayi gayi in order to tolerate hypoxia, and therefore the capability to stay in the OMZ by decreasing its swimming activity.