The dark ocean, the waters below 200 m depth, comprises about 95% of the volume of the ocean, but its contribution to the metabolism of the ocean is poorly quantified. Here we show that the respiration rate of microplankton declines exponentially at a rate of 0.53 km-1 in the dark ocean, and is enhanced at the interface between the mesopelagic and the abyssal layers (1,000--2,000 m). The respiratory CO2 production in the dark ocean, estimated at 20 to 33.3 Gt C yr-1, renders it a major component of the carbon flux in the biosphere.