Since the mitochondria consume oxygen, the oxygen concentration drops.

Under a few millimeters of water heterotrophic bacteria metabolize and consume oxygen.

For example, mitochondria consume oxygen which generates free radicals (reactive oxygen species) as an inevitable by-product.

The growing bacteria consume oxygen, which easily accepts electrons, present in the subsurface which increases reducing potential.

The decomposition process consumes oxygen, depriving fish and other organisms.