Simplified carbon cycle in sediment. Particulate organic carbon (POC) is delivered from the water column (blue) to the oxic sediment (orange) where part of it is oxidized to CO2 by aerobic heterotrophs. Part of the POC is further buried undegraded into anoxic sediment (black) where most of it is converted to dissolved organic carbon (DOC) by hydrolysing and fermenting bacteria. The produced DOC is then oxidized to CO2 by anaerobic respirers. A small fraction of POC is buried permanently in the sediment and CO2 is ultimately transported from the sediment to the overlying water by molecular diffusion.