The general view of marine sediments is that they are stratified heterotrophic communities in which the products of primary production are mineralised and recycled. Microbially catalysed oxidation and degradation of organic material cause a fine scale vertical zonation of chemical and biological components.
The vertical sequence of external electron acceptors used for respiration roughly follows the order of decreasing free energy yield. Thus oxygen, which has the highest energy yield, is first depleted followed by the sequential depletion of nitrate, oxidized manganese, oxidized iron and sulphate. However, stratified vertical zonation is only valid for sediment systems, which are entirely controlled by one-dimensional diffusion by which organic carbon and oxygen or other electron acceptors are supplied from above, while the reduced metabolites from anaerobic degradation of organic material are supplied from below.
The depth of each zone is dependent on the diffusive transport and availability of external electron acceptors (O2, NO3- Mn4+ Fe3+ and SO42-) and the demand of reaction rates. In most marine sediments aerobic respiration and sulphate reduction are the most important processes in the degradation of organic material. This is because oxygen respiration is noticeably the most energetically efficient process and sulphate is by far the most abundant electron acceptor in marine sediments.