A review of some important families of refractory macromolecules: Composition, origin, and fate in soils and sediments

Extensive knowledge about refractory biomacromolecules has been accumulating for 20 years. The components, characterized by a conspicuous resistance to drastic base and acid laboratory hydrolyses, also exhibit a relatively high resistance to degradation under natural conditions. These refractory biomacromolecules, identified in vascular plants and microalgae, probably play a major role in living organism protection, and they are thought to be important for organic matter sources, composition, and turnover in soils and sediments as well. In addition, some refractory macromolecules are formed in natural environments as the result of condensation/aromatization processes. This review is concerned with six families of refractory macromolecules: lignins, sporopollenins, aliphatic macromolecules (algaenans, cutans, suberans), tannins, black carbon, and proteins. The origin and composition of each family and its contribution to organic matter in soils and sediments are discussed, focusing on recent advances and on questions that are still pending.