Dissolved organic matter can be classified as labile or as recalcitrant, depending on its reactivity. Recalcitrant DOC is also called refractory DOC, and these terms seem to be used interchangeably in the context of DOC. Depending on the origin and composition of DOC, its behavior and cycling are different; the labile fraction of DOC decomposes rapidly through microbially or photochemically mediated processes, whereas refractory DOC is resistant to degradation and can persist in the ocean for millennia. In the coastal ocean, organic matter from terrestrial plant litter or soils appears to be more refractory and thus often behaves conservatively. In addition, refractory DOC is produced in the ocean by the bacterial transformation of labile DOC, which reshapes its composition.
Due to the continuous production and degradation in natural systems, the DOC pool contains a spectrum of reactive compounds each with their own reactivity, that have been divided into fractions from labile to recalcitrant, depending on the turnover times, as shown in the following table...
This wide range in turnover or degradation times has been linked with the chemical composition, structure and molecular size, but degradation also depends on the environmental conditions (e.g., nutrients), prokaryote diversity, redox state, iron availability, mineral-particle associations, temperature, sun-light exposure, biological production of recalcitrant compounds, and the effect of priming or dilution of individual molecules. For example, lignin can be degraded in aerobic soils but is relatively recalcitrant in anoxic marine sediments. This example shows bioavailability varies as a function of the ecosystem's properties. Accordingly, even normally ancient and recalcitrant compounds, such as petroleum, carboxyl-rich alicyclic molecules, can be degraded in the appropriate environmental setting.