The following table lists some reported reactions that are relevant to photogeochemical study, including reactions that involve only naturally occurring compounds as well as complementary reactions that involve synthetic but related compounds.  The selection of reactions and references given is merely illustrative and may not exhaustively reflect current knowledge, especially in the case of popular reactions such as nitrogen photofixation for which there is a large body of literature. Furthermore, although these reactions have natural counterparts, the probability of encountering optimal reaction conditions may be low in some cases; for example, most experimental work concerning CO photoreduction is intentionally performed in the absence of O, since O almost always suppresses the reduction of CO. In natural systems, however, it is uncommon to find an analogous context where CO and a catalyst are reached by light but there is no O present.