Posteriori reduction means a confirmation of a reduction. a reduction that you confirm without doubt.
This is because to reduce something you add electrons, thus reducing its overall charge
The chemical process of reduction, is any process in which electrons are added to an atom or ion (as by removing oxygen or adding hydrogen); always occurs accompanied by oxidation of the reducing...
Because it gains electrons ,therefore its valency decreases : it becomes Negative
the process of carbon reduction is called carbon reduction process
In Chem, reduction is the gain of electrons and I think it's the same in Bio.