In both nitrate reduction and sulfate reduction there are two types of pathways, assimilatory and dissimilatory. Assimilatory pathways are methods for taking a nutrient in the soil, moving it into the cell and using it for biosynthesis of macromolecules. Dissimilatory pathways use the substrate as a place to dump electrons and generate energy. Here we examine dissimilatory pathways. Assimilatory pathways will be explained in the context of cell biosynthesis.
Some microbes are capable of using nitrate as their terminal electron accepter.
The ETS used is somewhat similar to aerobic respiration, but the terminal electron transport protein donates its electrons to nitrate instead of oxygen. This reaction is not particularly efficient. Nitrate does not as willingly accept electrons when compared to oxygen and the potential energy gain from reducing nitrate is less. If microbes have a choice, they will use oxygen instead of nitrate, but in environments where oxygen is limiting and nitrate is plentiful, nitrate reduction takes place. Denitrification Nitrite, the product of nitrate reduction, is still a highly oxidized molecule and can accept up to six more electrons before being fully reduced to nitrogen gas.