Nitrogen makes up 78% of the atmosphere in volume and is an essential component of protein in living organisms. Although nitrogen is abundant in the atmosphere, most of it is in the form of unusable N2 gas for plant growth. It is only the NH4+ and (NO2 - + NO3-) ions that can be taken up by plants. Of which NH4+ is more preferred by algae.
In the figure, nitrogen-fixing bacteria convert the N2 gas into ammonia, nitrite and nitrate as inorganic nutrients. The inorganic nitrogen are then consumed by the phytoplankton during photosynthesis and enter the food web as organic nitrogen. The organic waste including dead plants or animals returns the nitrogen back to the water column which are later decomposed back to ammonia by bacteria. The ammonia can be uptake by the plants again or oxidized by nitrifying bacteria to nitrites and nitrate. Under low dissolved oxygen conditions (near the bottom), some of the nitrate is converted to nitrogen gas by denitrifying bacteria. This completes the nitrogen cycle in ocean.