They live in the sunlit surface oceans and are the base of the food chains in the ocean. Sinking dead organic matter is attacked by bacteria which decompose the organic matter using oxygen and regenerate the nitrates. Some organic matter passes though the water column and is deposited in bottom sediments. Part of this is reworked by bottom life and returned to bottom water as nitrates. Bottom water nitrates get back to the Earth's surface as part of upwelling where the process starts again.
Sulfur is another element that is key to biological life.
But sulfur is also a by-product of industrial processes and is the source of acid rain. Vertical Distribution of Nutrients in the Oceans. The following sketch shows the phosphate distribution in the water column of an equatorial latitude open ocean site. B. Note the phosphorus content is nearly zero at the surface because all phosphate is quickly used by phytoplankton. Phosphate content quickly rise below the photic zone because respiration continues without photosynthesis.