The most common limiting nutrients are phosphate and nitrate. Either in water or in the air, CO2 is used during photoshynthesis and recovered during respiration. Nitrogen is stable as N2 in the atmosphere. In water and sediment, N2 can be transformed into nitrates or ammonia by organic methods. Some nitrates are released by volcanoes and soil processes.
Diatoms are 'phytoplankton' or the 'grass of the oceans'.
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.