Plants absorb sulfur as sulfate ion (SO42-). It is - among others - essential for the formation of chlorophyll, photosynthesis, respiration, the utilisation of phosphorus and other nutrients. Sulfur is a part of sulfurous amino acids and thus also a basic element for proteins. There is about half as much sulfur as phosphorus in plant biomass. Large quantities of sulfate ions can be stored in the plant vacuoles.
Sulfate ions form a part of the so-called permanent water hardness.
There are many studies on the sulfur supply of plants and symptoms of sulfur deficiencies, however, aquatic plants with submerged growth hardly seem to have problems with a lack of sulfur. Tap water often contains a sufficient amount of sulfate ions, thus regular water changes help providing the plants with sulfur. Sulfate is also released when organic matter (e.g. plant and food rests) disintegrates in the tank, and sulfate is also added to the water by fertilizing salts like potassium sulfate, magnesium sulfate, ammonium sulfate and calcium sulfate (gypsum).