The major essential nutrients, nitrogen and phosphorus, limit primary productivity in many aquatic environments, though in some areas of the ocean (high nutrient low chlorophyll), productivity is limited by the availability of iron or iron and manganese. Planktonic cyanobacteria are major nitrogen fixers in marine and fresh waters; heterocystous cyanobacteria are common in fresh waters and occur as symbionts in marine diatoms. Non-heterocystous marine cyanobacteria occur free-living and as algal symbionts. Nitrogen fixation requires iron and molybdenum, which can be less commonly replaced by vanadium, as well as reductants and ATP. For combined nitrogen, the form assimilated into organic nitrogen is, as for diazotrophs, ammonium, which is taken up by specific transporters. Nitrate influx also involves an energised transporter. Nitrate reductase requires catalytic iron and molybdenum and reductant to produce nitrite; nitrite is reduced to ammonium by nitrite reductase using catalytic iron. Several forms of organic nitrogen can also be taken up and assimilated by algae. Phosphorus is taken up as inorganic phosphate; organic phosphate from the medium is hydrolysed by phosphatases secreted by algae. Aquatic rhizophytic macrophytes with rhizoids or roots in fine-grained substrates acquire various fractions of combined nitrogen and of phosphate from the sediment and from overlying water.