Macro algae are much more diverse than vascular or terrestrial plants in their ability to uptake and process nutrients. They generally require a larger range of nutrients in an inorganic form than vascular plants do, but are able to derive a few essential nutrients from organic compounds such as some aquarium substrates. Few studies have been done on individual species of macro algae to determine which major and minor nutrients are essential to the plants survival and growth. This is due largely to the fact that natural seawater is used in most research and aquaculture production, which typically contains such high concentrations of the major and minor nutrients needed for plants to grow that specific data is never collected. However, it is commonly accepted that macro algae and sea grasses need the same essential elements as higher plants do. Unlike vascular plants, marine macro algae must be supplied nutrients through the water column and not through the substrate. This is achieved by providing all needed elements through the aquarium water. 

Macro Nutrients

The macro nutrients, or primary nutrients, that marine plants need to achieve photosynthesis are: nitrogen, phosphate, potassium, sulphur, calcium, magnesium and carbon. Some marine algae that is heavily calcified, such as Halimeda optunia, need large amounts of calcium to grow. These plants in turn produce calcium based sand as a byproduct, which accounts for the majority of all Caribbean sand.


Nitrogen is one of the major nutrients required by all plants and algae, both aquatic and terrestrial. It is used by the plants or algae in the production of stored proteins. Marine plants take up nitrogen in several forms, some more effectively than others. In the aquarium they include ammonia, ammonium, nitrite and nitrate. Nitrate is preferred among all marine plants and is a readily available nutrient in most aquariums. In most marine planted tanks and refugiums there is usually sufficient amounts of nitrates available in the water column due to the nitrogen cycle and fish waste to sustain small populations of plants without adding any additional nutrients. However, in heavily planted or sea grass dominated systems it will become necessary to keep nitrates elevated just enough to encourage growth. It is commonly suggested that a range of 5-10 ppm of nitrates is acceptable in marine plant dominated systems, perhaps more in more densely populated systems. Nitrate can be added as a supplement such as KNO3 (potassium nitrate) or CaNO3 (calcium nitrate) in dry form or premixed as a liquid. Another alternative is to use treated tap water without removing the nutrients. Great care must be taken with any periodic dosing as it can be harmful to both the plants as well as fish and invertebrates in the aquarium.


Phosphorus is another important nutrient that marine plants need to grow. Marine plants readily absorb phosphorus in the form of phosphate (PO4). Phosphorus can be a limiting nutrient in the growth of marine plants but it is usually sufficiently present in the aquarium due to the addition of fish food, through detritus buildup and fish waste. In comparison with nitrogen, marine plants absorb far less phosphorus so care should be taken to limit the amount of phosphate in the aquarium Phosphate levels should be maintained below 0.07 ppm to rule out as a limiting nutrient. Over dosing this nutrient can be detrimental, causing algae blooms.


Potassium is absorbed by marine plants as an ion from aquarium water and is generally available in sufficient amounts in most salt mixes and tap water. Potassium is a key component that is used for photosynthesis in marine plants. Dosing is generally not needed or required.


Sulphur is used by marine algae and plants in the production of amino acids, proteins and chlorophyll. It is generally present in sufficient amounts in both salt mixes and tap water. Sulphur can be toxic in large quantities so it should never be added as a fertilizer to the aquarium.


Calcium is a very necessary element for all marine plants, as it is used in the formation of cell wall structure. As mentioned earlier, some macro algae such as coralline and calcareous algae readily absorb large amounts of calcium and are composed of almost pure calcium carbonate. Calcium, alkalinity and ph are all directly connected to each other, as the availability of carbonate mostly depends upon pH and alkalinity levels. So it is important to keep all three at acceptable levels so that calcium carbonate is freely available to the plants. Ideal dosing ranges are 7-10 DKH alkalinity, 8.4-8.5 ph and calcium levels between 380-450 ppm. Most commercially available products offer a simplified solution to maintaining the balance between alkalinity, ph and calcium levels. Calcium reactors can keep calcium levels high on a constant basis so are a welcome addition to any aquarium containing marine plants and invertebrates. Not only will keeping the ph and calcium levels high in a planted tank produce healthy plants, but the amount of micro algae will decrease as well.


Magnesium is another element that aids in the formation of cell wall structure within marine plants and especially coralline algae. It is often directly proportionate to the calcium levels found in both salt mixes and tap water. Because magnesium is an ingredient in most commercial salt mixes it is not normally necessary to dose this nutrient. However, magnesium will be depleted quickly with the formation of calcium carbonate in both plants and encrusting algae and needs to be monitored periodically. A target range of 1250-1350 ppm is sufficient for most marine aquariums containing calcareous and coralline algae.


Carbon is used by all living organisms in varying amounts and is particularly important in the growth and structure of vascular plants. Plants obtain carbon from carbon dioxide, which is turned into oxygen through the process of photosynthesis. Most species of sea grasses can absorb large amounts of carbon from the both their roots and leaves, while macro algae are thought to need varying amounts depending on the species. It is generally accepted that the aeration/circulation provided by the filtration system creates enough free carbon to satisfy the needs of marine plants. The process of introducing carbon into the aquarium water through carbon dioxide fertilization does little to help in the growth of marine macro algae, but it would benefit a sea grass dominated system. However, the danger of a drastic fall in ph due to over saturation of carbon dioxide could be harmful and unproductive at best so it is seldom done in the marine aquarium hobby.


Micro Nutrients

The minor nutrients marine plants need to grow are: iron, manganese, copper, zinc & molybdenum, boron, iodine, bromine. These nutrients are normally needed in very small amounts so are often referred to as "trace elements" . Most commercially available salt mixes contain all of the minor nutrients needed for marine plant growth, so they are generally replenished by regular water changes. 


Iron is an important micro nutrient absorbed by marine plants and is used in respiration and photosynthesis. The most readily absorbed form of iron is iron chelate. It is available in both dry and liquid forms and can be added safely to most aquariums without damage to its inhabitants. It is unknown how much available iron is used by marine plants but studies have shown that plants benefit from improved growth and color in systems that keep it at acceptable levels. There is some debate as to the quantity that should be dosed but most hobbyists add enough iron to rule it out as a limiting nutrient. 


Manganese is absorbed as an ion by marine plants and is used in photosynthesis. It is not normally a limiting nutrient in most marine aquariums as it is present in both salt mixes and tap water.


Copper is absorbed in very low concentrations as an ion and is used in respiration by marine plants. Marine invertebrates are very sensitive to copper so it should never be added to the aquarium water as it is present in significant amounts in both salt mixes and tap water.


Zinc is used by marine plants in the formation of chlorophyll and is present in both substrates and tap water. At high levels it is very toxic and should never be added to the aquarium water.


It is unclear what role this element plays in the growth of marine plants. In terrestrial plants it is used by the plant to break down nitrates into ammonium to be used as a source of nitrogen. Trace amounts are present in salt mixes and tap water.


Vascular plants require small levels of this nutrient for cellular membrane function, root growth and flower production. Boron has been shown to be essential for some marine algae, but not for most macro algae. Most salt mixes and tap water contain significant amounts necessary for the algae and plants that utilize them.


It is believed that some varieties of red and brown macro algae require iodine and bromine as essential nutrients for growth. It has an obscure place in the marine aquarium hobby but is available as a supplement for species that require it.

Vitamin B12

Several species of macro algae utilize Vitamin B as an essential element of growth. It is also thought to be necessary in the growth of several sea grass species including turtle grass. Not much is know on the quantities needed as most prepared salt mixes have sufficient amounts present.

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