The use of supplements in the (marine) aquarium has always been controversial,  dividing hobbyists into two camps.  Aquaria are a reflection of nature, but  differ dramatically  from the real thing. Nature can be regarded as a closed ecosystem; all its nutrients are recycled in the earth (lithosphere), water bodies (hydrosphere), air (atmosphere) and  organisms (biosphere). Aquaria, and marine aquaria as well, are open ecosystems, where nutrients are inserted (feeding) and removed (water changes, protein skimming, phosphate absorbants) manually, by the aquarist him/herself. Aquaria usually contain ample nutrients, caused by fish overstocking and heavy feeding. The question is; do supplements really matter? Do we really need to add more nutrients to improve coral health and growth? This article discusses the facts and figures about amino acids, a very popular additive nowadays.

An overview

Let's briefly discuss the ways in which corals take up nutrients on the reef. We distinguish between three main ways:

1. Corals receive sugars from their zooxanthellae, by means of photosynthesis. Up to 95% of their daily required energy is drawn from these so-called photosynthates.

2. Corals capture plankton; either phyto , zoo or bacterioplankton. The types of plankton uptake depend on species. Scientists believe that phytoplankton is the main food source for many soft corals (e.g. genus Dendronephthya).

3. Corals take up (in)organic nutrients from the water column. Nitrate, phosphate, ammonia, urea, DOC, trace elements and of course amino acids. Many marine aquarists use amino acid supplements nowadays, and it has become a popular market for manufacturers and aquarium stores. 

There still is controversy; some aquarists think aquaria don't require commercial supplements such as amino acids, trace elements and vitamins, whilst others don't leave their tank without. 

Uptake of nitrogen

Let’s take a look at the uptake of a very important element, nitrogen (N). The next pie chart shows the nitrogen uptake of Stylophora pistillata colonies in their natural environment:

Fig.1: Nitrogen budget for Stylophora pistillata colonies in their natural environment. Most of the nitrogen is provided by ammonia and nitrate; this is mainly due to the zooxanthellae (adapted from Renaud Grover et al, Journal of Experimental biology 2008). 

Note that ammonia and nitrate are the main N-source for this coral, both of which are abundant in marine aquaria. Scientists extracted the zooxanthellae from the colonies, and discovered that they are mostly responsible for this uptake. Amino acids are important as well, accounting for 21% of the nitrogen budget. Scientists also found that corals, and extracted zooxanthellae, are able to take up a lot more amino acids than they normally would in nature. Let's look at the types of amino acids and whether corals are able to extract them from the water. Figure 2 shows that at least 11 types of DFAA's (or  dissolved free amino acids) are taken up by this coral.

Figure 2: Uptake of 11 types of amino acids by S. pistillata. This species is able to actively take up all of the shown amino acids (adapted from Renaud Grover et al, Journal of Experimental biology 2008).

Now the question is; is this uptake passive, by diffusion, or does this species actively transport the DFAA's to its tissue? The answer lies in the next figure:

Figure 3: Uptake rate of nitrogen (N, nanomoles of nitrogen/hour/cm² of tissue) by S. pistillata colonies in relation to amino acid (DFAA in µmole/l) concentration. The red arrow shows normal seawater DFAA concentrations. The straight line (diamonds, linear line) shows what the coral in theory would be able to take up. In real life, the coral is able to take up a lot more (circles, saturable line) DFAA. The green arrow is an example of what could happen if we were to dose extra amino acids in the aquarium. Clearly, corals are able to take up at least 7 times the normal dosage (adapted from Renaud Grover et al, Journal of Experimental biology 2008).

Figure 3 shows the relationship between amino acid concentration (enriched ₁₅N DFAA's: dissolved free amino acids which have been labelled by a ¹⁵N isotope) in µmole/l and the uptake of ¹⁵N nitrogen. The ¹⁵N/¹⁴N ratio present in the coral tissue can be measured in the laboratory, by using a ratio mass spectrometer . The higher the ¹⁵N enrichment of the coral tissue, the more amino acids have been taken up.  It shows that this coral species is able to actively take up amino acids from the water column, even at low DFAA concentrations.

This means that corals really have adapted to the oligotrophic waters of the ocean, as the concentration of total amino acids fluctuates between 200 to 500 nmol/l. This is only 0.03 to 0.07 mg/l or ppm! It seems corals only need small concentrations of amino acids for normal growth. In fact; it seems that SPS corals like S. pistillata only need light and dissolved nutrients (next to nitrogen, their require phosphate and other elements as well) from the water column to survive. This is in contrast to corals which really need plankton to survive, like azooxanthellate gorgonians and Dendronephthya's. Now the big question is: does dosing extra amino acids in the aquarium help your corals grow? The answer is yes. Figure 3 shows that we can manipulate the amount of amino acids corals take up, by increasing their ambient concentration. It is known that amino acids are important for building the organic matrix; an important aspect of coral calcification². Next to this, amino acids are also used to build proteins, which have important functions in living cells.

Oligotrophic tanks

Many aquarists nowadays are fond of SPS corals like Acroporids and Montiporids. These tanks usually have strong protein skimmers (like Bubble Kings or large Deltec skimmers), to ensure the water stays nutrient-poor. Low nutrient levels probably causes the zooxanthellae to decrease in numbers (or reduce their photo-absorptive pigments, like chlorophyll ), reducing brown colouration. When these tanks do not receive enough nutrient input (few fish or moderate feeding), they can become too nutrient-poor. 

A tank which is poor in nitrogen (note that ammonia, nitrate and urea are not skimmed off, but that they can be kept low by food and waste skimming), might display corals which suffer from bleaching, as the zooxanthellae inside them are wasting away. They too need nutrients next to the sugars produced by carbohydrates. In much the same way, to over-usage of phosphate filters can be threatening to corals as well. Aquaria with heavy skimming which receive only moderate nutrient input could benefit from extra amino acid dosages, to prevent bleaching and improve coral colouration (corals produce beautiful pigments, in all kinds of colours. This is really a sunblock which allows corals to regulate the amount of light they receive, and to produce it corals need amino acids or proteins).

Scientists also found that amino acid uptake is stimulated by light, by using metal halide lamps at different heights. Above 160 micromole photons / second / cm² there was no extra amino acid uptake. For aquarists it means they don't need to put 400W bulbs above their (average sized) tanks to ensure amino acid uptake. However, it is interesting that amino acid uptake is a diurnal  event.

Figure 4: Corals may take up amino acids mainly during the day, even at very low concentrations (photograph: Hans Leijnse).

Concluding remarks 

Now that we know that coral aquaculture, and certain aquaria at home, could benefit from amino acid dosages, the next question is: how much do I need to add? This is difficult to answer. This depends on the aquarium volume, and moreover, on the nature and amount of biomass. Simply put; the amount, sizes and species of corals in the aquarium. On the coral reefs, the total DFAA fluctuates around 30 to 70 ppb, which really is quite low. The easiest way to determine the specific dosage for any tank would be to determine its total consumption per day (or week), or by measuring the total concentration of DFAA's at any given time. The equipment to do so is not really available to the average aquarist, yet. Finding out the stock concentration for the product you are using, and starting with the recommended dosage is good way to start. In the end, many aquarists develop 'salt water fingers', and learn how their aquarium responds to changes. 

If you consider using amino acids or other similar supplements for your home aquarium, please visit our main sponsor's website, which includes a list of distributors in your area:

References:

Renaud Grover, Jean-François Maguer, Denis Allemand and Christine Ferrier-Pagès, Uptake of dissolved free amino acids by the scleractinian coral Stylophora pistillata, Journal of experimental biology, 2008, pp 860-865(211)

Chalker, B.E., Taylor, D.L., 1975. Light-Enhanced Calcification, and the Role of Oxidative Phosphorylation in Calcification of the Coral Acropora cervicornis. Proc. R. Soc. Lond. B 190, 323–331.