The Reef Aquarium as a System. Nutrients Flow

A reef aquarium can be thought of as a system fed by inputs, which are processed to generate biomass, resulting in outputs. The system uses the inputs for its operation, transforming them into growth of organisms and producing waste products that must be reduced, removed or recycled in some way. Let’s see how this flow works in nature, to try to replicate it as far as possible.

In reef ecology, most of the nutrients exchange pathways are known as the carbon, nitrogen and phosphorus cycles, with an incredible number of organisms and variables involved. It can be considered that the three cycles belong to the same global process, where also geochemical cycles are relevant. Geochemical cycles carry out the transfer of nutrients on a large scale, driven by atmospheric, oceanographic or hydrological phenomena.

This gigantic network of nutrient transfer and recycling, has been consolidated over millions of years and functions with an efficiency superior to the most elaborate human engineering. All functions are performed by different species and none is indispensable; the absence of some provides opportunities for others. The waste products of some, are food for others and nothing is wasted. This complex network underpins the robustness and stability of coral reefs.

Fully replicating nature in a closed artificial system is impossible, but we can come relatively close. It is feasible to modify many of the natural processes and manage the water chemistry so that fish, corals and invertebrates have everything they need. A reef aquarium is a complete ecosystem where higher animals coexist with millions of microorganisms, so the interactions between them all condition the overall state.

The food web is a key component of this huge recycling network and includes all living things, from the tiniest bacteria to the largest planktonic crustaceans, fish, corals and invertebrates. All the organisms in the food web exchange nutrients with the water around and self-regulate their populations through competition and/or cooperation, depending on the environmental conditions. The members of the food web use nutrients as food or release them as waste products, closing the cycle.

This food web provides a good part of the aquarium capacity to process the waste products and recycle them into food. The first levels of the web are occupied by microorganisms like bacteria, protists, phytoplankton and microscopic zooplankton, which perform a great job of recycling, assimilating many of the dissolved ions and nutrients in excess. Figure 1 shows a simplified scheme of the food web in a reef tank.

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Figure 1. Food Web in a Reef Tank (Simplified)

Autotrophic microorganisms assimilate inorganic nutrients, e.g., ammonia, nitrate, phosphate and CO2, while heterotrophs feed on simple organic matter or digest other smaller organisms. DOC (dissolved organic carbon) is the basic organic food available in the aquarium. This tiny food comes from compounds released by bacteria, cyanobacteria, protists, phytoplankton, zooplankton, corals and algae. DOC is so elemental and minuscule that zooplankton is not able to assimilate it directly. But there is an extraordinarily abundant group of microorganisms, mostly heterotrophic bacteria and heterotrophic protists, which feeds on this simple organic matter and recycles it through its processes of growth, predation and excretion.

Bacteria, both autotrophic and heterotrophic, are the microscopic organisms that have the greatest impact on nutrient concentrations in the aquarium water. The reason is that their populations are gigantic compared to those of protists and phytoplankton. Appropriate techniques to stimulate the growth of beneficial bacterial communities in the reef aquarium are now well understood. This growth involves the removal of organic and inorganic nutrients from the water, which are recycled into bacterial biomass. The use of artificial organic carbon sources is one of these techniques.

Protists are very simple single-celled organisms, both autotrophic and heterotrophic, that cannot be classified as animals or plants. Most live drifting as part of the plankton or anchored to the substrate. The most common protists found in the aquarium are ciliates, coccolithophores, tintinnids, foraminifera, radiolarians and dinoflagellates. Foraminifera and radiolarians are heterotrophic or mixotrophic organisms that use a shell segregated with calcium carbonate or silica. They are major contributors to beach formation and some of them harbor microscopic algae within their tissues, just as coral polyps harbor zooxanthellae.

Both microorganisms and higher organisms in the food web modify nutrient concentrations in water through a series of natural processes. Table 1 identifies the names of these and their effect on dissolved nutrients. The upward arrows indicate generation and the downward arrows indicate consumption.

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Table 1. Natural Processes Impacting Nutrient Concentrations in Reef Tank's Water

Nitrogen fixation. By nitrogen fixation, diazotroph organisms break down the nitrogen molecule dissolved in the water (N2), releasing ammonia (NH3). This process does not significantly contribute to the tank total ammonia concentration, but it is interesting to understand how it works, because it is carried out by organisms such as cyanobacteria, some of which are undesirable and sometimes constitute pests. Cyanobacteria are autotrophic organisms capable of thriving in waters with very low nitrate and phosphate concentrations, directly assimilating dissolved nitrogen from the water, to release ammonia.

The ammonia produced by this process is an excellent nutrient which can be used by other autotrophs present in the sand and rocks, for example, diatoms and some dinoflagellates. For this reason, when a cyanobacteria plague occurs, there are other invasive species associated, which coexist in the same favorable environment. Hence the difficulty of eradicating them when the bloom is consolidated.

Feeding. The contribution of food to the tank implies a directly increase in all organic, and indirectly, inorganic nutrients. The food wastes suspended in the water are organic matter which begins to decompose by the action of heterotrophic bacteria, releasing ammonia, that triggers the nitrification processes. The food also contributes phosphate to the aquarium.

Heterotrophic decomposition. The decomposition of organic matter is a process that begins when any life form dies into the tank. The work is done by heterotrophic bacteria, which feed on organic debris floating in the water column, into the sediments or live rocks. It is the process with the greatest impact on the aquarium water ammonia concentration.

Respiration. It is carried out by all heterotrophic organisms to oxidize the organic matter assimilated during feeding. In this way they obtain energy for their metabolic processes. In respiration, oxygen is assimilated and carbon dioxide is released. CO2 released contributes to pH decrement. During the night, this effect is more pronounced, since algae, zooxanthellae and other organisms that perform photosynthesis, also release carbon dioxide.

Excretion. Excretion occurs when any heterotrophic organism, e.g., zooplankton, fish or corals, release ammonia and organic debris as waste material. This ammonia is “leftover nitrogen” from their tissue-building activities and other metabolic processes. The organic wastes resulting from excretion serve as food for many other organisms, either directly as organic matter for heterotrophs or later, once transformed into nitrate and phosphate for autotrophs.

Biological nitrification. For the aquarium to be able to process all the ammonia generated in the processes of fixation, excretion and heterotrophic bacterial decomposition, a nitrifying microbial community must exist. This community consists mainly of autotrophic bacteria, which assimilate carbon from dissolved CO2 and use ammonia and nitrite molecules as energy sources. The end product of nitrification is the nitrate ion (NO3-). The transformation of ammonia to nitrate takes place in two stages.

  • Stage 1: Transformation of ammonia to nitrite, which involves species such as Nitrosomona europaea, Nitrosococcus oceanus y Nitrosomonas mobilis.
  • Stage 2: Transformation of nitrite to nitrate, involving species such as Nitrobacter winogradski, Nitrococcus mobilis and Nitrospira gracilis.

Nitrification, which is probably the most relevant process in an aquarium, takes place in oxygen-rich environments, so adequate water movement is essential to facilitate gas exchange with the air surrounding. In these aerobic conditions, nitrifying bacterial populations are responsible to avoid the accumulation of ammonia and nitrite beyond toxic concentrations.

Biological denitrification. In this process, bacteria use the nitrate and phosphate dissolved in the water to grow, releasing molecular nitrogen as a waste product. Denitrification involves autotrophic and heterotrophic bacteria, under aerobic and anaerobic conditions. It is of extraordinary importance for the reduction of nitrate and phosphate concentrations in a reef aquarium. An example of systems that implement autotrophic denitrification are sulfur reactors, where bacteria consume CO2 and sulfur compounds. Some useful strains for this process are Paracoccus denitrificans, Pseudomonas denitrificans, Thiobacillus denitrificans and Thiomicrospira denitrificans. These reactors have lost popularity over the years in favor of heterotrophic denitrification, which is more efficient and easier to implement. In the heterotrophic mode, bacteria assimilate organic carbon (DOC) and use the nitrate molecule (NO3-) releasing molecular nitrogen (N2).

In an aquarium under normal conditions, biological denitrification takes some time to work efficiently. The reason is that the denitrifying heterotrophic bacterial communities are limited by the low organic carbon content of the water. Under these conditions, denitrification takes place exclusively in anaerobic zones, such as the interior of live rocks or deep sediment zones, where oxygen is scarce. When artificial organic carbon is added, the denitrification efficiency is significantly increased. For this purpose, it is convenient to use very simple molecular structure organic compounds, so that the bacteria can assimilate them quickly, grow, reproduce and remove both nitrate and phosphate form the water. It is widely known that a mixture of methanol, ethanol, acetic acid and glucose works quite well. Aquarists also use vodka, which contains glucose and ethanol, with also good results. There are also commercial products for this purpose that provide excellent performance.

An artificial source of organic carbon favors the aerobic heterotrophic denitrification pathway, which involves polyphosphate-accumulating bacterial communities, which assimilate phosphate in oxygen-rich environments and release them in anaerobic ones. Examples of aerobic denitrifying bacteria are Nitrosomonas eutropha, Pseudomonas aeruginosa, Paracoccus denitrificans or Microvirgula aerodenitrificans. The process consumes a large amount of oxygen, so vigorous water movement is necessary to avoid suffocating the fish and corals. One of the signs that we can observe when these bacteria reproduce, are accumulations of light gray biofilms on the mechanical filters, recirculating pumps, tank walls and sump.

Photosynthesis. Zooxanthellae perform photosynthesis to generate organic compounds, which are transferred to the coral as food (translocation). This process is also carried out by other photoautotrophic organisms, such phytoplankton and all types of algae. Photosynthesis removes ammonia, nitrate, phosphate and CO2 from the aquarium water, transforming them into plant tissues and organic carbon compounds, e.g. amino acids or carbohydrates. Algae refugia and algae reactors use photosynthesis for inorganic nutrient reduction in the aquarium.

In figure 3, a nutrient flow map is shows, including some of the natural recycling key processes. Nutrient transfers pathways are depicted on top of some of the typical physical environments where the processes occur.

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Figure 3. Nutrients and Natural Processes are Linked Through Transfer Pathways

An “ideal reef aquarium” would behave like an open ecosystem, i.e. with perfectly balanced nutrient recycling. It would be able to transform into biomass all the food and energy supplied from the outside, keeping the concentrations of organic and inorganic nutrients in their ranges. In Figure 4, we can see a schematic for the nutrient flow in this ideal aquarium. The inputs of matter and energy would be: food, animals, light, oxygen and atmospheric CO2 surrounding the tank.

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Figure 4. Nutrient Flow in an "Ideal" Reef Aquarium

Within a real aquarium, the natural recycling network processes the inputs, facilitating the growth of microorganisms, corals, fish and invertebrates. The recycling network is formed by the natural processes that we have previously depicted, plus the artificial systems that the aquarist uses to reduce of export organic and inorganic nutrients, like protein skimmers, mechanical and chemical filtration media, organic carbon or algae refugia. If the recycling network is not properly adjusted, or there is a very rapid increase in inputs, e.g., overfeeding or overpopulation, excessive accumulation of organic and inorganic nutrients occurs. These additional nutrients are not needed and may even be detrimental. Similarly, if inputs are low, or the aquarium has an excessive capacity to deplete them, a nutrient deficit result. Figure 5 shows the nutrient flow scheme for a real aquarium.

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Figure 5. Nutrient Flow in a Real Reef Aquarium

Excess nutrients can be removed in two ways: by reduction or by export. Reduction involves techniques that modify the water chemistry to favor processes that consume them, e.g. denitrification. Exporting, however, involves extracting them with filtration media, chemical filtration, water changes or specific devices as mechanical filters and protein skimmers. As a matter of cost efficiency, the reduction and export systems must be sized according to the inputs and the aquarium’s capacity to process them.

In short, it is an incessant inflow, recycling and outflow, which must be adjusted to reach a stable equilibrium situation, where all organisms have available food and the concentrations of organic matter, ammonia, nitrite, nitrate and phosphate are within the recommended ranges. If the flow balance is positive, it is necessary to limit inputs or increase reduction and/or export. If the flow balance is negative, it is necessary to increase inputs or limit reduction and/or export. The nutrient flow can be described according to the following equation:

Inputs + Recycling Network = Growth + ΔNutrients

Where ΔNutrients, are the excess nutrients, both organic matter, ammonia, nitrite, nitrate and phosphate. Then, for the real aquarium to behave as an open system, ΔN utrients must be zero. If ΔNutrients is positive, reduction and/or export must be implemented, while if ΔNutrients is negative, it is necessary to increase or add them artificially. Generally speaking, the natural recycling processes in aquariums are capable of reducing excess amounts of ammonia and nitrite within hours, however, nitrate and phosphate take much longer to eliminate, accumulating indefinitely if nothing is done.

By encouraging the development of a robust food web and regulating inputs appropriately, it is possible to bring the aquarium to a state where some of the usual nutrient export systems can be reduced, with maintenance savings. For example, there are aquarists who experience a noticeable reduction in the amount of organic matter removed by the tank skimmer, some months after set-up. This leads one to think that the device is losing its efficiency, but the reality is that the aquarium is consolidating its maturation; so that the natural processes are capable by themselves of processing all the provided food and waste products that are generated.
About author
Angel Cegarra is the author of the book “Reefkeeping Fundamentals”, where he presents a fascinating journey through the science and technology that govern life inside a reef tank. From nutrient recycling processes, through coral biology, project initiation, feeding techniques and maintenance of water chemistry. The book includes world class reef aquariums description along with priceless advice on how they were planned and how they are kept healthy and vibrant. Covering the biology of coral reefs and relating it to the biological, chemical, and physical parameters considered when creating a reef aquarium. Home aquarist moving into an area of knowledge that was once the exclusive domains of scientists. Order at Amazon.

Link to Amazon:ÅMÅŽÕÑ&dchild=1&keywords=reefkeeping+fundamentals&qid=1619508382&sr=8-1

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