Coral Growth - All You Need to Know

In this article, I want to comprehensively describe the issue of coral growth. To make it easier for you to understand the topic, I will first focus on discussing what coral growth is in general and what factors are crucial for this process. Then I’ll move on to some useful tips to help you care for your coral growth properly.

Hope you find this article helpful 🙂 Happy reading!

What does coral growth depend on?

There are three main physiological processes that have a significant impact on coral growth: photosynthesis, heterotrophic nutrition and calcification. So let’s move on to discussing each of the above processes separately.

Coral growth and photosynthesis

Corals that contain symbiotic algae in their tissues can obtain the main part of their energy resources from photosynthesis. These algae, called zooxanthellae, use light energy to produce organic compounds from carbon dioxide and water. As part of this symbiosis, the corals provide shelter and nutrients to the algae, and in return the algae provide them with a significant amount of organic compounds, including sugars and amino acids.

In the process of photosynthesis, algae also produce oxygen, which is used by corals for respiration. Corals take in oxygen through diffusion, which is the natural movement of molecules from a place of higher concentration to a place of lower concentration. Corals are also able to take up oxygen from the water through the tissue on the polyps. The polyps are surrounded by mouth tissue called mesoglea. The mesoglea transports oxygen from the polyps to other parts of the coral, including bone tissue, where it is used for respiration and metabolism.

The growth and functioning of corals is very dependent on the availability of oxygen. In low oxygen conditions, corals may have difficulty surviving and reproducing. In some cases, when oxygen levels are too low, corals are able to switch to anaerobic functioning, which leads to the production of harmful chemicals and can lead to their death.

Since the process of photosynthesis provides energy and oxygen, it is crucial for coral growth.

What affects the efficiency of photosynthesis in corals?

The efficiency of photosynthesis in corals depends on several factors, such as: the quality of light, the concentration of carbon dioxide (CO2), the amount and type of zooxanthellae, and the presence of chlorophyll, which is found in these symbiotic algae.

Light quality is one of the most important factors affecting photosynthesis. Corals living in shallow marine waters, where there is a lot of light, tend to have more zooxanthellae in their tissues and achieve higher photosynthesis efficiency than those living in deeper waters, where there is less light.

The presence of chlorophyll in corals is also important for photosynthesis efficiency as chlorophyll is a key photosynthetic pigment that absorbs light and converts it into chemical energy needed for photosynthesis.

The concentration of carbon dioxide (CO2) also affects the efficiency of photosynthesis in corals, as it is necessary for the photosynthesis process. Low CO2 concentrations can limit the efficiency of photosynthesis, which can lead to a decrease in coral growth.

The amount and type of zooxanthellae is also a factor affecting the efficiency of photosynthesis in corals. There are many different types of zooxanthellae, each with a different sensitivity to environmental conditions. The optimal conditions for one type of zooxanthellae may not be suitable for another, which can lead to reduced photosynthesis efficiency and coral growth.

Heterotrophic feeding of corals

In the vicinity of coral reefs or in aquariums with poor lighting, the amount of light reaching the corals may be limited, and this may affect their ability to carry out photosynthesis and obtain energy. In such cases, corals can increase their heterotrophic activity, i.e. taking food from the surrounding environment, to replenish their energy reserves.

Heterotrophic coral nutrition involves taking in organic matter from the surrounding environment, usually by filtration of the water. Corals can feed on zooplankton, detritus, phytoplankton, bacteria and other small organisms. There are also predatory corals that prey on smaller organisms such as small fish and crustaceans.
However, among corals, heterotrophic nutrition usually accounts for only a small percentage of their total diet. Corals are mostly photosynthetic, which means they produce their energy through photosynthesis by the aforementioned symbiotic zooxanthellae algae. Heterotrophic nutrition complements the corals’ diet and helps them maintain their health and growth.

What affects the efficiency of the heterotrophic nutrition process (i.e. heterotrophy) in corals?

The effectiveness of the process of coral heterotrophic nutrition can be due to many factors. Below I list a few of them:

  1. Availability of food in the aquarium water – corals need the right amount of food to successfully feed heterotrophically. A lack of food or a low concentration of food in the water can limit the corals’ ability to feed.
  2. Type and amount of food – corals prefer to eat plankton and detritus, but can also eat larger food particles such as fish and crustaceans (depending on the type of coral). Different types of food affect the ability of corals to take up and digest food differently.
  3. Duration of exposure to food – corals have a limited amount of time during which they can take up and digest food. Excessive exposure to food may prevent corals from taking up enough food.
  4. Water temperature – Water temperature can influence the efficiency of the heterotrophic nutrition process in corals. High temperatures can increase the rate of food digestion, but at the same time can cause internal damage to the coral’s body. The recommended temperature is 24-27 °C / 72-80 °F.
  5. Presence of toxic substances – the presence of toxic substances in the water can inhibit the ability of corals to take up and digest food, the same applies to dirt in the water or overdoses.
  6. Oxygen deficiency – low oxygen levels in the water can inhibit corals’ ability to take up and digest food by slowing down their metabolism.

Calcification, i.e. the process of creating lime skeletons.

Calcification is the third important process responsible for coral growth. Calcification is the process of creating calcium skeletons by corals. Calcium is essential for the construction of coral skeletons, and the calcification process consists of depositing calcium ions (Ca2+) and bicarbonate ions (HCO3) on an organic matter that is secreted by the corals.

The calcification process depends on many factors, including the concentration of calcium and bicarbonates in seawater, the presence of appropriate enzymes, as well as the presence of appropriate bacteria and symbiotic algae.

We will now look at the key areas of calcification.

Enzymes affecting the calcification process

Enzymes play an important role in the formation of the calcium skeleton as they control the rate of calcium excretion and deposition and control the internal pH of coral cells.

One of the most important enzymes involved in the calcification process is carboanhydrase. This enzyme catalyzes the reaction between carbon dioxide (CO2) and water, which leads to the formation of bicarbonate ions (HCO3) and hydrogen ions (H+). The hydrogen ion is expelled outside the cell, which reduces the acidity inside the cell, and the bicarbonate ion is used to produce calcium carbonate (CO32-) by reacting with calcium ions (Ca2+), leading to the formation of a calcium skeleton.

Another important enzyme is ATPase. This enzyme catalyzes the reaction that leads to the breakdown of the ATP molecule, which generates the energy needed to transport calcium ions inside the cell, where it is used for calcification.

In addition, corals contain enzymes that control the pH inside the cells. An example of such an enzyme is the Na+/H+ exchanger which helps maintain a constant pH in the cell.

All these enzymes are crucial for an effective calcification process. Changes in their functioning, caused for example by changes in pH or temperature, can lead to a weakening of the calcification process and affect the rate of coral growth.

Bacteria affecting the calcification process

The calcification process in corals depends not only on the presence of appropriate enzymes, but also on interactions with microorganisms, including bacteria. These bacteria, known as endosymbiotic bacteria, can affect coral calcification in several ways.

Firstly, endosymbiotic bacteria may help provide calcium to corals through biological processes such as mineralization or decomposition of organic matter. For example, bacteria can secrete enzymes that break down organic matter near the coral, releasing calcium from its constituents. In this way, the bacteria can help increase the concentration of calcium in the water, which contributes to the formation of calcareous skeletons in corals.

Secondly, some endosymbiotic bacteria can produce organic compounds that corals use to make calcareous skeletons. For example, bacteria can produce acetic acid, which corals use as a substrate to produce calcareous skeletons.

Finally, endosymbiotic bacteria may help neutralize the hydrogen acids produced during coral metabolic processes. Acidity can reduce the calcium concentration in the water, which makes the calcification process more difficult. Bacteria that neutralize the acidic environment can help maintain the proper pH for calcification processes.

All in all, endosymbiotic bacteria may play an important role in the calcification process in corals by supplying calcium, producing organic compounds and neutralizing the acidic environment. However, the exact mechanism and role of these bacteria in coral calcification is still being investigated.

Photosynthesis and the process of calcification

In corals that contain zooxanthellae algae, the process of calcification is related to photosynthesis, as coral skeletons are made of calcium carbonate (CaCO3), which is a by-product of algal photosynthesis.

During photosynthesis, zooxanthellae algae take carbon dioxide (CO2) and water (H2O) from seawater and convert them to sugars and oxygen (O2) through photosynthesis. Photosynthesis also produces bicarbonate (HCO3), which is transported from the algae to the coral. In coral, bicarbonate reacts with calcium ions to form calcium carbonate (CaCO3), which is deposited in the coral’s organic matrix to form coral skeletons.

The calcification process is very important for corals because it allows them to form hard skeletons that form the basis of their structure and enable them to survive in conditions of constant environmental changes. However, the calcification process is also very sensitive to environmental changes, such as changes in pH or seawater temperature, which can lead to disturbances in the calcification process and negatively affect the development of corals.

What affects the calcification process in corals?

The effectiveness of the calcification process, i.e. the formation of calcareous skeletons in corals, depends on many factors. And here are some of them:

  1. Availability of calcium and bicarbonate ions – corals need calcium (Ca2+) and bicarbonate (HCO3) to build their skeleton. The absence of any of these ions can limit the calcification process.
  2. pH of water – high pH favors the calcification process, because it facilitates the dissolution of calcium and bicarbonate ions in water. In turn, low pH can negatively affect the calcification process, as it can lead to the dissolution of the coral skeleton. The recommended pH in seawater is 8.1-8.4.
  3. Water Temperature – high temperatures favor calcification as they increase the rate of chemical reactions needed to build the skeleton. However, too high temperatures can lead to bleaching and weakening of corals, which may limit their ability to calcify. As I wrote above, the recommended temperature is 24-27 °C / 72 – 80 °F.
  4. Nutrient availability – corals need adequate amounts of nutrients such as amino acids, sugars and fats to produce proteins and other organic compounds that make up the living skeleton matrix.
  5. Availability of light – corals feed on photosynthesis from their symbiotic algae, which provide them with energy. Therefore, the quality of light is an important factor in the ability of corals to calcify.
  6. Water flow – water flow is an important factor in calcification efficiency. Proper water flow enables the supply of nutrients and calcium to the corals and the removal of the waste they produce. Too little flow can cause the accumulation of harmful substances such as ammonia and carbon dioxide, which can adversely affect the calcification process.


In the article, I described the processes that affect coral growth and what affects their effectiveness. As you can see, corals need factors such as high-quality light, adequate food supply and the right level of chemical elements for proper growth. In addition, it is necessary to ensure the stability of the environment, the appropriate level of temperature, pH and proper water circulation in the aquarium. In other articles on ReefPedia you will find comprehensive information on the right choice of light and tips to help you maintain corals and provide conditions for their proper growth. We invite you to have a read! Soon you’ll also be able to start a discussion in threads dedicated to individual topics on the Social Reef platform.

About the author

Picture of Marek Protasewicz

Marek Protasewicz

Reefkeeping has been my passion for over 10 years now. I love learning. The hobby has taught me many valuable lessons, patience being the best example.
Combining work and passion is my path. I run Crazy Coral, a marine aquarium shop, for a number of years. Building this business from the scratch I learnt from my own mistakes at a heavy cost.
Later I managed a project aimed at development of methods for quick growth of Corals in non-natural conditions. The project was carried out by Get Sales, Poland.
Presently, I am responsible for distribution strategy at Reef Factory, of which I am a
co-founder. The company produces smart devices for marine aquaristics.
The last projects I have been involved in are Social Reef and ReefPedia.

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