Why do corals lose their color?
Many of the questions I get asked relate to diseased corals. Often the problem is related to varying degrees of paleness in color compared to normal or the presence of noticeable white areas on the body of the coral. In any case, it is very difficult to determine the nature of the problem, but one of the most common mistakes is misidentifying coral bleaching. In the next few articles, I’m going to look at the causes, manifestations and effects of discoloration, and then talk about the various types of problems associated with tissue death. These situations are difficult to distinguish: the causes and impacts may be similar or different, and they may even be interrelated. 
Trachyphyllia geoffroyi often susceptible to discoloration in aquariums. This may be partly due to the fact that brightly colored specimens (red and green) are caught at great depths. The coral in the photo is bleached and showing signs of tissue loss, possibly due to starvation resulting from the loss of zooxanthellae. BACKGROUND What is discoloration? Bleaching can occur in corals that live in symbiosis with various types of dinoflagellates known as zooxanthellae. By general definition, bleaching is the removal, suppression, or loss of zooxanthellae from coral tissue. A coral obtains zooxanthellae in two ways: first, it can receive a “starter culture” from a parent colony if the parent colony incubates its planula. On the other hand, in corals that release sperm and eggs into the water when fertilization occurs in the water column, the planula (which does not have zooxanthellae) can obtain algae from the water. Once ingested, the algae is not digested, but rather enters the cells and is placed in a small intracellular sac called a vacuole. Once in the vacuole, they are “captured” and are at the mercy of the coral polyp. Golden brown algae reproduce inside the cell and form a dense but very thin layer inside the polyp. Zooxanthellae are found primarily in the inner layer of coral tissue, called the gastrodermis, although they can sometimes be found in the outer layer of tissue (ectoderm) and in the tentacles of day-feeding corals. Corals that feed at night have transparent tentacles and usually lack zooxanthellae. 
Bleached copy Goniopora. The tissues are transparent, indicating a significant reduction in the number of zooxanthellae. This coral is most likely not bleached by excessive light or too much heat because it was collected at great depths with low light levels and the temperature is relatively stable. Once inside the polyp, the zooxanthellae receive nutrients that are regulated, and usually limited, by the host coral. In turn, the algae use sunlight for photosynthesis and supply the coral polyp with photosynthetic products. The nutrients for zooxanthellae are mainly the products of metabolism (metabolism) of the coral; in particular, carbon dioxide and nitrogen. One of the benefits of being in coral tissue is that zooxanthellae have constant access to nitrogen in the form of metabolic waste from the coral. In comparison, seawater, which typically contains insufficient nitrogen, may not be able to provide enough nitrogen for growth and reproduction. However, the coral is able to and does control the amount of substances supplied to the zooxanthellae, dumping the excess back into the seawater. Under normal conditions, a fairly even balance is maintained, with very little deficiency or excess, with virtually all of the coral’s metabolic waste being consumed by a regulated population of zooxanthellae. Types of discoloration
However, situations arise that disrupt the balanced symbiosis of algae and coral. In situations of acute or chronic nutritional deficiency, the coral produces insufficient amounts of metabolic waste to support the functioning of the zooxanthellae. Conversely, zooxanthellae will be unable to produce enough photosynthetic products to support the coral. In cases of significant deficiency, the density of zooxanthellae decreases. There are three options: the zooxanthellae simply die and are removed from the coral; the coral can absorb zooxanthellae in order to satisfy its own energy needs (if we are talking about species that can digest plant material, especially cell wall components); or the coral sheds some of the zooxanthellae from its tissues into the water. This is discoloration. 
This Cynarina lacrymalis severely discolored. The tissues are clearly visible and swollen, although without zooxanthellae pigmentation. The white color comes from the skeleton, visible under the tissues. This coral will have to grab food or need to be fed to prevent starvation and restore a full complement of zooxanthellae. Similarly, although for different reasons, acute or chronic excess of nutrients (especially nitrogen) can lead to discoloration. Since corals are able to absorb dissolved nutrients directly from the water, they benefit from the energy obtained in this way. However, when dissolved nitrogen is absorbed into tissues and cells, zooxanthellae also gain access to the substance. In this case, an excess amount of nutrients is available to the zooxanthellae; therefore, the coral has less control over the nutrition of the zooxanthellae, which means that the zooxanthellae can use nitrogen for growth and reproduction. Increasing the density of zooxanthellae does not always benefit the coral; growth can become unbalanced and uncontrolled. In this case, bleaching becomes necessary to maintain the correct algae density in the coral tissue. The life cycle of zooxanthellae is limited, so at any given time there are a certain number of zooxanthellae that age and become useless to the polyp. Such zooxanthellae are excreted, and this is also a type of bleaching. The above descriptions reflect the adaptive and productive behaviors used to maintain a balanced symbiosis. Therefore, discoloration, as described, is not necessarily a harmful phenomenon, as is commonly believed. However, there are varying degrees of discoloration and other factors that can cause discoloration. We are talking about situations where bleaching is not a normal regulatory mechanism, but is caused by various factors that threaten not only the symbiosis, but also the health of both parties (coral and zooxanthellae). Another definition of discoloration
Coral bleaching has another, more popular definition. According to this definition, a coral is considered bleached when there is a noticeable lightening of the characteristic coloration, which is interpreted as a loss of approximately 50% of zooxanthellae density. Most people associate bleached coral with pictures of all-white corals on reefs. This is considered a severe bleaching event, with mass bleaching defined as the partial or complete bleaching of an entire community of corals. When I talk about complete (total) discoloration, this is still an exaggeration. To my knowledge, there have been no known cases of complete bleaching, except under experimental conditions (difficult to achieve) and some temperate corals can survive with or without zooxanthellae. In the worst case, the density of zooxanthellae is reduced to a minimum, to the point that their brownish coloration becomes invisible, and the coral tissue becomes largely transparent, revealing the underlying white skeleton. Mechanisms and effects of bleaching
The presence of small amounts of zooxanthellae in the tissue of bleached coral causes bleached corals to often recover. It is unlikely that they are significantly reduced by obtaining zooxanthellae from water, but rather by multiplying zooxanthellae remaining in the tissues. However, if the density of zooxanthellae is too low, the coral does not receive enough energy from the primary products of photosynthesis of its symbiotic algae. An energy deficit arises that must be replenished either through nutrition or directly obtaining nutrients from seawater. At the same time, severely bleached corals often do not recover and die. Why? Often, seawater does not contain enough nutrients so that direct absorption of nutrients cannot provide the required degree, or rate, of nutrition. Secondly, even if there is enough food in the water for corals, maintaining the mechanism for capturing and absorbing particles, such as nematocysts, is energy intensive. The coral may not be able to effectively maintain the functionality of these structures, and therefore may not be able to feed effectively. Moreover, energy is required to absorb and digest feed. Aquarists may notice that bleached corals lose interest or the ability to capture food particles offered to them. Ultimately, bleached corals operate in an energy deficit mode and must metabolize their own tissues to survive; this is recession and tissue death. This process is also called “starvation” (complete exhaustion). 
Representative of Sinularia sp. in the photo it is discolored, although on some branches (on the left) the zooxanthellae are still visible. The best solution to severe bleaching, other than removing the stressors that led to the bleaching in the first place, is to provide sufficient nutrients to support the coral and restore the zooxanthellae population, and to provide them in a form so that the coral expends minimal energy to their receipt and use. The optimal solution to meet all these requirements is to provide a sufficient amount of dissolved nitrogen in the water. High nitrogen levels are unlikely to be beneficial once bleached corals have recovered, but they are beneficial during the recovery process. Bingman rightly points out that in many aquariums the nitrogen content in usable form is already several times higher than in reefs. In such cases, increasing the content of suitable forms of nitrogen (nitrates and ammonium) most likely will not play any role. However, many aquarists now try to maintain nitrogen levels in their systems around natural reef levels or even below the reef average, in which case ammonium or nitrates can become a resource that promotes zooxanthellae proliferation. For more information on the role of nitrogen in zooxanthellae reproduction, see Marubini and Davies (1996), Hoegh-Guldberg (1994), Hoegh-Guldberg and Smith (1989), Mueller-Parker et al. (1994). Another problem with bleaching is the way in which the zooxanthellae are lost. Aquarists may be familiar with the brown slimy strands of zooxanthellae that come out of a coral’s mouth. Often, the excretion or loss of zooxanthellae is a relatively controlled process whereby vacuoles containing algal cells move towards the outer cell membrane, fusing with it and then releasing their contents into the gastric cavity. However, in cases of rapid discoloration or severe stress, a destructive shed occurs where the entire contents of the cell are discharged into the gastric cavity or, worse, the entire dermal (skin cell) is detached and lost. It is clear that such traumatic reactions cause even more harm than just the loss of algae cells. In such cases, the discoloration is severe with associated damage, and then the chances of recovery are very low. Corals can bleach for a number of reasons, some of which were described above as regulatory processes. More specifically, the quantity and quality of the primary products of photosynthesis are the main factors. In particular, oxygen production by zooxanthellae may be problematic. Excess oxygen, especially in the form when single oxygen radicals are produced or when they combine with water to form hydrogen peroxide, causes damage to coral tissue. Corals actually produce enzymes that neutralize these forms of oxygen, but in conditions that trigger bleaching, they may not be able to keep up with the amount of oxygen produced. Therefore, bleaching occurs to prevent oxygen poisoning of the coral cells. It is still unclear whether coral or zooxanthellae control the bleaching. There is evidence for both hypotheses, and it is possible that various cases of bleaching, depending on the situation, are controlled by both or one of the parties. Further research is needed to determine these and other aspects of the bleaching reaction. Other aspects of bleaching
There are many factors that can cause discoloration. In the natural environment, a known factor associated with mass coral bleaching is a prolonged increase in temperature above normal levels. Temperature, as a cause of bleaching, can be combined with other factors, including low current, light and nutrients. Various studies have identified the following factors that can lead to discoloration: • Bacterial discoloration – Vibrio shiloi
• Low light or darkness
• Chemicals – pollutants, metals, pesticides, contaminants
• Endolithic fungi
• Ciliates – unknown type and role
• Increased salinity
• Coccideans – unknown type and role
• Reduced salinity
• High temperature – short or long term exposure
• Medicines
• Weak current (still/stagnant water, calm)
• Competition
• High light levels – short or long term exposure
• Sedimentation
• UV radiation – sudden increase or high levels for a long time
• Fasting (exhaustion)
• Sudden change in temperature – increase or decrease
• Physical damage or stress “But my coral is still a pale blue color,” the aquarist clarifies, “it cannot be bleached.” Not true! The bright colors of many corals are due to the presence of fluorescent proteins that are not part of the zooxanthellae. These pigment complexes are found in vesicles either above or below the zooxanthellae in animal tissues. They serve to regulate visible and UV light and perform either an enhancing or a protective role. If zooxanthellae are lost, these pigments are still present in the tissue for some time. Because they no longer serve their function and are metabolically expensive to produce, these pigments will be lost over time until they are needed again. If the coral recovers, it will produce these pigments again when needed. However, it takes time for them to metabolize (produce pigments as a result of metabolism) (unless the contents of the cells have been completely lost due to bleaching or the cells have completely separated), so the coral is able to partially retain bright pigmentation even if it has lost almost all of its zooxanthellae. Conclusions and notes for aquarists:
In conclusion, bleaching is a common phenomenon among corals, both in the wild and in aquariums. In many cases, minor discoloration may go undetected when zooxanthellae and pigments are present in large numbers, making observation difficult. When the bleaching is severe, the coral’s tissues become pale or translucent, causing the coral to appear pale or appear white. In such a situation, it can be very difficult to determine whether coral tissue remains on the skeleton. In some cases, you can notice the disintegration of the tissues, so it is obvious that the tissues still remain, but they are transparent. In other cases, especially when the coral is still under stress, the coral tissue may not expand and becomes smaller and remains compressed. In this case, it is very difficult to determine whether the coral has tissue left or whether tissue loss has occurred. One of the quickest ways to figure it out is to observe the rapid colonization of diatoms and other algae. These algae will not settle on the coral tissue, but they will readily colonize the exposed skeleton and should be visible during the daytime within one day of the exposed skeleton. However, this aspect can also be deceptive. Sometimes a bleached coral will have tissue left behind, but it will then die as a result of the bleaching, exposing the skeleton. In addition, coral recovery from bleaching sometimes begins quickly, and the recovering and multiplying brown zooxanthellae within the tissues can be mistaken for diatoms and other brown algae on the bare skeleton of the coral. Conversely, brown diatoms are often mistaken for a recovering zooxanthellae population. Moreover, the original population of diatoms is quickly replaced by other algae, many of which may well be single-celled green algae, misleading aquarists who mistake them for regenerating coral tissue. Sometimes aquarists believe that their coral is recovering because the green color of the tissues is returning, but often it is just green algae growing on the exposed skeleton. 
Euphyllia parancora shows bleaching but retains fluorescent proteins. Whether or not a coral recovers from bleaching depends primarily on subsequent conditions and the extent of bleaching. There are no strictly defined rules for determining whether a coral will recover or not, in most cases only time will tell. Because even if the coral appears white, it is not always a sign of bleaching. Pale and white color of coral can also indicate tissue recession, competition, predation, environmental stress and disease. Although bleaching is difficult to recognize, it is the easiest of all white coral problems to identify. In the next article I will talk about other reasons for the appearance of “white” corals and their recognition in aquariums. Original source: advancedaquarist.com
Translated specifically for ReefCentral.ru If you saw this material on another site, it means it was stolen.
Please report any observed facts to info@reefcentral.ru 
- My name is Aleksey
- From: Moscow
Sent 21 October 2018 – 13:29
Good afternoon, dear Aquarists!
All our SPS turned white. First the calendrum, then the montipora plate turned from green to white, now the remaining SPS is turning white.
What’s wrong with the water? What measures should be taken?
All indicators were measured. We measured the TDS of water – 14 (can this have such an effect?). There are no algae.
Please help me with advice!
#2 
cutmaster
- My name is: Alexey
- From: Krasnogorsk
Sent 21 October 2018 – 13:46
#3 odin.
- My name is Sergey
- From: Moscow m Skhodnenskaya
Sent 21 October 2018 – 14:00
All indicators were measured.
Or is measuring indicators enough?
#4 Orion
- My name is Aleksey
- From: Moscow
Sent 21 October 2018 – 14:14
Good afternoon, dear Aquarists!
All our SPS turned white. First the calendrum, then the montipora plate turned from green to white, now the remaining SPS is turning white.
The Scat80 light was set to minimum, I also thought it was very bright, but they still continue to turn white.
#5 Orion
- My name is Aleksey
- From: Moscow
Sent 21 October 2018 – 14:15
Or is measuring indicators enough?
I understand your irony, but since I am an amateur in this matter, I ask, what else needs to be checked?
#6 bourbon
- From: Moscow
Sent 21 October 2018 – 14:18
What nitrate, phosphate and kH
#7 SHUN
- My name is: Shura
- From: Cherepovets
Sent 21 October 2018 – 14:27
Trolling again) recently there was a topic with the same name))) in the same section just below
How would you first describe the entire system with the meaning of all the tests?
#8 Colobot
- My name is: Konstantin
- From: Moscow
Sent 23 October 2018 – 22:49
Maria, good evening!
If the corals have not “flyed around”, they have simply lost color, I would increase water changes, for example from 10% once a week to 20%.
There was a similar situation with the “plate” montipores; substitutions helped.
Yulia’s advice helped (many thanks to her!)
https://reefcentral. vaia-montipora/
- nnf200 likes this
#9 vaniamix
- My name is Ivan
- From: Moscow
Sent 24 October 2018 – 19:36
