Excess Light Doesn't Cause Coral Bleaching

I'm in the mood for a good debate, so there you have my opinion. I've thought it over and done a lot of observation on the matter, and I think this is yet another aquarium myth that's ready to be proven wrong.

Have at it!

 

Excessive light of a certain wavelenght? such as UV ?
Im sure I have seen more than 1 account of corals expelling Zooxanthellia as a result of failing to acclimatise to new lighting adequately

many of these accounts are related to upgrades to stronger lighting option
or changing existing type of lamps to ones with stronger light output

seen so many of these
and experienced a couple myself

possibly coincental or that there in fact are other aspects in play, that you cannot measure accurately , and as such the changes in the lighting simply encouraged the Zoo exodus , but may not have created it, in its own right

I think you mean a gradual increase to intense lighting does not cause bleaching?
or "you cannot in fact provide to much light "

its the speed of acclimatisation, that IMO can be the deciding factor with relation to a coral bleaching

Steve

 

Coral Reef Bleaching

 

Generally temperature is the cause of bleaching, however light <400nm, alone, can cause coral bleaching. Light <400nm or >=400nm can cause photoinhibition. This is a net reduction in photosynthesis, but does not necessarily result in bleaching. Efficient photosynthesis will result in oxidation stress though, which combined with other factors, such as insufficient gas exchange, reduced superoxide dismutase activity, insufficient fluorescent Protein to quench radicals etc... can lower the thermal threshold for coral bleaching. In other words, this is generally a combination of synergistic effects. Photoinhibition in itself, for example, reduces oxidative stress, as photosythethis is reduced. So, more factors are generally required. Of course in our systems, there are inherently a lot of extra stressors, compared to the ocean, so, light may be more correlated with bleaching than in nature...

 

As I understand it, corals are equipped with the ability to expel excess dynoflaglets when food production from photosynthesis is more than needed. This natural reaction results in damage our death when the coral is stressed from other conditions such as temperature which seemed to be supported in all of your responses.

The thing that gets me is not really the specifics of why it happens, but the amount of people out there saying you can't run your leds at full power or your corals will bleach and that you need to light acclimate all new corals. I look at it like a three strikes rule. Your corals and other animals can sustain having two system weaknesses against them, but stack one more on the pile, and you will have a problem. It would seem to me that if excess light causes your coral to bleach, it is a symptom of an unknown weakness in your system.

I didn't have time to clarify what I meant by my observations earlier, so I'll add that on now. I maintain several large coral flats with 400 watt halides. All frags are a couple inches under the surface of the water, and after the several thousand corals moved instantly in to these conditions, I've yet to notice a trend of bleaching unless otherwise linked to additional poor conditions. I've also gained quite a bit of experience placing corals directly under leds such as ai run at full power with no bleaching trend.

This is a subject that I find more and more interesting every time I hear it recommended not to run sols at full power.

Michael Rice
Marine Engineers Aquarium Blog
Sent from Tapatalk so excuse my poor English

 

Actually it's a survival reaction caused by the production of excess oxygen as a result of over-photosynthesizing by the zoanthellae, which is bleaching. Without the other stress factors, the coral can in fact recover from it.

 

Man, I just typed up this whole thing and then I accidently hit back. Now it is gone. Do I feel like typing that up again? Should I be studying? Yes. Oh well I'll do my best to say it again...

From what I have read, I think the question here is not what the high lighting does to the coral, but what is does to harm the zooxanthellae. The high lighting disrupts the function of the symbiotic zooxanthellae - which harms the cells of the coral due to the toxic agents mentioned by m2434. The coral must get rid of these harmful zooxanthellae and has been observed to do so in two ways.

First, the corals cells undergo apoptosis. They basically kill their cells that have been harmed. They do so in order to remove zooxanthellae that are no longer functioning properly which will prevent further tissue damage. Another reason they may do this is part of their innate, or initial, immune response. They phagocytize, or eat and breakdown, the zooxanthellae that are now recognized are recognized as their enemy. We do the same thing. Many of our white blood cells are phagocytes, which literally ingest toxins like bacteria in order to remove them. This breakdown itself can result in bleaching.

Another related potential cause is that the coral, without the zooxanthellae is in starvation mode, because it is no longer receiving the proper nutrients that it once received. This initiates a process called autophagy. The coral is basically eating its own cells in order to survive. We do this too. Our bodies eat our fat and eventually muscle when we are starving to death. The cells that were once there causing the beautiful colors we once saw, are no longer there because they have been broken down into nutrients for the coral.

 

As to mechanisms of bleaching, I think there is even still some debate whether or not it is host or symboint initiated. However, it's safe to say corals can make the environment less habitable for the symboints. They can limit access to certain nutrients (presumably if they are not membrane permeable and therefore diffusion limited). Also, they can produce pigment to block light. Probably other mechanisms as well. Without these mechanisms, the zoox would grow wild. When we start talking about LEDs, the problem doesn't seem to be intensity so much (PAR), but rather spectrum. The ocean may get murky and certain wavelengths may get filtered out (I.e dimmer), but wavelengths don't exactly "increase" in intensity. The sun puts out a certain spectral range at a consistent intensity and the atmosphere, water etc... Filter some out in a predictable manner. The fundamental properties of the light dosn't change per say. When you sworn lighting sources it does and it therefore takes time for the animal to adjust it, photoprotective mechanisms in response. Also, I brouht up GFP, it's recently been shown GFP quenches O2 radicals, protecting the coral from it's effects. If photosynthesis runs wild because pigments arn't in place and this mechanism isn't in place, then that's sort of the double wammy your talking about. The change spectrum Is the additional stressor.


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Gabbyr189, sorry I think you posted as I was typing. I don't have good reception, but I'll try to quickly mention a few thing. The accumulation of radicals is harmful to the coral and symboint. Both have various mechanisms to deal with them. As to apoptosis and autophagy, I think that was observed in aptaisia. It's my understanding that There is a lot more to the story though that is still being worked out.


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Excess light causes higher levels of photosynthesis in the zooxanthellae which causes them to produce toxic amounts of oxygen. It is poisonous to the coral. The coral getting rid of its own gastrodermal cells is thought to be one of the reasons they can die after bleaching. They just don't have enough of these cells to function.
And I believe that the whole GFP thing is very species selective. I'll have to check on that though, I haven't read too much about it yet. More info on GFP would be great!