02 and corals bleaching

[elegance coral]

I am trying to understand coral bleaching. Is there a connection between temp and lighting? I had assumed that it was 02 that caused them to bleach, but 02 levels drop as temp increases, so why do corals bleach if temp gets to high? I have always thought that corals bleach because the algae produce to much 02 under bright lights. If this is true, why do they bleach under hight temp situations where there should be less 02 in the water? I would think there is some connection between the high temps and bright lights, but I can't figure out what it is? I hope someone here could help me out with this.
Thanks Darrell

[elegance coral]

Can algae produce more 02 at higher temps? Can 02 be more damaging at higher temps, and if so, why?

[Ninong]

Corals bleach (expel their zooxanthellae) in response to extreme stress, usually in the form of rising water temperature.

From AIMS (http://www.aims.gov.au/pages/research/coral-bleaching/1997-98-mbe/mbe-00.html):

Mass bleaching can occur when sea-surface temperatures (SST) rise well above the average for summer and this stresses corals. Frequently the warmer temperatures coincide with more solar radiation, particularly during very calm periods. Bleaching also occurs during extreme low tides or heavy freshwater runoff onto reefs. This time, bleaching has been seen both in shallow water down to very deep water, for example down to 50 m deep.

There are some correlations between the widespread coral bleaching in 1997-1998 and one of the strongest El Nino events of this century, but the patterns are unclear with many exceptions. The correlation exists for the east Pacific, but the bleaching in southeast Asia coincides with a strong La Nina (the complete reverse of El Nino), and the bleaching in the Indian Ocean and parts of the Caribbean do correlate with either El Nino or La Nina.

Follow the link above for the full article and many other articles on coral bleaching.

[wtac]

It's not so much O2 being a problem but the production of superoxides from photosynthesis. The enzyme peroxide dismutase is hampered from "neutralizing" superoxides as high temperature does affect it's activity. Thus the coral expels zooxanthellae to minimize cellular damage from photosynthetic produced superoxides.

JM2C

[hahnmeister]

Corals can start to bleach/RTN from low calcium and alkalinity levels as well. They also can from 'light shock' getting too much light. Sure, the extra light could be used to create more 02, but my understanding is that its a bit like trying to put jet fuel in a car... its just too much. I mean, more power is usually seen as a good thing, but even in chemotherapy or sun tanning, too much can kill you. I would imagine that the light increases the coral's temps beyond that of the surrounding water due to radiation... just like sitting in the sun on an otherwise cool day can make you sweat.

[elegance coral]

<a href=showthread.php?s=&postid=10229919#post10229919 target=_blank>Originally posted</a> by wtac
It's not so much O2 being a problem but the production of superoxides from photosynthesis. The enzyme peroxide dismutase is hampered from "neutralizing" superoxides as high temperature does affect it's activity. Thus the coral expels zooxanthellae to minimize cellular damage from photosynthetic produced superoxides.

JM2C

I hope you're still reading this and you can help me understand it. Am I correct in assuming that the coral has the ability to neutralize superoxides to an extent? The superoxides can be produced by the algae under bright lights at rates greater than the coral can neutralize causing them to bleach. Heat hampers the corals ability to neutralize superoxides so we end up with the same effect under high temp situations. Is this close to correct?

[wtac]

Hahn: that is true as well. Low Ca and alk can contribute as well as the lag time required for the coral to adjust from a low to a high light intensity environment.

I'm kind of like that w/alcohol ;). If I haven't had a drink in a few weeks, a beer gets me nice and toasty. If I have had a beer or two everyday after work during the week, the weekend I can have 3-4 beers before I get the same buzz...LOL.

EC: You got it :).

[Ninong]

<a href=showthread.php?s=&postid=10223949#post10223949 target=_blank>Originally posted</a> by elegance coral
I am trying to understand coral bleaching. Is there a connection between temp and lighting?
Yes, there is a connection between elevated water temperature and UV-B radiation. Exposure to elevated temperature and UV-B radiation causes a depression of photosynthesis (Lesser 1996).

The coral is able to protect its zooxanthellae from elevated UV-B radiation provided the water temperature does not exceed normal maximum levels. Once the water temperature gets around 31 degrees C, the coral can no longer protect its zooxanthellae.

Various hypotheses have been advanced to explain the cellular mechanism of bleaching when extreme sea temperatures are one of the causative factors. Bleaching can also be caused by other factors, such as heavy rainfall, sedimentation, etc.

High temperature and irradiance stressors have been implicated in the disruption of enzyme systems in zooxanthellae that offer protection against oxygen toxicity. Photosynthesis pathways in zooxanthellae are impaired at temperatures above 30 degrees C, this effect could activate the disassociation of coral / algal symbiosis. Low- or high-temperature shocks results in zooxanthellae loss as a result of cell adhesion dysfunction. This involves the detachment of cnidarian endodermal cells with their zooxanthellae and the eventual expulsion of both cell types.

It has also been hypothesized that bleaching in some cases is an adaptive mechanism which allows the coral to be repopulated with a different type of zooxanthellae, possibly conferring greater stress resistance.

[elegance coral]

Thank You all!!!!!!! I think I got it now. You have been a great help.

[Dell2go]

Cool stuff.