Coral bleaching

[Mr31415]

I asked about this before, but I would now really like to get some pointers to good articles on the subject of coral bleaching, in specific I want to understand the factors that can cause corals to bleach, the duration those factors have to be present before bleaching starts and how it gets reversed, if at all.

 

[greenbean36191]

There are lots of good reviews in the scientific literature, but I don't know of any articles written for the general public that really address the issue well IMO. Try to get ahold of these:

Douglas, A.E. 2003. Coral bleaching- how and why? Mar. Pol. Bull. 46:385-392.

Brown, B.E. 1997. Coral bleaching: causes and consequences. Coral Reefs. 16: 129-138.

West, J.M. and R.V. Salm. 2003. Resistance and resilience to coral bleaching: Implications for coral reef conservation and management. Conserv. Biol. 17:956-967.

Coles, S.L. and B.E. Brown. 2003. Coral bleaching-capacity for acclimatization and adaptation. Adv. Mar. Biol. 46:183-223.

Some general answers to your questions are that bleaching is a general stress response that can be caused by many types of environmental stress and different mechanisms, none of which are well understood. The duration it takes to cause bleaching depends on the magnitude of the stressor. If the stressor goes away the coral can recover by the remaining zoox repopulating the tissue or taking in new zoox from the water. If you have more specific questions I might be able to answer them or at least help you find the answer.

 

[m2434]

greenbean36191, I have a somewhat related question for you... It's not really my field, but I've been thinking a little about gene regulation, in corals, in response to environmental stressors (hence the relation to the bleaching topic . Do you know of any good microarray studies (preferably with publicly available data)? Proteomic studies would also be interesting, but I have a stronger background in genomics and transcriptomics... so this is probably a better place to start.

Thanks -
m2434

 

[dwd5813]

NOAA has some interesting info on bleaching events.

http://www.coris.noaa.gov/activities/reef_managers_guide/

 

[Mr31415]

greenbean36191 - Thanks for the references.

Basically I am confused as to the cause of the bleaching of my corals recently. About 2 months ago I upgraded my lighting from 8 x T5 to 4 x T5 + 2 x 250W MH. I did not start the pendant from a higher position - I immediately added the extra light. I furthermore removed the glass cover plates so even more light reached the coral.

Everything was doing fine for about 2 months. I then started dosing Kent Tech-M to raise my magnesium to 1740mg/L (TWallace's method of killing Bryopsis). Well, it worked. Very well. All my bryopsis are 100% gone. However 3 days in to the process I noticed my colt coral started to bleach. This was followed by another soft coral, then some of my button polyps, then my galaxy, then my one Euphyllia (the other one is fine), then lastly my Porites with christmas worms.

Now normally I'd assume it was due to the magnesium supplement, however my Porites has only bleached on the surface areas perpendicular to the light. The vertical sides of the porites has not bleached at all. Now if it was solely due to the magnesium supplement, then surely the whole porites would have bleached?

So my question is - what caused the Porites (and other coral) then to bleach?

 

[m2434]

Changes in lighting are notorious for causing problems. The fact that it took 2 months is a little strange, but not really... Some people say that you should decrease the lighting duration after adding new lights. This will help corals adapt. It might help with your SPS corals, although never did anything useful in my situation **. Also, I would definitely look into a UV shield. If your Mhs are double-ended, I'm told this is necessary. Mine where single ended, but I still think I should have had them.

** Background info - My situation:
I found that some corals would lived fine under a 250MH for months and even grew well, but all of a sudden for no reason, they would start bleaching and dieing. I'd move them lower in the tank and they'd come back. But when I tried to move them up again they would bleach again... Occasionally they'd do okay for a month or so and then bleach again. With some such as zoas, I'd observe the polyps get smaller and smaller until they disappeared, almost like they were starving...

Whats really odd is that I finally gave up and switched to PC lights and the same corals are doing great. Within 3 months or so, they have grown and average of 3x their size. In the 4-5 years I had most of them under the MH's they lived, but never grew.... Granted these are things like zoas palythoa, an LT anemone, etc... but still these corals are soft corals, that are supposed to like fairly high levels of light....

My thinking is that the issue was something to do with the UV light produced by the MHs. I did notice when they started bleaching, their tissue would start to flake off, like a sunburn. Maybe the constant sunburning just stressed them out too much or some light protective nutrient was being used up. The other possibility, is that the zooxanthellae, grew to fast and poisoned the corals with excess byproducts. (This is actually the exact subject that sparked my interest in gene regulation).

 

[greenbean36191]

Do you know of any good microarray studies (preferably with publicly available data)? Proteomic studies would also be interesting, but I have a stronger background in genomics and transcriptomics... so this is probably a better place to start.

Wow, that's a great question and I don't have a good answer. I'm more focused on the interspecies to community levels so I don't really know a whole lot about what's going on in the genomics world other than people are working with corals and yes, they are using microarrays. I don't know of any studies specifically looking at changes in gene expression associated with bleaching/stress though. A google scholar search would probably be more helpful than I am, since I have no clue who, if anyone is working on that problem.

So my question is - what caused the Porites (and other coral) then to bleach?

It was probably a combination of the light and the supplement. There are a lot of reports of corals bleaching only on the sides that get the sun or of corals in shaded areas surviving bleaching events that killed neighbors. As it turns out, excessive light can magnify the effects of other stressors. Toxins such as heavy metals are known to be stressors that can contribute to bleaching and I suspect that in the process of getting your magnesium that high you also ended up overdosing one of the other metals that are in Tech-M.