Other factors in bleaching

[futrtrubl]

Yes heat is the big one and you'll probably never see bleaching because of one of the other factors, but together with heat or other factors these can be the difference between recovery and death. Besides, heat as a cause has been dealt with by others.
A researcher came down recently to do a study on a possible mechanism for bleaching and the role of light. After seeing the results I asked if I could post them to this forum, he enthusiastically said yes and gave me a paper/poster I could post for all to see. download 159kb
In summary the proposed mechanism is that zooxanthellae produce hydrogen peroxide as a normal part of photosynthesis and the rate increases with increasing light levels. This hydrogen peroxide interferes with a calcium exclusion pump in cells and an increased Ca+ concentration in the cells causes the expulsion of the algae. Heat also interferes with the pump so in the presence of light the pump can break down at lower temperatures.

What does this mean for aquarists? I know most people turn off their lights to control their water temperatures if it rises too much, Ubut this shows that it is doubly imperative to turn lights down or off if temperatures start to rise. By my estimation turning off the lights can give you an extra ~3C/5.5F head room (~33C in light to 36C in the dark for complete fragmentation in 2 hours. These raw temperatures are valid only for his experiment and shouldn’t be used for your aquarium, different coral/zooxanthellae combinations have different tolerances) on your water temperature as well as help control your water temperature.

But what about the other factors he lists; cold shock, increased UV, vibrios, reduced salinity and oxygen stress?
Cold Shock: I wasn’t able to find an abstract for Muscatine et al’s paper, but what I get from those that cited him is that if the coral is exposed to colder water then returned to the original or higher temperature in a short period the coral may bleach. I don’t know what temperature regimen we are looking at here but watch out for this with any emergency cooling measures you take (iced bottles, transfer to a cooled container etc).
Increased UV: I think many know about this already, but it’s worth stating. Fix those UV lenses!
Vibrios: No worries, your corals are safe from the sound of your pumps. Vibrios are a group of rod shaped bacteria which includes cholera. Kushmaro et al found that all and only bleached areas of a certain coral were infected with a certain bacteria and when healthy corals were injected with the bacterium they became bleached. Don’t really have a recommendation here, though a UV sterilizer may help.
Reduced Salinity: I very much doubt anyone will dump uncontained ice in their tanks to drop temperatures, but this is something to think about if when it comes to hyposaline treatments. Not sure how reduced it has to be for effects.
Oxygen Stress: We’re coming back to the peroxide thing again. But things that can create “reduced oxygen intermediates� (superoxide, peroxide, or the hydroxyl radical) may warrant some attention. I’ve never looked into getting an ozone system so I don’t know if there is a tolerance for some or no ozone in the tank itself though I do know that ROIs (my own acronym, gotta have one ;’] ) are quickly reduced to harmless forms in water. Also UV can produce free radicals but again, they may not persist long enough. But there is good news! I just saved… ehem… peroxide only becomes harmful if it builds up in the coral faster than it can be gotten rid of, which it can do through too methods. First it has a suite of enzymes designed to reduce peroxide harmlessly. The second is the simple diffusion of the peroxide out of the polyp. This can be increased with increased water flow over the coral.

Help! My corals are bleaching/may bleach! What can I do?
- First off, you probably need to get those temps back down (it is the most common reason after all) just step away from the fire extinguisher and ice tray (unless you plan to contain it).
- If your temps are fine check your lights for UV shield problems.
- For extra insurance or if you think things are bad enough turn off/down those lights.
- If your corals can take it you can increase water flow over them to take away the peroxide.
- If you are a natural sea water kind of guy (I am) you may want keep off the stuff for a while as it may introduce vibrios (they are exclusively aquatic and many are salt loving).
- I don’t really think I have enough evidence to recommend turning off ozone systems.
- If your corals have bleached you may notice them extend feeding tentacles at times they normally don’t. This is because they have lost a food source, the primary one for many hermatypic (reef building) corals, so one of the things you can do for them is step up any feeding regime you have for them. In the wild some corals can recover after being bleached for 2 months, probably due to filter feeding.

FITT, W.K. & M.E. WARNER (1995).
Patterns of bleaching of four species of Caribbean corals. Biological Bulletin,189: 298-307. download 896kb
Goreau, Thomas F. (1964)
Mass Expulsion of Zooxanthellae from Jamaican Reef Communities after Hurricane Flora. Science, 145(3630):383-386 abstract
Jokiel, P. L. (1979)
Solar Ultraviolet Radiation and Coral Reef Epifauna. Science, 207:1069
Kushmaro, A; Loya, Y; Fine, M; Rosenberg, E (1996)
Bacterial infection and coral bleaching. Nature, 380(6573):396 abstract
Lesser, M. P. (1997)
Oxidative stress causes coral bleaching during exposure to elevated temperatures. Coral Reefs, 16(3):187-192 abstract
Muscatine, L., D. Grossman, and J. Doino (1991)
Release of symbiotic algae by tropical sea anemones and corals after cold shock. Marine Ecology Progress Series, 77: 233-243.
SANDEMAN, I. (2006)
What Happens When Corals Bleach: A New Perspective. Download 159kb

Edward

[Sk8r]

Excellent post!

[jdieck]

Great post I hope a mod chims in and makes it a sticky.

[mhurley]

Great information!

[Sk8r]

bump

[futrtrubl]

Thanks for the comments guys. Please feel free to post disagreements, clarifications and anything you think I missed.

Edward

[futrtrubl]

Oh, I should mention that in the article on oxidative stress the author added anti-oxidants to the water which reduced the bleach rate of the coral... not that I'm saying you should add vitamin C to your tanks though ;']

Edward

[affan]

What about stressed corals giving algae fewer nutrients and thereby forcing the algae to expel itself from the polyp? I've also read that when in stress this algae produces oxide toxicity, and these toxins are extremely unhealthy for the polyps.

[futrtrubl]

From everything I've read it's the coral expelling the algae and not the algae itself leaving. There may be a situations where stressed algae produce peroxide but I don't know of any.
Sandeman's paper also reports that the algae in corals produce much less peroxide than similar non-symbiotic species, indicating that there is probably selection for low peroxide strains.

Edward

[HippieSmell]

Nothing to add, I just think this should be a sticky somewhere. I've read a lot of posts and have never seen this info. Thanks.

[futrtrubl]

Unrelated to coral bleaching, but yesterday I took some Halimeda out of a touch tank to show some students. When I came back an hour later both spp. were almost completely white. I got distracted and when I came back about 8 hours later they were back to being a healthy green.
Well this is actually somewhat related to coral bleaching. When the coral expells the algae, it doesn't necessarily kick it out of the polyp completely, the algae can float around and re-enter the polyp elsewhere. If conditions are just as bad in the new loacation it will be chucked right back out, but if conditions are ok it will stay there. In bleaching that has to do with light (perhaps with heat as well) often you'll find that all the algae has migrated down into the skelleton of the coral where they recieve less light. These can rapidly recolonise the upper portions once conditions return to normal.


Edward

[John Kelly]

Here is a good downloadable article by M. P. Lesser from 1996:
Elevated temperatures and ultraviolet radiation cause oxidative stress and inhibit photosynthesis in symbiotic dinoflagellates

This year, he had an article published in the Annual Review of Physiology, vol 68, entitled:
OXIDATIVE STRESS IN MARINE ENVIRONMENTS:
Biochemistry and Physiological Ecology

If Discovery Bay Marine Lab has an account with http://annualreviews.org, you may be able to download it. It is a super article that pretty much sums up the past and current understanding of reduced oxygen intermediates, which cause oxidative stress.

[futrtrubl]

Thanks! I'll be looking into that.

Edward

[reefshadow]

yesterday I took some Halimeda out of a touch tank to show some students. When I came back an hour later both spp. were almost completely white. I got distracted and when I came back about 8 hours later they were back to being a healthy green.

i have a large growth of halimeda in one corner of my display. it turns white every night and regains all the colour shortly after the lights come on. weird.

re the bleaching of sps, that is some pretty cool info.

what about pests; ie red bugs, flatworms, etc...

[boxfishpooalot]

Good information!

IIRC infared light can increase the surface temperature of the coral signifigantly higher than the surrounding water, causing it to bleach. Increasing the flow of water decreased the surface temperature of the coral. Low water flow caused the coral to raise in temperature resulting in bleaching.

Maybee its not infrared light, well watever light is responisible for heat , espcially light from metal halide lighting was given emphasis. Maybee uv light?

I also think this is a good sticky.

[jdieck]

Originally posted by reefshadow
i have a large growth of halimeda in one corner of my display. it turns white every night and regains all the colour shortly after the lights come on. weird.

I read somewhere that in halimeda the pigment (chlorophyl) migrates to the inside and back to the surface of the leaves with the light cycle.

[futrtrubl]

Drew, E.A. & K.M. Abel. 1990. Studies on Halimeda III, A daily cycle of chloroplast migration within segments. Botanica Marina, 33, 31-45.
Sorry, no link to an abstract. I just witnessed this last night too.

boxfishpooalot: While absorption of radiant energy could well increase the temperature of the coral itself it wouldn't be enough to account for the difference in bleaching temperatures in light and dark.
Even so, if you have high water temps then it is another good reason to turn off/down lights or increase water flow. Thanks for pointing that out.

Edward

[boxfishpooalot]

water was at 22c and the coral was at 23.75c(71.6f and 74.75f) as a maximum value reported in this study, at slowed water flow. That is a diffence of 3.15 F raised on the corals surface.

If we had a temperature of 85f(already a stressfull surrounding water temp) and raised it by 3.15f(the maximum value reported), that would be a temperature of 88.15 f on the coral.

Is that not going to contribute to bleaching corals? Can you explain this?

Here was the experiment, thanks Box
http://www.advancedaquarist.com/2006...20light%20heat

[John Kelly]

I think first and foremost that light by itself can and will cause bleaching in corals regardless of the surrounding water temperature or coral body temperature. It depends on the sensitivity and exposure of the certain zooxanthellae species to light and the excitation that occurs within the zoox itself. It is the excitation that raises the metabolic rate and temperature within the zoox. Increased water temperature would create the same bleaching reaction (as seen in the oceans), but the cause is from a different source (water temperature working from outside of the coral vs. light energy working within the zoox). Corals obviously "color up" as a reaction to increased light, but I think it is a STRESS reaction to protect the coral tissue and zooxanthellae and not a sign of increased health. If you observe and study corals closely, you will usually find that bright coloration and brown zooxanthellae manifest themselves in opposition of each other, not with each other. A brightly colored coral may actually be "bleached" of most or all zooxanthellae and when the consequences of having little or no zooxanthellae begins to effect the coral physiologically, it eventually loses the bright coloration, turns white, and dies.

Sometimes I think the only thing that keeps some people's SPS corals alive in their overlit "reef" tank is the fact that the skeleton of the coral casts enough of a shadow for some of the zooxanthellae to survive in. With many LPS, there is none or little shadow created by a skeleton; therefore, because the zooxanthellae have no place to hide, LPS are more sensitive to lighting than SPS (generally speaking).

[futrtrubl]

boxfishpooalot: I'm not saying that local temp spikes don't happen, don't contribute to bleaching or aren't significant. I'm just saying that the temperature increase due to lighting will not be enough to fully account for the difference in bleaching temperatures between lit and unlit corals (~3C/5.5F by my estimation) I have updated my document to include your info. Just wish I could edit the first post...
John Kelly: Damn, I knew I forgot to mention some stuff! Yes, increased coral (not zooxanthellic) pigmentation is a stress responce to too much light, as the coral tries to shade the zooxanthellae. Note that this responce will not cool the coral in any way as it will still absorb the same amount of radiation, or more. Another thing to note is that increased deposition of the calcium carbonate skelleton is not always a sign of a healthy coral as it is also a way for the coral to reduce intracellular calcium concentrations (the cause of bleaching) under near bleaching conditions.

Edward

[futrtrubl]

reefshadow: Damn, I completely missed your question about pests. Unfortunately I have no information on how pests affect coral bleaching. And while we have exteme amounts of the small red/brown flatworms in or display we haven't had any bleaching. Red bugs I've never experienced. One thing that would be interesting to look into would be inter-coral chemical warfare, but that doesn't seem to affect bleaching either.

Edward