Light Intensity vs. Light Duration

[silverwolf72]

Can you get away with less light if the photo period is longer? What effect would it have on corals? If longer light periods are an acceptable replacement for higher intensity would this not be a better way to go? (less heat generated, less radiation produced, less expensive equipment)

 

[demonsp]

No. Increasing light time wont help but keeping water quality high would help best.

 

[silverwolf72]

From reading several articles it seems that corals are able to photosynthesize at a lot lower lvls than the amount full day sun that most aquariums simulate. By lengthening the exposure to light they would still be able to produce adequate food for themselves.

 

[MCsaxmaster]

Re: Light Intensity vs. Light Duration

<a href=showthread.php?s=&postid=12060936#post12060936 target=_blank>Originally posted</a> by silverwolf72 Can you get away with less light if the photo period is longer?

It depends on what you mean by "get away with." If you mean, "Can a coral end up with the same net amount of photosynthate with a long photoperiod of low intensity light compared to a short photoperiod of high intensity light (assuming similar total amount of light when integrated over the photoperiod)?" then the answer is a resounding yes.

Over the lower end of the light intensity corals see in nature (e.g., what they see in the early morning and the late afternoon) the rate or photosynthesis varies linearly with light intensity. As light intensity becomes saturating the rate of photosynthesis levels off and an increase in light intensity doesn't result in an increase in light intensity. See a typical P/E curve below:

http://www.okstate.edu/artsci/botany/bisc3034/lnotes/lightsat.jpg

If we are on the lower end of the curve, below saturating intensity, then an change in light will result in a proportional change in photosynthesis, although exactly how much depends on the organism an the light intensity to which it is adapted. In other words, it isn't a 1:1 relationship.

For example, increasing light intensity from 50 uE/m2/s to 100 uE/m2/s on a shade-adapted coral might double the rate of photosynthesis whereas in a light-adapted coral it might only increase the rate 20%. It depends on what the organism is adapted to, and hence where photosynthesis becomes light saturated for that particular critter.

While it is theoretically inescapable that one could achieve the same net daily photosynthesis by using a long photoperiod of low intensity as a shorter photoperiod of high intensity, without doing physiological experimentation, one doesn't have a good way of knowing how much longer the photoperiod needs to be for a given reduction of light intensity.

As above, by reducing the light intensity from 100 umol/m2/s to 50 umol/m2/s we would need to double the photoperiod for the shade-adapted coral but quintuple it for the light-adapted coral to achieve the same daily net photosynthesis.

What effect would it have on corals?

Depends

If longer light periods are an acceptable replacement for higher intensity would this not be a better way to go? (less heat generated, less radiation produced, less expensive equipment)

Possibly, but to really be sure of what we're getting requires a lot of work and physiological experimentation.

In general I think it best to shoot for somewhere around the middle of the range for what we might think is acceptable light intensity for the coral: if we are either on the high end or low end we have a reduce margin of error.

A photoperiod of 10-12 hrs is a good starting point I think. One can certainly go longer if one wishes, but I'd do so cautiously and watch the corals to ensure that they are not responding negatively. Perhaps increase photoperiod an hour a week and make sure everything continues to look well--back down if something starts to look 'off.'

Moya et al., 2006 looked to see if there was a diel cycle of calcification in corals. One of the ways they did this was by varying the photoperiod the corals were exposed to. At a light intensity of 175 uE/m2/s, which was enough to saturate calcification and easly obtained using metal halides or T5's, the corals experienced the highest net rates of calcification after the longest photoperiod they examined (20 hrs). They only did this for a couple of days, so it's not clear the corals would have remained "happy" under this photoperiod continuously, but for a couple of days they did just fine.

Chris

 

[Poorcollegereef]

Should I toss out the idea of using very strong lighting for a shorter time? oops, I guess I just did. Anyways, MC: Great answer!

 

[MCsaxmaster]

Actually, short photoperiods of very high light intensity don't work well for photosynthetic critters. As you can see in the P/E curve above, eventually photosynthesis becomes light saturated. For many corals, even those adapted to very strong light, we can easily attain saturating light intensities in captivity. If, for example, a coral adapted to strong light becomes saturated at 400 uE/m2/s then bumping the light intensity to, say, 600 uE/m2/s won't increase the rate of photosynthesis--that will remain essentially unchanged. If we shorten the photoperiod, the net daily production will decline. The increased light intensity will do quite literally no good at all since the photochemistry is already operating at its maximum and is not wanting for light.

And in fact, if we raise the light intensity enough the corals will begin to incurr photoinhibition. The capture of energy for photosynthesis is an inherently "dangerous" process. It results directly in the production of reactive oxygen species (ROS) and other nasties. All photosynthetic critters have physiological mechanisms to deal with normal levels of ROS (if they didn't they would be oxidized to death at the dimmest flash of light), but those mechanisms only go so far. Eventually these systems become overwhelmed at high light intensities. For, you see, light absorption continues to go up essentially linearly with light intensity, but the proportion of the light that can actually be used for photochemistry levels off. Thus, as the light intensity climbs the corals (or whatever photosynthesizer) is left with more and more excess light energy which it has absorbed and has to get rid of somehow. When the natural means of dumping light (e.g., xanthophyll cycle) are overwhelmed the production of ROS shoots through the roof and starts burning up the very photosynthetic apparati we want to be stimulating.

Thus, at some point above the saturation intensity the light intensity becomes strong enough to actually cause photoinhibition and thereby reduce the rate of photosynthesis relative to the rate at a lower light intensity. Using a short photoperiod of very high intensity is a double-whammy: not only is the photoperiod shortened, and hence net production reduced, the rate of photosynthesis is lower than near the saturation intensity. The effect would be similar to reducing the light intensity while simultaneously reducing the photoperiod, at least in terms of the effects on net primary production.

Light intensities in the neighborhood of what might be saturating and a photoperiod of 10-12 hrs a day is the place to start, I would suggest.

p.s. Thanks for the kind words

 

[Agu]

Chris, Great answers !

I have observed in some LPS corals, primarily euphyllia, that polyp extension is also a function of light cycle. This has been corroborated by other anecdotal comments on RC. The corals show full polyp extension fron 1/4 to 3/4 through the light cycle (12 hrs in my case) and then the polyps start to retract as the end of the light cycle nears.

Would you care to comment on this phenomena in relation to light cycles/intensity ?

 

[Poorcollegereef]

Awesome, thank you MC. I sometimes feel obligated to ask the inverse of the original question. Let it be knowm that it was better than todays discussion/lecture about mudflats and Redox potential discontinuity layer. By the way, what are you concentrating in at uncwil?

 

[MCsaxmaster]

<a href=showthread.php?s=&postid=12072083#post12072083 target=_blank>Originally posted</a> by Agu
Chris, Great answers !

I have observed in some LPS corals, primarily euphyllia, that polyp extension is also a function of light cycle. This has been corroborated by other anecdotal comments on RC. The corals show full polyp extension fron 1/4 to 3/4 through the light cycle (12 hrs in my case) and then the polyps start to retract as the end of the light cycle nears.

Would you care to comment on this phenomena in relation to light cycles/intensity ?

Nope...






















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I don't know why they do that and without some serious study I think haphazard guessing is fairly useless. I used to have some Xenia that seemed to "know" when 8 pm was. They would close up everyday at 8 pm for about 1/2 an hour and would reopen again before lights off (10 pm). I'm not sure why they did it, but they did it everyday for months.

Corals do have mechanisms to keep track of time, just as most organisms do, but not much is known about these mechanisms. They simply aren't entirely easy to study, and not a lot of work has been done yet.

I will relate one story of my own though. Two years ago I was working on the Acropora palmata spawn in Puerto Rico. We had two sites with A. palmata where we intended to collect gametes. We currently understand enough about the spawn cycle to predict a period of ~5 days when it is likely to occur, and with past experience we can usually predict the time of the day for the spawning.

On one of the first nights we went out to our site in Rincon my advisor and a collegue were discussing the likely timing of the spawn.

"Around 8:30," he said.
"Really, that early," my advisor asked, "they spawn on Margerita around 9:00?"
"No, no, not that late--8:30, 8:45 here."

I half thought they were joking, but when the corals did spawn a couple of nights later they started at ~8:35 pm and the whole thing was over before 9:00 pm.

I don't know how the corals keep track of time for things like that, but nonetheless they do it. As with other organisms, there are probably a number of interacting zeitgebers or zeitgebers that are affected differently by the same stimuli. I don't know how they do it, but they do it.

 

[MCsaxmaster]

<a href=showthread.php?s=&postid=12073051#post12073051 target=_blank>Originally posted</a> by Poorcollegereef
Awesome, thank you MC. I sometimes feel obligated to ask the inverse of the original question. Let it be knowm that it was better than todays discussion/lecture about mudflats and Redox potential discontinuity layer. By the way, what are you concentrating in at uncwil?

Ha, thanks for the kind words

I'm working toward my MS in marine biology here. My work is on the effects of ocean acidification on coral calcification with reference to the likely mechanism of calcification. I'm working not only to understand how acidification ultimately affects calcification, but also some of the underlying mechanisms by which alterations in SW chemistry affect calcification.

Chris