Photoinhibition and Temperature

[bristle]

I'm not an expert, but I have a bit of experience in animal and plant physiology, and this came up in a lecture this afternoon.

Photoinhibition is caused by two main factors:

- Too much sunlight
- Cold temperatures

What if we were to increase the temperature of our aquariums from the standard 78F to somewhere around 83 or 84? This is within a tolerable range and I have been keeping my aquarium at these temperatures for about 8 months now.

This might mitigate a bit of the photoinhibition and thus result in longer photosynthetic and thus growth periods.

Comments?

 

[BeanAnimal]

I have run at 82-84 for over 3 years

 

[MCsaxmaster]

Photoinhibition can be caused by a variety of stressors and certainly is not limited to excess light or cold stress. Heat stress can and does cause the same sort of damage, hence why heat stress can and does cause bleaching There are many other stressors that cause photoinhibition though (UV stress, salinity stress, dessication, some herbicides, toxins, etc.).

While I think it is probabably preferable to keep most corals/reef tanks in the low 80's most of the time (as opposed to high 70's), I would not cite this as a reason to do so.

 

[bristle]

In an aquarium, dessication, herbicides, etc. aren't really relevant. Unless something goes horribly wrong, most factors are relatively stable in a well maintained aquarium (some flux with water changes, increasing nutrients).

Salinity stress can be mitigated by use of a top off. I don't know about UV. I know our bulbs emit tons of it but much is captured by the glass shields (how much I cannot say).

I don't know much of the absorptive power of carbon but I hear it thrown around here that it can absorb toxin warfare from corals.

My question is really...can tank at a stable salinity and other physical factors achieve more growth at a higher temperature by prolonging the onset of photoinhibition? I don't know and I was hoping that someone might have done something like this before.

 

[greenbean36191]

Yes, everything else being equal, higher temperatures (with limits) will give slightly faster growth rates. The thermal optima has only been measured for a handful of species, but it's usually in the neighborhood of 82-84 (this will depend on acclimatization). Higher than that doesn't really offer any benefit. This faster growth has nothing to do with photoinhibition though. It's just a matter of kinetics.

It sounds like you're under the false impression that photosynthesis becomes less efficient with time though. This isn't the case. At a given, non-damaging light level and temperature, photosynthesis will continue at the same rate for as long as the light is on.

 

[ReefEnabler]

greenbean, could you shed some light on the topic of "photosynthetic saturation" please????

It's a term I've seen thrown around here ALOT but with very little in the way of science to back it up. Can a coral reach saturation before the lights go out?

 

[BeanAnimal]

Photosynthetic activity increases as light levels increase. However, at some point, increased light does not increase the level of photosynthesis. Beyond this point, photoinhibition may or may not occur.

Think of it this way... different camera sensors (and films) have differnet light sensitivites. At some level of light the sensor (or films) are maxed out. In the case of a camera, the image turns white, it can't get any whiter even if you increase the amount of light shining into it. It may or may not damage the sensor, that is another subject

Bean

 

[MCsaxmaster]

I'm not Mike, but I'll give it a shot

<a href=showthread.php?s=&postid=13365716#post13365716 target=_blank>Originally posted</a> by RyanBrucks
greenbean, could you shed some light on the topic of "photosynthetic saturation" please????

Here's a graphic to help make sense of what I'm talking about:

http://www.petsforum.com/personal/Trevor-Jones/IdealisedPICurve.gif

If a photosynthetic organism is in the dark the rate of photosynthesis will be 0. Without light (or also with extremely dim light) photosynthesis can't occur. Let's say we start to give light to a dark adapted plant. As it starts to get light, it will begin to photosynthesize. At low light intensity we get a linear increase in photosynthesis with an increase in light intensity. As light intensity continues to increase, the rate of photosynthesis eventually starts to level off. Eventually the rate of photosynthesis reaches a roughly constant level despite an increase in light intensity. That is, even though we are increasing light intensity, the rate of photosynthesis is staying the same.

At low light intensity, the organisms have all the machinery and resources they need for photosynthesis, except light to make the process work. Hence, if light intensity is higher, the rate of photosynthesis is higher by a proportional amount. However, eventually we get to a point when the photosynthetic machinery is working as fast as it possibly can. More power in the way of light can't make the machinery work faster, hence photosynthesis becomes light saturated.

<a href=showthread.php?s=&postid=13365716#post13365716 target=_blank>Originally posted</a> by RyanBrucks
It's a term I've seen thrown around here ALOT but with very little in the way of science to back it up. Can a coral reach saturation before the lights go out?

The CRITICALLY important thing to understand here is that, because photosynthetic reactions are incredibly fast (the entire process can be completed thousands of times per second) photoperiod has absolutely NOTHING to do with photosaturation. If the photoperiod in question is longer than tiny fractions of a second (10^-12 s to absorb a photon) what determines whether an organism will reach photosaturation is the light intensity, not photoperiod. Photoperiod is totally and utterly irrelevant here. Hence, a coral will reach photosaturation in a tiny fraction of a second if the light intensity in the tank is high enough. I will NEVER reach photosaturation if the light intensity is too low. However, keep in mind it could be significantly photoinhibited if the light intensity is far too high.

Chris

 

[MCsaxmaster]

<a href=showthread.php?s=&postid=13363765#post13363765 target=_blank>Originally posted</a> by bristle
My question is really...can tank at a stable salinity and other physical factors achieve more growth at a higher temperature by prolonging the onset of photoinhibition?

Within the range you're talking about, no.

Studies performed with tropical corals generally find that the species studied over a wide geographic range attain max. growth rates within the range of 77-86 F with the majority falling in the range 80-83 F. As mentioned above, however, it has nothing to do with avoiding photoinhibition in the low 80's instead of the high 70's. As above, I think it is probably preferable to maintain most tropical corals/reef tanks in the low 80's as opposed to the upper 70's most of the time, but it has nothing to do with the onset of photoinhibition.

In fact, what we might consider "optimal" temperatures for most corals are only ~3-6 F below their upper thermal limits, but at least 10+ F above their lower thermal limits (and up to 30 F above the lower thermal limits for some tropical corals).

Chris

 

[ReefEnabler]

ok thanks for clearing that up Chris!
when I heard the term before I thought it had more to do with photoperiod than light intensity, but it was either A) my lack of understanding or B) somebody throwing around terms too easily. I can see my assumptions were way off but its good to learn something.

 

[MCsaxmaster]

:thumbsup:

No worries. This is a very, very common misunderstanding among aquarists IME. In fact, it is very rare I've encountered anyone in the hobby that has heard or uses the term "photosaturation" and does not attach photoperiod to it. So please, do me and your fellow aquarists a favor and spread the word

 

[BeanAnimal]

well that makes my simple explanation look kinda lame...

 

[bristle]

It makes sense now...

I was forgetting some simple principles photosynthesis and of the electron transport chain...electrons move quickly! So photoinhibition does not occur over the length of a day after saturating itself. It will occur as soon as it has too much light.

Will the threshold for photoinhibition then increase at warmer temperatures? So...photosynthesis will evolve more oxygen in the low 80s and more growth?

 

[BeanAnimal]

The warmer the temperature, the faster the chemical and biological functions work. I would imagine that the faster the coral can process the energy, the more it can take in.

 

[bristle]

To a point, of course .

 

[MCsaxmaster]

<a href=showthread.php?s=&postid=13367558#post13367558 target=_blank>Originally posted</a> by BeanAnimal
well that makes my simple explanation look kinda lame...

Ha, I like your analogy, and think it's a good one

 

[MCsaxmaster]

<a href=showthread.php?s=&postid=13367746#post13367746 target=_blank>Originally posted</a> by bristle
Will the threshold for photoinhibition then increase at warmer temperatures? So...photosynthesis will evolve more oxygen in the low 80s and more growth?

Not in the range we're talking about. If we were looking at something like a temperate alga and comparing photosynthesis at, say, 5 C and 25 C we might see real differences since the physiology will be somewhat slowed down at the low temp, slower to quench PS II and hence may yield more photoinhibition. On the other hand, photosynthetic membranes would be more rigid (less fluid) due to temp effects on the lipids, maintaining proteinrotein interactions and thereby reducing the incidence of photodamage. It depends...

But, again, with the temperatures we're talking about (high 70's vs. low 80's) the differences will be negligible.

Chris

 

[hahnmeister]

I would cite flow as a major component of photoinhibition as well, and not just for its feeding component (which helps corals maintain their pigments to defend against excessive light). But when you have a 80 degree tank, and then some bright bulb beating down on a coral, its pigments are going to be experiencing radiant heating... like blacktop in summer... and the best way to combat that heat is with a good amount of flow to help the coral 'cool off'. Otherwise it can just sit there and bake. The only thing that I know of that can stand that is a clam.

 

[mhaith]

I hope this isn't a hijack or I missed the answer above...Assuming a point below photosaturation, would prolonged photoperiod be a benefit? In the same vein, do corals enjoy some benefit of a dark period?

 

[BeanAnimal]

Similar to most living organisms, corals need some "time off" to process waste and do other house keeping. That line (when more is too much) is dependent on many variables. In our case those would be water parameters, food sources and availability, etc. What works in YOUR tank as an extended photoperiod may be a stressor in my tank. But the short answer is that they do NEED a dark period.