Ack! Yes, the article linked is the one I was meant to refer to. This is Moya et al., 2006, not Meibom et al., 2006. Whoopsie daisy.
Airinhere,
You raise several interesting questions.
MCsaxmaster. I read through some of the articles posted recently regarding light cycles. You are correct about light having a positive effect on calcium absorbtion. The study I read suggested a 4X increase roughly. Learn something new everyday. Thanks!
No worries :thumbsup: Light:dark calcification ratios in zooxanthellate corals have been reported from around 1 to 26. The mean is about 3 and most corals are around 2 or 3. I think it is important to note that these are changes in calcification, not merely the uptake of calcium. Corals don't really maintain a pool of calcium except a tiny amount in the tissue (calcium is important for cell function, but must be kept at very low concentrations--1/10,000th as much as is in sea water) and what is in the gut. Light does not affect the uptake of calcium from sea water (into the gut) but rather the rate of calcification, that is, the rate calcium carbonte is precipitated.
This is why we all come here to post. Somewhere there is someone who has spent the time and energy to seriously study all these experimental methods we are practicing. Without your insight, I would have no lead for finding the actual research regarding coral calcification and would be reliant upon the experience of other reefers who I can regard as dependable sources of information.
:thumbsup:
About the saturation of light on corals, I think we are talking about different things. What I am refering to is the end result of several hours of photosynthetic activity.
As I understand it, while a coral is exposed to light, its zooxanthela is conducting photosynthesis. The zooxanthela essentially (and this will be a gross simplification I am sure) utilizes Carbon Dioxide and metabolizes it into simple sugars and other products that are then utilized by the coral as a source of energy (and catalyst for calcium absorbtion).
Yes except for the terminology "catalyst for calcium absorption." The corals get a lot of energy-rich photosynthate (mostly as glycerol) from the zoox. Calcification by corals requires a lot of energy. It seems very likely that energy obtained from the photosynthate is used during calcification, but it's unclear if that has anything to do with light enhanced calcification (LEC).
There have been 3 major types of hypotheses to explain LEC. 1) the zoox. provide stuff needed for calcification--this could be simply food (energy), O2, precursors of organic matrix, etc. 2) the zoox. remove "bad stuff" that could interfere with calcification--here primarily people are thinking nutrients (e.g., phosphate) that could interfere with precipitation of CaCO3 chemically. 3) the zoox. modulate the chemistry inside the corals in a way favorable to calcification. Since calcification removes alkalinity it causes the pH to drop inside the polyp. Photosynthesis produces OH- (due to the fixation of CO2). This can counteract the drop in pH caused by calcification and make the internal chemistry more favorable for calcification.
These hypotheses are not mutually exclusive, but we don't really have good data suggesting which mechanism or set of mechanisms is/are really the important ones. Maybe adding O2 (lots of O2 is needed by the calicoblastic cells) is the most important part? Maybe providing energy is the most important part? Maybe buffering against a drop in pH is the most important part? We really don't know, so we really shouldn't suggest why photosynthesis aids calcification but merely recognize that it does.
A waste product produced by the photosynthesis is Oxygen. If the Oxygen is allowed to build up for extended periods of time, it has an oxidising effect on the coral and can injure the coral.
Yes, to an extent. The big issue is not really the oxygen as in O2, but rather the production of reactive oxygen species like O2- and H2O2. Those are really nasty oxidizers (just straight O2 isn't too bad). The production of reactive oxygen species is favored by certain conditions like high O2 concentration in the precense of photosystem II in the zoox., or exposure of O2 to UV. Corals, more than most any other sort of animals other than other symbiotic ones (e.g., tridacnid clams) produce TONS of the enzymes needed to get rid of these reactive oxygen species (superoxide dismutase, catalase, etc.) so that they can protect themselves from serious oxidative stress and, well, death.
Also, water flow is critically important in removing O2 from coral tissue in the light. With strong flow O2 is removed quickly, fewer reactive oxygen species form and the coral is able to deal with the situation without bleaching. In stagnant water (or just lower water flow) the coral can't get rid of O2 fast enough, the reactive oxygen species build up, and it bleaches when exposed to exactly the same stress as above.
The corals response to the damage being done by the oxidization is to expell some of its zooxanthela.
That's bleaching, and that only happens when the oxidative stress (due to reactive oxygen species--ROS) is sufficient to overwhelm the systems meant to deal with them (SOD, CAT, etc.) and the damage is sufficiently great.
This is often observed and reffered to as light shock.
Yup, zoox. adapted to lower light suddenly exposed to higher light will collect way too much light energy, produce a whole lot of ROS--more than the corals can deal with--and the corals bleach.
The saturation point I refer to is the point where satisfactory results of photosynthesis have occured to keep a coral alive and healthy, but before any quantities of oxidizing agents have a chance to begin accumulating.
I would
strongly encourage you to avoid using the term "saturation" or "saturation point" in that case. These terms already have well defined and very specific meanings when talking about photosynthesis, and using the same terms to describe a totally distinct process could be...confusing
Please do consider though, if the light is not too bright and the coral can get rid of O2 fast enough then you will always remaind below the threshold where serious damage from ROS occurs. There is always some damage--that is unavoidable (if you expose a photosynthetic organism to light they incurr cellular damage as an inherent physical property of light absorption--one in a thousand excited electrons goes into a "triplet state" which is where you get your ROS). As long as the amount of damage is at or below the threshold where it is repairable the corals will be fine indefinitely--photoperiod won't matter. If you are causing net damage (light is too bright, there's too much UV, there's not enough water flow, etc.) then photoperiod WILL matter and the longer the period of time the lights are on the worse the damage.
I'm under the impression this saturation point can occur within 5 or six hours of light exposure of a constant nature.
Hmmmm, it is more complicated than that I'd say. Most corals, even those living in shallow water in nature that are definitely not light-limited, can't fix enough photosynthate to satisfy all of their needs. They might fix enough for 150% of their metabolic rate, but they use half of it to produce mucus. Thus, even though they are producing a heck of a lot of photosynthate they still might only be getting 3/4 of their energy from the zoox. (the rest comes from food). If we go to a shorter photoperiod the zoox. will fix less carbon and the corals will have a smaller energy budget. Some corals adapt to this fine and just up their intake of zooplankton while other corals are less adaptable and either slow down their growth and metabolism, or, well...die as a result of starvation.
My concept behind the dual light cycle is to disregard the natural light cycle and go for two shortened light cycles at very high intensity. (dual 250W halides). As opposed to a 10-12 hour light cycle with the dual 250W halides. By reducing the exposure time to six hour incriments, I should be able to avoid light shocking my corals and I should be able to trick their metabolisms into an accelerated growth cycle.
You aren't going to shock your corals with too much light unless you are expsoing them to more light than they are adapted to or can adapt to (some zoox. are shade adapted and simply can't tolerate high light no matter what). As long as the light is not too bright (not bright enough to really cause damage) and the corals can get rid of O2 fast enough (and there isn't another stress on the photosystems like high UV, high temp, etc.) then it shouldn't matter how long the lights are on--you aren't causing any lasting damage to the corals or the zoox. I'm not sure I follow what you mean by "trick their metabolisms into an accelerated growth cycle."
I believe their response to light is almost mechanical so by giving them sufficient light for a day then providing them a dark period to resituate themselves, They should show accelerated growth by introducing a second light cycle. Any growth they would experience would be limited to the amount of time they are in the light or dark cycles.
But they shouldn't be experiencing any lasting damage unless there is something otherwise "off" with the husbandry. They shouldn't need or even benefit necessarily from a dark period because they have not experienced lasting or irreparable damage.
I fully relize this is likely dodgy science at best, but I honestly think there is something to it. I think 12 hour light/dark cycles are a function of the size of our PLANET and are not the best light/dark cycles for our corals.
No worries on your interpretations. I really enjoy when folks think freely and intelligently, but I also think that we all must be careful not to allow ourselves or others to think that our hypotheses are facts.
Agreed on the diurnal cycle--that's what's available so that's what they get, whether or not it is the most ideal paradigm physiologically.
Chris
