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Photosynthesis/Irradiance (P/I) Curves and Why They Are Important to ReefKeepers

By Dana Riddle and Andy Amussen. Presented on September 5th, 1999 on #reefs IRC.

We wish to thank Rich for inviting us to speak tonight. Our topic for tonight concerns one of the more important aspects of maintaining a successful reef aquarium.

Lighting is an important consideration in the reef hobby, yet it is one surrounded by confusion. Perhaps it is because there are so many choices in types of lighting. Perhaps it is because we are only beginning to understand lighting requirements of many marine animals containing symbiotic dinoflagellates – zooxanthellae. Debates have raged over which lighting is most suitable. Some hobbyists claimed that metal halide lighting is the only way to keep certain corals while others, using fluorescent lighting, pointed to their successes with these same animals. Observations such as these led to confusion. We’ll discuss lighting (irradiance) and how it promotes photosynthesis. Then we’ll look at a few case studies.

Photosynthesis is the light-driven process by which carbon dioxide (and/or bicarbonates in aquatic environments) is converted to simple sugars. This process takes place in green plants, algae and (of most interest to reef hobbyists) the dinoflagellates (called zooxanthellae) living within the tissues of many corals. As hobbyists, we want to make certain that our captive corals get enough light to promote photosynthesis and, at the same time, we don’t want to over-light the aquarium either (this results in needlessly high electric bills, unwanted heating of the aquarium, etc.)

Let’s examine an imaginary (but true-to-life) zooxanthellate coral to understand what happens within it when it is exposed to light: The small coral colony is only a year old; as a coral planula, it had settled on a small lava rock in 1 meter of water off the coast of the Big Island of Hawai’i. It is dawn now; the sun is casting its rays over Mauna Loa and soon the tropical sunlight will dance in ever changing patterns across the small reef. But, for now, the sea is dark, and the symbiotic algae within the coral’s tissues are at rest. They, like the coral animal, are extracting dissolved oxygen from the water. In this sense, they are no different than all other living things – they need oxygen to survive.

The sun begins the day’s journey toward its zenith and the clear ocean water begins to assume the color of the reflected light. What was once a black ocean turns to gray; when the sun is higher, the sea will turn to blue. The morning sun falls upon the coral. Within the tissues, the zooxanthellae awaken and begin to use the small amount of energy. They begin to produce small amounts of oxygen through the process we call photosynthesis. Radiant energy increases as the solar disk climbs higher in the sky. The photosynthetic pigments within the coral’s zooxanthellae work harder and produce more oxygen. At some point, the oxygen produced by the zooxanthellae will meet their and the coral’s respiratory needs and they will no longer gather dissolved oxygen from the water (Scientists would say that they have reached the Compensation Point).

At noon, when the sun is directly overhead, the shallow water offers but little protection from the fierce tropical sun. The zooxanthellae are almost overwhelmed by the amount of radiant energy falling upon them – they have been “working” as hard as they can and the increasing amount of sunlight no longer increases the rate of photosynthesis. They have reached the Saturation Point.

As the sun begins its slow march to dusk, less energy penetrates the warm ocean and once again the coral’s zooxanthellae photosynthesize at a rate proportional to the amount of light falling upon them. At sunset, just as earlier in the day, they will again reach the Compensation Point and begin to extract oxygen from the water in order to survive. They will continue to do so until the next morning’s dawn and the process will begin anew.

So, to review our story and the points I tried to convey: Corals and zooxanthellae need oxygen in order to survive. At night, they draw dissolved oxygen from the water column. When radiant energy is sufficient, the zooxanthellae will begin to make oxygen. When enough oxygen is produced, the zooxanthellae will “share” it with the coral animal. Once the coral animal’s (and zooxanthellae’s) oxygen needs are met (the Compensation Point), the oxygen will be released to the water column. Zooxanthellae use only a fraction of the energy available to them, so when the rate of photosynthesis doesn’t increase with increasing sunlight, we say that they have reached their “Saturation Point.” There is also something called “Photoinhibition.” This occurs when the rate of photosynthesis slows down (or stops altogether) when there is too much light.

A hypothetical P/I curve can be seen here.

We should be concerned about all three phenomena – Compensation Point (how much light is just enough); Saturation Point (when switching to metal halide lamps is senseless because fluorescent lamps will provide enough light) and Photoinhibition (when there is simply too much light).

At this point, I am reminded of a true story – I was invited to view an aquarium and give some opinions as to why the captive animals weren’t doing very well. Upon seeing it, I appreciated the amount of engineering that this fellow had incorporated into the tank. It was state-of-the-art. Down-draft skimming, remote Jaubert-type plenum, computerized monitoring for the lighting and chiller systems. It seemed that nothing had been left to chance. The owner was dismayed – all this effort and expense (the metal halide/fluorescent lighting combination alone was an expensive investment and costly to operate). He had wanted to start with colorful and easy-to-keep marine invertebrates. He chose some colorful “mushroom” corals ( Discosoma spp. ) – and couldn’t keep them. They shriveled upon being placed in the aquarium and eventually died. He was distraught – if he couldn’t keep the “easy” animals, how could he ever hope to keep some of the more “difficult” stony corals? The answer was easy – there was simply too much light for the “mushrooms” – he probably could have kept the Acropora specimens to begin with!

So, how much light to some common coral inhabitants need? To answer these questions, we designed and built some highly specialized equipment for use in our lab. The most important apparatus is called a respirometer. It is a clear “plastic” container and holds slightly less than a gallon of water. We place a coral colony within it and slowly increase (and measure) the amount of light falling upon it. At predetermined time increments, we also measure the amount of dissolved oxygen in the water (using an electronic dissolved oxygen meter). At the end of the test, we graph the amount of light and dissolved oxygen. This is called a P/I Curve (P/I stands for Photosynthesis/Irradiance).

In this manner, we are able to estimate the amount of photosynthesis and how much light corals and algae actually need. Some of our research suggests that the more colorful coralline algae need relatively little light. In fact, just one or two 9-watt fluorescent lamps will provide enough light. Place these same algae under a 400-watt metal halide and you’ll likely kill them. Same thing for corals – we used the respirometer to determine Compensation Points, estimate Saturation Points and observe possible Photoinhibition for several corals under artificial lighting.

Our results compare favorably with those of other researchers investigating “low” light corals. For example, we’ve found that the Compensation Point of a pretty, green tank-raised stony coral ( Stylophora pistillata ) seems to be about 2,950 lux (or a PAR- Photosynthetically Active Radiation - level of about 59 µEinsteins per square meter per second. All results are tested for statistical significance; this is not simply anecdotal information. These experiments are on going and there seems to be some important information arising from these trials. Consider, though, that one set of Compensation Point experiments took three weeks of lab work…. We should have solid data to present at one of the major conferences this Fall (MACNA XI in Louisville or possibly the Marine Ornamentals conference in Hawai’i.)

It seems to be a common misconception that lighting of insufficient intensity can be overcome by lengthening the amount of time the lamps are on (the photoperiod). This isn’t so, as can be easily illustrated. What happens when zooxanthellae are starved for light? These guys are remarkably adaptive but it takes them awhile to adjust to a new, “low light” environment. Zooxanthellae store energy reserves for “hard times.” They will use some of this energy while adapting to a new, lower light intensity. If the time required to adapt is longer than the time for which they have reserves, they will starve and die. In some cases, the zooxanthellae could never adjust. Once the zooxanthellae die, the coral’s chances of survival are diminished. The moral of this story – don’t try to make a coral adjust to very low light conditions (all the more reason you should have a light meter!)

For our purposes here, we’ve disregarded the spectral output of different lamps and reported just intensity levels. The necessary brevity of this article will prevent us from discussing peripheral collected data such as ultraviolet radiation, temperature, water motion, inorganic carbon sources, etc. As for Compensation and Saturation Points determined by other researchers (in: Kirk, J.T.O., 1983. Light and Photosynthesis in Aquatic Ecosystems. Cambridge University Press, Cambridge. 401 pp.), these are for Stylophora pistillata:

Coral Compensation Point* Saturation Point* Temperature**
“Low-light” S. pistillata 2,000 10,000 28°C
“High-light” S. pistillata 7,000 30,000-100,000 28°C

*These were originally listed in µEinsteins per meter per second; I multiplied the original numbers by 50 to obtain lux. **Temperature plays a part in determining Compensation and Saturation Points and is therefore listed.

What lighting systems can deliver enough light to keep the “low light” Stylophora? Our research indicates that just two, new 40 watt lamps can deliver enough light to the bottom of an 18” deep aquarium! However, the output of these lamps will rapidly diminish and the coral will suffer due to lack of light. Two or four 110-watt VHO lamps are much better choices, as are 175 metal halides (we’re assuming these lamps will be about 4 inches above the water surface in an efficient reflector.) Meeting the Compensation Point of the “high light” Stylophora (on the bottom of an 18” deep aquarium) will require, at a minimum, 4-110 watt VHO lamps or, better, 175 watt metal halide lamps. Meeting the Saturation Point of even the “low light” Stylophora will take some doing (use 4-110 watt VHO lamps and change them every 6 months, or use metal halides). This information is just the tip of the iceberg. An article of this type could easily turn into a small book (and probably will). I’ll now try to answer any questions.

Editor's note- after the talk (and immediately before the Q&A;) took place, Dana and Andy lost their connection to the internet. They answered these questions by email later on that evening once they were able to re-connect to the 'net.

Ok, I use 400watt halides. Am I giving my corals "too much" light therefore slowing growth?

It is possible that your 400 watt lamps could produce too much light (it depends upon many factors including type of lamp, lamp orientation, lamp height, type of reflector material, etc., etc.) The area of "too much light" is likely to be directly below the lamp and would affect corals just a few inches deep in the aquarium. Recent studies have shown that photosynthesis slows during periods of maximum light intensity. This occurs around noon and these high light levels are of short duration. In an aquarium, a coral could be subjected to very high light intensity for as long as the metal halide lamps are on. Therefore, it is possible to "overlight" a coral within an aquarium.

I find it interesting you used S. pistillata as a high light/low light example, is there any way to tell what lighting a coral needs other than experimenting? I've had my pistillata for 1 1/2 years, and it hasn't grown, yet all the other sps are growing like weeds. It's in the upper 1/2 of tank lit by 2x250w iwasaki, and 2x110w actinic.

Light distribution patterns are difficult to estimate, hence our recommendation that every hobbyist (or club) invest in a relatively inexpensive lux meter. It is generally accepted that low light will inhibit growth, we are suggesting that high light levels will do the same. If you could supply some lighting data, I think we all could learn a lot.

I have a 30 gal tall tank- 2' long 2' high and 1' deep. Currently it has NO lighting 1 15w full spectrum and 1 15w actinic...what would you recommendation be to upgrade the lighting, and secondly what lighting schedule would you put the tank on?

Your current lighting setup is fine for certain corals ("Mushroom corals"- Discosoma , "Green Fuzzy Polyps"- Rhodactis spp. , etc.) and many calcareous algae species. I like VHO lamps because they provide "good" amounts of light and distribute it very evenly. I have the tank lights on a timer and they are on so I can see the tank before I leave for work. They're on for 14 hours so I can enjoy the tank in the evening as well.

What is the relationship between lighting and alkalinity levels?

We recommend that alkalinity levels should be around 3.6 meq/l. Strong lighting may promote photosynthesis and hence calcification thus depleteing alkalinity and calcium. Alkalinity levels may also drop due to its destruction during nitrification.

For your measurements of PAR underwater, do you have any data showing the attenuation with respect to depth ?

Yes, this data is being published in FAMA in a 4-part series on lighting (Parts 1 and 2 have already been published). We'll also discuss recent lighting findings at the Louisville MACNA.

Other than the tried and true method of frying corals, are there any generalities of shape or color indicating preferred lighting levels?

Do frying corals smell better than, say, poached corals? .... Your question is a good one and one that I personally have invested a great deal of time and money in pursuit of an answer. Generally, stony corals exhibiting a solid green coloration prefer less light than many others (this green color will "bleach" under strong light). On the other hand, intensely purple (not violet) SPS corals seem to prefer much more light (the color will fade under weak light). Stoutly built Acropora specimens are probably from areas with very good water movement and, as a rule, these animals receive a lot of light. Finger-like Porites corals are probably from deeper areas and receive less light. Our respirometer experiments are continuing....

Is there a PAR reading for each of the 3 points photoinhibition..compensation..and saturation?

Yes, Compensation will always occur at a relatively low PAR value (about 60 µEinsteins or 3,000 lux, in our experiments with soft, stony corals). Saturation is higher and Photoinhibition is at the high end of PAR values.

What's a good reference (besides Baensch) for determining light levels for different animals?

If you have access to a good science library, you probably can't do much better than Kirk's "Photosynthesis in Aquatic Ecosystems" - and he'll list other references. If it's hobby literature, I'll shamelessly plug my book "The Captive Reef" - it recommends specific lighting systems for some of the more popular aquarium corals. Of course, Delbeek and Sprung's books are a good source of info (altthough a little less specific). There are others still that list light requirements as "low", "medium", and "high" light. These terms are rather subjective, but may give you a good ballpark idea.

What is a good lighting system for 12" deep tank (20 gal) for a wide range of corals?

If it were me, I put as many VHO lamps above that tank as I could (2 daylights to each actinic).

So is the important issue watts supplied vs depth?

Since watts indicate the "strength" of the light at a given depth, yes, you are correct. Getting good light penetration into the aquarium and delivered to your animals is the name of the game. Of course, light strength or intensity can be reported in other units, such as lux or PAR units.

So the only use for zooxanthellae is for oxygen in corals?

No, zooxanthellae supply (and take) many things... they uptake waste products from the coral animal (carbon dioxide, nitrogenous wastes, phosphorous than can inhibit calcification) and "recycle" these wastes as potentially useful products including amino acids, sugars, fatty acids, oxygen and others. We measure oxygen in our experiments since this has been the standard in so many scientific trials. Oxygen is easily measured as well.

Does the temp of the bulb matters if you add a separate actinic blue bulb?

Your question is "Does spectrum matter?" Spectrum does not play a significant role when oxygen production is the only parameter examined. However, blue light is known to promote protein synthesis. Red light may promote undesirable algae growth. I could go on and on... but here's my recommendation: Use an actinic lamp if its a display aquarium; if its a "grow-out" tank, actinic lighting probably isn't necessary.

Would a dino outbreak be indicative of overlighting?

I'm going to read between the lines here... I suppose you mean "cyanobacteria outbreak"... good question ( if that's what you mean ).... Charles Delbeek reported recently that increased lighting made a cyano outbreak "disappear"... was it because of an increased redox level or did bubbles made during an extended period of photosynthesis simply lift the mats of slime away? Charles has the first-hand observation, but he suggests that more lighting got rid of the "problem." If you mean "dinoflagellate"... gosh, you've stumped me here...

Again, thanks to all who participated in this chat. And very special thanks for inviting us. Dana and Andy

Thank you for a great talk Dana and Andy!

Created by liquid
Last modified 2006-11-26 05:20