# PAR and UV



## Bufo Bill (15 Jun 2013)

Quick question: does PAR include UV light, specifically UVA & UVB?

Many thanks from Bill.


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## ceg4048 (15 Jun 2013)

Hi,
	No PAR is only relevant to visible light. Plants reject UV so it actually has a negative effect on photosynthesis. P.A.R stands for Photosynthetically Active Radiation. So since UV does not activate photosynthesis it is not included in the measurement.

Cheers,


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## Bufo Bill (15 Jun 2013)

Right, thanks very much, very helpful. 

All the best from Bill.


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## Yo-han (15 Jun 2013)

With the original question answered I'm hijacking this thread. How about algae and PAR? Dennerle has some amazing UV coating around their t5 bulbs with the claim that algae uses UV and plants don't. Never really believed it but can algae use UV?


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## ceg4048 (16 Jun 2013)

Hi,
I don't think so because algae have exactly the same Chlorophyl as plants do. I don't know all the possible pigment types, but I'm not aware of any pigment that absorbs UV energy for the purpose of passing it's energy on to the Chlorophyll. Here is a typical response curve:






On the far left is Violet with a wavelength of about 400 nanometers. Ultraviolet is around 300 nanometers, so that's off the chart to the left. The typical plant response when radiated with UV is actually a breakdown in the Chlorophyll molecule, so each plant and algae have different response mechanisms. Normally the response is to generate pigments that specifically reflect away the UV.

The mechanism of photosynthesis involves a carefully orchestrated movement of electrons from a dedicated area of the Chlorophyll molecule. These electrons are passed on to other proteins which ultimately produce sugar in certain chemical reactions. The dedicated reaction area, where the visible light photons collide with the Chlorophyll is within the area called the "Porphyrin Ring" where the Magnesium atom (Mg) sits like a Queen Bee. When struck by sufficient numbers of photons of the correct energy levels (400 to 700 nanometers) atomic bonds are broken in this area and the electron is ejected. However, UV photons are at a higher energy level, and so when they collide with the Chlorophyll molecules they break the atomic bonds in other areas of the Chlorophyll, thereby ionizing the Chlorophyll and turning it into a material that is something OTHER THAN Chlorophyll. This is sometimes referred to as "photobleaching". UV also energizes electrons in unintentional molecules such as DNA. These electrons get ejected and the DNA are ionized, causing genetic mutation and often, death. That is why UV is used for sterilization. So it's very unlikely that algae use this energy to photosynthesize. A more likely motivation would be the temptation to use UV as an algecide, which for example works brilliantly with green water algae. UV bulbs are often used to eradicate a green water attack., and in fact is the best know way of treating green water.




Cheers,


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## Samjpikey (17 Jun 2013)

Ceg obviously worked hard at school  
Don't google it .... ceg it !


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## Yo-han (17 Jun 2013)

ceg4048 said:


> Hi,
> I don't thinks because algae have exactly the same Chlorophyl as plants do.


 
My first thought too.

But UV killing algae is mainly UV-C. In reef tanks they use UV to make the colours of their corals (the algae within) pop. They are not killed by it. Emersed plants in paludaria receive extra UV-A/B light as well without killing plants/algae. So is the chlorophyll killing of UV for all kinds or just UV-C? Eitherway, the coating won't add much I guess...


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## ceg4048 (17 Jun 2013)

Yes, I agree that the coating is more a marketing issue than reality. The UVC wavelengths are from about 100nm to just under 300nm. The energy per photon at 100nm is about three times higher than it is at 300nm so that's why the UVC has germicidal properties.

I'm not really sure about saltwater, but our vision also fails to record data at UV wavelengths, so I'm not sure whether the corals are fluorescing the UV back into the visible spectrum, whether the corals develop pigments to reflect the UV, or whether the bulbs emit partially in violet and that we simply are seeing the reflected violet/indigo.

Again, when we are looking at objects illuminated by black light, generally we can only see the reflected violet/indigo. We can't see the UV. The UV-A/B lights used in the way you mention have a negative effect on the plants but it's simply that the plants are able to adapt and repair the damage or to develop pigments which ameliorate the effects of those wavelengths. I would be much more worried about what those lamps are doing to the eyes. Forget about the plants.

Cheers,


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## gmartins (18 Jun 2013)

My view of using UV in saltwater was that it induced a somewhat controlled reduction in zooxanthelid algae living in the corals resulting in colour enhancement. So they are actually deliberating causing "harm" to the corals (less algae, less food)... but I might be wrong here...


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## ian_m (18 Jun 2013)

UV is used in salt water aquariums because the corals fluoresce. Anyone who has been night diving on a coral reef with a UV torch can confirm this.


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