greenbean36191
Premium Member
PAR is simply the measure of photons within the the spectrum from 400-700 nm. It's not corrected for action spectra, nor is there a useful way to do that.
I think this whole discussion reflects a general misunderstanding about the significance of action spectra.
First of all, make sure you're not looking at the spectrum of chlorophyll b. This is the form with the major spike around 470, but it isn't found in corals. They use chl a, which absorbs roughly equally in the red and blue. More importantly though, the chl activity spectrum isn't the activity spectrum of zooxanthellae. Zoox use a wide variety of pigments and the combined spectrum tends to be pretty flat across the whole PAR spectrum. You don't get the major drop off in the yellow like you do with chl. Generally there is some skewing towards one end or the other, usually towards blue, but not always, and this tends to be pretty minor. The activity spectrum also varies from coral to coral and even in the same coral over time. In other words, there is no single ideal spectrum for a mixed tank.
Also another point that seems to be misunderstood is that the activity spectrum simply represents the likelihood that a given photon will be absorbed- not how useful it is for photosynthesis after it's absorbed. All photons are relatively equal in that regard. The activity spectrum is really only relevant when the ability to collect photons is the rate-limiting step. In other words, it's important when photosynthesis is running below the maximum rate.
As far as efficiency goes, yes, the corals might absorb a higher percentage of the photons in the blue spectrum than the red, but once they're absorbed, a blue photon or a red one are equivalent in their ability to perform photosynthesis. The extra energy carried by the blue photon actually has to be purged as heat or fluorescence to make it equivalent to a red photon, so that energy is essentially lost. Since that extra energy that's being lost is ultimately coming from the electricity you're paying for, I'm not sure that it actually ends up being any more efficient in terms of energy use.
So what does all this mean? Well in theory, if you used a bluer light you would usually have slightly higher photosynthesis than a redder light with the same PAR, but the difference would be negligible. To get the best growth though you would simply provide saturating light intensity and the actual spectrum of the light has very little to do with that. In the real world, we're paying to produce that PAR and it costs more to do it towards the blue end. Since blue photons are more energetic, you get less of them per given energy input, therefore less PAR. This is the reason bluer bulbs tend to be lower in PAR.
I think this whole discussion reflects a general misunderstanding about the significance of action spectra.
First of all, make sure you're not looking at the spectrum of chlorophyll b. This is the form with the major spike around 470, but it isn't found in corals. They use chl a, which absorbs roughly equally in the red and blue. More importantly though, the chl activity spectrum isn't the activity spectrum of zooxanthellae. Zoox use a wide variety of pigments and the combined spectrum tends to be pretty flat across the whole PAR spectrum. You don't get the major drop off in the yellow like you do with chl. Generally there is some skewing towards one end or the other, usually towards blue, but not always, and this tends to be pretty minor. The activity spectrum also varies from coral to coral and even in the same coral over time. In other words, there is no single ideal spectrum for a mixed tank.
Also another point that seems to be misunderstood is that the activity spectrum simply represents the likelihood that a given photon will be absorbed- not how useful it is for photosynthesis after it's absorbed. All photons are relatively equal in that regard. The activity spectrum is really only relevant when the ability to collect photons is the rate-limiting step. In other words, it's important when photosynthesis is running below the maximum rate.
As far as efficiency goes, yes, the corals might absorb a higher percentage of the photons in the blue spectrum than the red, but once they're absorbed, a blue photon or a red one are equivalent in their ability to perform photosynthesis. The extra energy carried by the blue photon actually has to be purged as heat or fluorescence to make it equivalent to a red photon, so that energy is essentially lost. Since that extra energy that's being lost is ultimately coming from the electricity you're paying for, I'm not sure that it actually ends up being any more efficient in terms of energy use.
So what does all this mean? Well in theory, if you used a bluer light you would usually have slightly higher photosynthesis than a redder light with the same PAR, but the difference would be negligible. To get the best growth though you would simply provide saturating light intensity and the actual spectrum of the light has very little to do with that. In the real world, we're paying to produce that PAR and it costs more to do it towards the blue end. Since blue photons are more energetic, you get less of them per given energy input, therefore less PAR. This is the reason bluer bulbs tend to be lower in PAR.
Each of us has a different idea of what looks good, but most of us lean towards a bluer looking tank and the way that the corals look under bluer light.
) is geared toward the spectral curve that photosynthesis occurs (in a broad sense, covering many photosynthetic compounds).
