I haven't gone looking too much into the specifics of algal growth under various light conditions at this point, but oreo54's post on page 1 here gives some idea of the complexities involved with this discussion/research (i.e. what the "white" in the light itself is, the kind of algae involved, etc.). That said, from what I've found, there are currently only two things I've come across that are scientifically researched that seem to possibly support the idea that lowering "white light" and/or running just blue light decreases algal growth (and one of them I'd feel a lot more confident about the assertion that it might lead to a decreased amount of algae if Randy thinks there a chance it would be impactful/meaningful from the chemistry side of things):
1 ) Most algae use Chlorophyll A and some other kind of Chlorophyll (B, C, or D), while corals basically use just Chlorophyll A (this is an extreme simplification - there are a number of other important pigments involved as well, and some corals technically use a little Chlorophyll C as well); in theory, by hitting just the spectrum for Chlorophyll A by using just blue light, it could potentially provide corals an edge over the algae (I haven't seen studies done on if it actually does provide an edge or not though, and I would assume any edge given is small).
2 ) This is the one where I don't know enough about the chemistry involved yet to know for sure if it would even matter, but:
Corals will still grow under just blue light, yes. As mentioned, changing from white and blue or just white to just blue will likely impact the coral's growth, coloration, and possibly health (if the coral is from deep water rather than shallow water, using just blue light will likely increase coral growth and health - if the coral is from shallow water, switching to just blue may decrease the growth and health).* Switching to blue light from white or white/blue will also impact any photosynthesis going on in the tank:
"More importantly, under blue light calcification is very high, even higher than under full spectrum, while photosynthesis remains below the compensation point, meaning that CO2 production is higher than consumption (Figure 4B)." **
*Source:
Scalar irradiance microsensor measurements performed inside the tissue of living corals show that absorption and fluorescence emission by host pigments produce dramatic spectral alterations in the light environment experienced by the symbionts.
elifesciences.org
**Source:
Light enhanced calcification (LEC) is a well documented phenomenon in reef-building corals. The main mechanism proposed for LEC is that photosynthetic CO2 up...
www.frontiersin.org
***This third link reinforces both of the points listed with the other links:
SUMMARYDepth zonation on coral reefs is largely driven by the amount of downwelling, photosynthetically active radiation (PAR) that is absorbed by the symbiotic algae (zooxanthellae) of corals. The minimum light requirements of zooxanthellae are related to both the total intensity of downwelling...
journals.biologists.com
In my mind, photosynthesis remaining below the compensation point could potentially mean that the blue lighting rather than white lighting helps limit algae by reducing available oxygen in the tank, but - again - I'd run this one by Randy to see if it might be valid.
(P.S. the quote above is - I'd imagine - why many big coral sellers run full blue lighting: maximizing coral growth; importantly, however, as noted, this maximized growth under blue light may only apply to deeper-water corals.)
Anyway, as mentioned, there's also a chance that lowering PAR by decreasing the white channel on your lighting could also decrease the amount of algal growth simply by limiting the potential energy uptake of the algae.
So, there are a few things that indicate that decreasing white lighting could potentially reduce algal growth, but whether it actually would reduce it or not has - to my knowledge - not actually been studied at this point.
Regardless, at this point in time, since there's not really any solid evidence that running just blue reduces algae growth, I'd suggest it may be better to decide if you want white lighting or blue lighting based on 1 ) whether your corals are from deep water or shallow water, and/or 2 ) if you want your coral fluorescence to really pop or if you want to be able to see your fish/other inhabitants' colors better.
Some other, pertinent information:
From what I've read, it's because algae generally uses different light wavelengths for photosynthesis than coral zooxanthellae do - corals using more blue light, and algae using more red and green/yellow.
For a simple explanation, here's a graph from Khan Academy about photosynthesis and light absorption:
I wish I could find the graphs that I've seen specifically for corals/algae, but I can't remember where I've seen them. Anyway, basically the "Windex" blue light tanks use that spectrum (and some people recommend it over using white/red lights) because - at least theoretically - the corals do fine using primarily the Chlorophyll A provided by the deep blue while essentially starving out the algae by not providing the green/yellow and red lights for Chlorophyll B and Beta Carotene. I've seen some more complicated charts of this with more absorbing pigments than just these three (Chlorophyll C, Chlorophyll D, PSC, and PPC to name a few), but - again - I can't find the ones I've seen for corals which would be more useful, and this one gets the point across.
So, the theory is that you feed the corals with blue light, and the algae with other lights. By reducing the other lights, you stop feeding the algae/give the corals the competitive edge. That's why people say to reduce intensity of other colors to stop algae. How accurate that theory is would likely depend on the specific corals and algae involved, but the theory
seems relatively sound when speaking broadly. Since many (possibly most) algae species utilize Chlorophyll A, I have no idea if it's effective or not, but there is a chance that it gives corals the edge they need to beat out algae. I'm not aware of any studies that actually show one way or the other though.
With regards to the "... old ones grow algae!" - here's a quote from Dana Riddle on it that basically says it's possible following the same lines of reasoning listed above, but he's not (or at least wasn't) sure if it was true:
In case you're curious, here's a graph from the paper linked below that shows different classes of algae use different spectrums of light for their photosynthesis:
May also have something to do with the harsher (salt) environment. We all wish our LED lighting was less, but what's a person to do...?
