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Re: Light intensity and water depth



> I'm not sure if this is why but from the talk I have with a marine storekeeper,
> he mentioned that in reef, the light need to penetrate the corals to be useful. 
> In planted tank, the light just need to fall on the leaves. That is why they 
> need more intense lighting.

I think I follow, and this is mostly correct:

A plant like algae has chloroplasts.  Light hits, penetrates cell wall,
and hits the chloroplast.  There's the minimum for the photon to excite
activity as the initial point of photosynthesis.

With many corals, the invertebrate actually absorbs algae and holds
it within the coral itself.  This is symbiotic mutualism:  The algae
gets the advantage of protection from predation, and access to the
coral's nitrite waste for growth.  In return, the coral partially 
feeds off the sugars produced by the algae, and actually consumes 
surplus algae.  In effect, the algae's NPP "subsidizes" the 
coral's diet.  Thus, a coral individual can adapt to a lower external
feeding requirement if it has access to good light (and has
algae internally), or can depend more on external food sources if
lighting is inadequate (or it could not opportunistically acquire algae
for mutualistic retention).  Recent studies show that algal/coral 
relationships are highly species sensitive (a specific coral species 
will retain only a specific species of algae).

However, because the algae resides INSIDE the coral, the photon must
go through the coral to the algal cell, through the cell wall, and
into the chloroplast as a first step in photosynthesis.  That's more
penetration required.

Hence the need for brighter light in a reef.

--charley                           Fort Collins, Colorado USA
charleyb at gr_hp.com	or	charley at agrostis_nrel.colostate.edu