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Correct lighting for optimal photosynthesis
I have read many postings here concerning the correct lighting for aquarium plants. Many
people are saying that one can make up for an inadequate spectrum of light by increasing
the intensity of light. This is partially true, but not the best thing for the plants (and isn't
this why we are all here?). Inherent in the photochemistry of cyaanobacteria, algae, and
higher plants are two photosystems, termed PSII and PSI. The 'special pair' of
chlorophyll molecules associated with each photosystem and actual release of electrons
absorb light maximally at different wavelenghts. The break is around 682 nm. Below this
wavelength one is preferentially exciting PSII (responsible for oxygen production and
generation of the proton gradient used to generate ATP). Above ths critical wavelength,
PSI is preferentially excited (responsible for the reduction of NADP to NADPH).
Classical experiments by Emerson (and others like Blinks, Duysens, and Kok) provided
experimental evidence for the two photosystems, with Emerson showing that algal cells
illuminated with a combination of light at wavelengths above and below the critical value of
682 nm produced oxygen evolution rates that were greatly enhanced above the sum of the
rates produced when the cells were illuminated with one or the other wavelengths alone.
Notice that there is no one wavelength that can only excite PSII or PSI. Below the
wavelength of 682 nm PSII is excited somewhat and vice versa above 682 nm. This is why
increasing light intensity partially solves the problem. However, this is not the best
solution if one is concerned about optimal growth of the plants. One should choose a
combination of lights that optimally excites photosynthesis to produce rich, luxurious
growth. Nowadays, this is not a problem as many choices for lighting are available on the
market that are relativley inexpensive. If we are going to go all out as far as CO2 addition,
correct substrate additives, and addition of trace elements, why not go all the way with our
lighting too?
Robb D. VanPutte
Dept. of Biology
Texas A&M University