an additional and essential energy

an additional and essential energy source available for photo-
synthesis in ultrahigh cell densities, greatly expanding the very

limited photic volume in such cultures and seemingly support-
ing photosynthesis in the absence of blue and red light a small

distance underneath the surface. Since the green range of the

spectrum, which represents approximately one-third to one-
quarter of the entire photosynthetically active radiation range,

corresponds to the maximal flux of solar energy, the intensity

of the green light when mutual shading is extreme is sufficient

to facilitate photosynthesis at a level corresponding to that

induced by the red and blue ranges of the spectrum. Further

investigation is required to determine the efficiency of green

light for photosynthesis in our system.

Conclusion. The general conclusions pertaining to determi-
nation of the inherent optical properties required for calculat-
ing light penetration depth into algal suspension are as follows.

(i) A similar absorption coefficient for Chl was found for Spir-
ulina cultures grown in widely different reactor types (5). The

absorption coefficient, however, was quite sensitive to the phys-
iological state of algae (not shown), opening the possibility of

monitoring some aspects of the physiological state of algae in

real time by using nondestructive remote-sensing techniques.

(ii) Absorption and scattering coefficients for Chl were quite

close to those reported for algae in ocean and inland waters.

(iii) The vertical attenuation coefficient of downwelling irradi-
ance can be used to estimate the depth of light penetration into

a reactor supporting ultrahigh cell densities. (iv) In ultrahigh-
cell-density cultures, the photic volume, which is only a small

fraction of the reactor volume, greatly differs for the blue and

red light compared with the green light. A photosynthetic role

for green light under these unique circumstances is suggested.