That's neat project, and a colorimeter will be a
useful tool for you in measuring algae
concentration. As you probably know already,
cyanobacteria get their energy from
photosynthesis. To capture light for this
process, they use a special protein called
phycocyanin. This protein is what gives
cyanobacteria their distinctive bluish hue. Not
coincidentally, phycocyanin absorbs orange and
red light, particularly light with a wavelength
of about 620 nanometers. Because cyanobacteria
are full of phycocyanin, I think you should use
a wavelength of 620 nanometers. If you can't be
that specific, use a wavelength in the visible
orange-red range between 580 and 630 nanometers.
Reference: Glazer, A.N., 1989. Light guides.
Directional energy transfer in a photosynthetic
antenna. J. Biol. Chem. 264:1-4.
Best of luck!
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