Nanodrop is a great technology for measuring solutes absorbance (and, by proxy, concentration) but a poor choice for measuring cell density (unless you use the cuvette model/mode), regardless of the cell type (cyanobacteria, algae, yeast. E. coli, etc.).
The reason is the construction of the nanodrop itself - the very short measurement distance, usually 1 mm, and the way cell density is measured.
When measuring solubles the light pass through many 'molecules' and we see the accumulated absorbance. If we have soluble molecules this will work fine in 1 mm path length as there will be enough molecules.
But when we measure cells light scattering come to play, and much of the light is backscattered and never makes it to the next cell. If we use regular cuvette or even 96-well the width of the beam (in cuvette ~1 cm^2) and the light path partially make up for it (but don't go over 1OD for E. coli in 1 cm cuvette or shading by neighboring cells will be an issue). In a nano drop, the very narrow beam (~1 mm^2) and very short path length will likely rack havoc in your measurements.
That said, cyanobacteria are usually measured at 730nm to avoid chlorophyll absorbance, as chlorophyll content might change in different growth conditions and skew your result.
See https://www.researchgate.net/post/What_is_the_wavelength_which_is_suitable_for_taking_OD_of_Cyanobacterial_culture2#:~:text=All%20Answers%20(14)&text=ancestor%20and%20share%20many%20similar,mostly%20used%20for%20cyanobacterial%20suspensions.
Thanks for the information. I just let you know I am using the Cuvette Mode for Microcystis aeruginosa PCC 7806 which is cyanobacteria. Can you please let me know 750 nm or 800 nm which one will be more perfect?
Both should be OK. Now comes the question, do you want to follow growth or measure cell density?
Translating OD(750 or 800) to cell concentration will require calibration by microscopy counting.
Ideally, you should use FACS, or if you must, microscopy counting.
You can do calibration curve of the above against OD750nm and use this calibration
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