@ Maziar: I recently posed similar questions to Professor Wim Vermaas and got such answer: " As the OD is scattering and therefore depends on the layout of someone's spectrophotometer (particularly on the distance and slit size between the cuvette and photodiode), I have never wanted to specifically link OD and cell number/ml. You will need to determine that for your system with a hemocytometer. However, for most systems 1 OD730 is in the range of 10^8 cells/ml. " Please keep me posted about further findings. IB

thanks friends!

Hey Igor, it depends on microalgael species, growth state, pigmentation, process parameters etc. A more exact ralationship may be given by a correlation between bio dry mass and optical density.