Dear Hamed, the growth of microalgae can be determined at 680 nm.

Best regards.

I have used 680nm to measure the growth of Chlorella vulgaris and it worked perfectly.

Best regards.

you can use 680nm or 750nm.

I think 680 nm is not a good idea because at 680 nm chlorella shows the peak of chlorophyll a and b and it depends on so many other variables like light intensity, nutrients and other environmental factors. I use to measure the growth of microalgae at 650 nm because   it is not specific to any photosynthetic pigments so its gives the exact idea of density or turbidity or number of cells present in a particular time duration.