How does the OD correlate with the number of algae cells?
As with bacterial cells, number of cell and OD are correlated. I am not a specialist, but it seems that you can use microscope to visually count number of cells, then correlate it with optical density of the solution you have. This will allow you to first establish how many cell correspond to 1 unit of OD, then use OD to estimate number of cells quicker than by counting.

Just a few details: counting of cell number is done by dropping given volume of culture on a special slide with grid, then manual counting using microscope. 

If we assume that 100 cells/mL (or whatever) solution gives you OD of 1, then OD of 2 will correspond to 1000cells/mL concentration. That us due to logarithmic relationship between OD and light absorption by the cell culture.