A UV-B stress tolerance of the four airborne microalgae and the two waterborne microalgae used as controls. About 500 cells in the early stationary phase were spread onto each agar plate and irradiated with 302 nm UV for the specified durations. The survival rate of each strain was defined as the colony numbers on the UV-treated plates compared to those on the non-treated plates (n = 6, mean ± SE). F5, Desmodesmus sp. F5; 2219-4, Neodesmus sp. UTEX 2219-4. B Different autofluorescence intensities from the cell wall of the six microalgal strains.