Deep ultraviolet light emitting diodes (LED) were studied in water disinfection using Asterionellopsis glacialis as a model microorganism. Ultraviolet treatment reactors were constructed and LEDs with peak emission wavelengths 256, 262, 268, 274 and 278 nm and combinations 256 + 262, 262 + 268, 268 + 274 and 274 + 278 were investigated. The photosynthetic activity of the algae was measured using PAM fluorometry and flow cytometry. Dose-based efficiency evaluation with PAM fluorometry did not present any significant distinction between different wavelength LEDs, best performance being with 256 + 262 combination (0.0017 cm(2)/mJ). Best performance using flow cytometry analysis was observed with 256 nm LEDs. Best time-based performance with PAM fluorometry was observed using 274 and 278 nm LEDs with inactivation constants of 0.0715 and 0.0680 1/min respectively. The combination 274 + 278 nm performed best according to flow cytometry analysis with inactivation rate constant of 0.1296 1/min. The dose-based efficiency of lower wavelength LEDs can be significantly compensated with higher output power, lower energy consumption and longer lifetime of LEDs with higher wavelengths. (C) 2016 Elsevier B.V. All rights reserved.