Abstract: The influence of irradiance on the nitrifying activity in photobioreactors of a bacterial consortium enriched from a wastewater treatment bioreactor was assessed using independent ammonium oxidation kinetic batch tests and respirometric assays. Culture irradiance below 250 μmol m(-2) s(-1) did not show a significant effect on nitrification activity, while irradiance at 500 and 1250 μmol m(-2) s(-1) caused a decrease of 20 and 60% in the specific total ammonium nitrogen removal rates and a reduction of 26 and 71% in the specific NO3- production rates, respectively. However, no significant influence of irradiance on the affinity constant of NH4+ oxidation was observed. The increasing nitrite accumulation at higher light intensities suggested a higher light sensitivity of nitrite oxidizers. Additionally, NH4+ oxidation respirometric assays showed a decrease in the oxygen uptake of 14 and 50% at 500 and 1250 μmol m(-2) s(-1), respectively. The experimental determination of the light extinction coefficient (lambda) of the nitrifying bacterial consortium (lambda = 0.0003 m(2) g(-1)) and of Chlorella sorokiniana (lambda = 0.1045 m(2) g(-1)) allowed the estimation of light penetration in algal-bacterial high rate algal ponds, which showed that photo inhibition of nitrifying bacteria can be significantly mitigated in the presence of high density microalgal cultures. 2016 Elsevier Ltd. All rights reserved.