Oxygenation by photosynthesis in a mixed culture of algae and nitrifiers was investigated for its potential to support nitrification. An open photo-bioreactor (1 L fermentor; 30 degrees C) was inoculated with an enriched culture of nitrifiers obtained from activated sludge and a pure culture of Scenedesinus sp. The reactor was illuminated (60 mu mol/m(2) s) and operated as a sequencing batch reactor with 50% discharge per cycle and sludge retention time of 15 or 30 days. Full nitrification of synthetic wastewater (50 mg NH4+-N/L) without mechanical aeration was achieved after less than 14 cycles (18.5 h React per 24 h cycle) and the biomass steadily increased until a concentration of 1.9 g VSS/L with 29 mg chl-a/L. The maximum observed nitrification rate was 7.7 mgNH(4)(+)-N/L h. A mass balance showed that ammonium removal was primarily by nitrification (81-85%) rather than by ammonium uptake by algae. (C) 2013 Elsevier Ltd. All rights reserved.