Removal of nitrogen from wastewater without using electricity consuming aerators was previously observed in photo-bioreactors with a mixed algal-bacterial biomass. Algammox is the particular process based on algae, ammonium oxidizing organisms and anammox bacteria. In this research the activity of anammox bacteria in such an oxygen-producing environment was tested, as well as the effect of short-duration increase in dissolved oxygen (DO) to values potentially inhibiting anammox activity. Sequencing batch photo-bioreactors were fed with settled domestic wastewater enriched with ammonium (200 mg NH₄ ⁺-N/L) and exposed to light within the photosynthetic active range with intensity of about 500 μmol/m²·s. Each cycle consisted of 12 h illumination and 12 h darkness. A well-settling biomass (10 days solids retention time) developed that carried out nitritation, nitrification and anammox. Ammonium removal rate during the light period was 4.5 mg N-NH₄ ⁺/L·h, equal to 858 mg N-NH₄ ⁺/m²·h or 477 mg N-NH₄ ⁺/(mol photons). When the reactors were aerated for 3 h to temporarily increase the DO, anammox was inhibited at bulk DO values larger than 0.4–1.0 mg/L. For almost oxygen saturated conditions, recovery time was about 9 days. Algammox photo-bioreactors are therefore able to overcome short periods of oxygen stress, provided they occur only occasionally.