The use of biogas for grid injection or vehicle fuel requires purification steps to obtain biomethane, process normally called biogas upgrading. The use of microalgae cultUres has been proposed as a new alternative for CO2 removal from biogas. Full-scale systems for biogas upgrading using microalgae should be able to deal with natural existing day/night photoperiods. This research evaluated the effect of a light/ dark photoperiod on the operation of a photosynthetic biogas upgrading system, at lab -scale conditions. A system based on an open-photobioreactor connected to a mass transfer column was Used for that purpose. Using a continuous biogas flow, an upgraded biogas with a CO2 concentration between 2 and 4.5% was obtained throughout light and dark periods. O-2 concentrations below 1% in final biogas were observed. Mass balances showed that CO2 desorption was the main process behind CO2 removal. CO2 removal during the dark phase was possible, under the tested conditions, as a result of inorganic carbon desorption from the photobioreactor and accumulation in the liquid phase. (C)2017 Elsevier Ltd. All rights reserved.