The effect of partial oxygen pressure on growth of Neochloris oleoabundans was studied at sub-saturating light intensity in a fully-controlled stirred tank photobioreactor. At the three partial oxygen pressures tested (P-o2 = 0.24; 0.63; 0.84 bar), the specific growth rate was 1.38; 1.36 and 1.06 day(-1), respectively. An increase of the P-co2 from 0.007 to 0.02 bar at Po-2 of 0.84 bar resulted in an increase in the growth rate from 1.06 to 1.36 day(-1). These results confirm that the reduction of algal growth at high oxygen concentrations at sub-saturating light conditions is mainly caused by competitive inhibition of Rubisco. This negative effect on growth can be overcome by restoring the O-2/CO2 ratio by an increase in the partial carbon dioxide pressure. In comparison to general practice (P-o2 = 042 bar), working at partial O-2 pressure of 0.84 bar could reduce the energy requirement for degassing by a factor of 3-4. (C) 2011 Elsevier Ltd. All rights reserved.