Alleviation of oxygen stress on Neochloris oleoabundans: effects of bicarbonate and pH

This study investigated the deleterious effects of oxygen stress in a range of 100-400 % air saturation and verified the alleviation effects of the addition of bicarbonate and adjustment of culture pH. In the cultures with imposed dissolved oxygen (dO(2)) stress at 400 % air saturation, the highest dry cell weight (DCWmax) of 1.04 +/- 0.17 g L-1, lipid content (LC) of 151.2 +/- 4.8 mg g(-1) DCW and lipid productivity (LP) of 19.1 +/- 0.8 mg L-1 day(-1) were obtained in the cultures supplemented with 160 mM NaHCO3, compared to a DCWmax of 0.76 +/- 0.018 g L-1, LC of 116.3 +/- 0.2 mg g(-1) DCW and LP of 9.96 +/- 0.02 mg L-1 day(-1) in the control (with 0 mM NaHCO3). The results indicate that oxygen stress was hazardous to cell growth and impaired lipid accumulation in Neochloris oleoabundans. The addition of bicarbonate alleviated oxygen stress through increasing the ratio of dCO(2) to dO(2) to balance the photosynthesis and photorespiration, inhibiting the production of reactive oxygen species (ROS) and activating cellular antioxidant mechanisms such as enhanced lipids as an electron sink and ROS-scavenging antioxidant enzymes. Studies also show that the increase of pH in the range of 7.5-9.5 enhanced cell growth and lipid accumulation, although accompanied by increasing ROS concentration in cultures.