Effect of temperature and dissolved oxygen on gravity sedimentation of the unicellular alga Dunaliella salina

Abstract Purpose The microalgae Dunaliella salina is mainly cultivated as a source of natural β-carotene, but it can also be an important source of other nutrients such as protein, carbohydrate, and lipids. Harvesting and dewatering are considered the most expensive processes in the biomass production, so we proposed gravity sedimentation as a cost-effective method. The effect of temperature and dissolved oxygen concentration on the gravity sedimentation of normal cells and carotenoid-accumulating cells of Dunaliella salina was investigated in this study. Methods Dunaliella salina was cultivated in an f/2 culture medium at two different salinities, 12.5% (2.2 M NaCl) and 17.5% (3.3 M NaCl). Carotenoid-accumulating cells were cultivated in a medium without a nitrogen source by removing NaNO3 from the f/2 medium and at two different salinities, 12.5% and 17.5%. For gravity sedimentation tests, 10 mL of the suspended culture media were transferred to conical tubes and wrapped with aluminum foil for shading. The tubes were incubated at 4 °C, 10 °C, 15 °C, 20 °C, 25 °C, 30 °C, 35 °C, 40 °C, 45 °C, and 50 °C for 4 h and 18 h. For the gravity sedimentation under aerobic and anaerobic conditions, 10 mL of the suspended culture media were transferred to conical tubes and then purged with 20% oxygen and 80% nitrogen or 100% nitrogen for 10 min and wrapped with aluminum foil. The tubes were incubated at 25 °C for 0.5 h, 1 h, 2 h, and 3 h. Result Recovery rates differed with temperature, salinity, and time. The recovery rate of normal cells and carotenoid-accumulating cells reached 79 to 96% at 20 °C, 25 °C, and 30 °C at 12.5% and 17.5% of salinity and after 4 h. The recovery rate of both normal and carotenoid-accumulating cells in gravity sedimentation experiments was not significantly affected by changing initial dissolved oxygen in the cell-cultured media. Conclusion Considering that gravity sedimentation at the optimum growth temperature for D. salina presented high recovery rates, gravity sedimentation is expected to be a cost-effective method to harvest D. salina from open pond cultivation systems.