Balancing Photosynthesis and Respiration Increases Microalgal Biomass Productivity during Photoheterotrophy on Glycerol

Glycerol was used as an organic substrate to enhance the biomass production rate of a Dactylococcus microalga during photoheterotrophy while simultaneously reducing the need for cell culture gas exchange. Photoheterotrophic cultivations were carried out at concentrations of 6, 30, and 150 mM glycerol in parallel with photoautotrophic and heterotrophic cultivations. The highest biomass productivity was with 30 mM glycerol, the concentration where the net oxygen exchange between the cells and the culture medium was minimized, thus implying a balance between respiration and photosynthesis and internal recycling of O2 and CO2. The maximum specific growth rate and biomass productivity were subsequently increased by 43 ± 9% and 108 ± 16%, respectively, compared to the photoautotrophic cultivations. The net oxygen production rate could be modeled as a function of the light intensity, chlorophyll content, the photosynthetic efficiency as measured by PAM fluorometry, and the respiration rate. Glycerol addition decreased the cellular chlorophyll a content and the photosynthetic efficiency, but increased carbon fixation by respiration. These results show the addition of a waste product, e.g., glycerol from biodiesel, could be used as an algal bioprocess cosubstrate to reduce or eliminate the aeration energy demand while simultaneously increasing biomass production.