Characterization of CO2 fixation on algal biofilms with an infrared gas analyzer and importance of a space-rich structure on the surface

An attached microalgal culture system, or a biofilm system, which can reduce the cost of biomass recovery, is an attractive approach to solve the CO2 problem. In this study, we developed a method for precisely measuring CO2 fixation on algal biofilms, and optimized the photosynthetic capacity of a unicellular microalga, Chlorella. The CO2 concentration in the sample gas, measured with an infrared gas analyzer, was corrected by considering the difference in H2O concentration between the sample gas and the reference gas. The corrected CO2 concentration coincided with that of the reference gas, for which there were no cells. Using the established method, the photosynthetic capacity of Chlorella attached to a glass fiber membrane filter was characterized. Interestingly, the CO2 fixation rate per chlorophyll was almost the same as in liquid, at cell densities of up to 120 mg chlorophyll m−2. The light-dependency on the filter was very similar to that in liquid, while the photosynthetic rate was more markedly limited under low CO2 conditions than in liquid, suggesting that CO2 is a more severe limiting factor than light on the filter. The cell concentration-dependency of photosynthesis and microscopic observations suggest that photosynthesis is only performed by cells in a few top layers on the filter. To improve the CO2 supply, the solid surface was changed to a towel, which increased the photosynthetic rate in spite of its low optical transparency. The maximum rate of CO2 capture per area was estimated to be over 110 tons ha−1 year−1. These findings suggest that the Chlorella solid-surface culture system is a promising system for carbon capture, and that it can be further improved if the solid surface is converted into a steric structure with a high optical transparency.