In situ carbon supplementation in large-scale cultivations of Spirulina platensis in open raceway pond

The objective of this study was to estimate the CO₂ absorptivity provided by an in situ carbon supply system using a photosynthetic culture of the cyanobacterium Spirulina platensis in an open raceway pond. The effects of initial total carbon concentrations (ranging from 0 to 0.1 mol/L), suspension depths (ranging from 5 to 20 cm) and pH values (ranging from 8.9 to 11.0) on the CO₂ absorptivity were studied. The results indicated that CO₂ absorptivity was positively correlated with pH value, negatively correlated with total carbon concentration, and only negligibly affected by the suspension depth. The optimum total carbon concentration range and pH range were 0.03~0.09 mol/L and 9.7~10.0, respectively. An average CO₂ absorptivity of 86.16% and average CO₂ utilization efficiency of 79.18% were achieved using this in situ carbon-supply system in large-scale cultivation of Spirulina platensis, with an initial total carbon concentration of 0.06 mol/L and pH value of 9.8. Our results demonstrated that this system could obtain a favorable CO₂ utilization efficiency in outdoor, large-scale cultivation of Spirulina platensis in open raceway ponds.