How to combine CO2 abatement and starch production in Chlorella vulgaris

Microalgae production has gained attention in recent years as promising systems for CO2 abatement as well as a source of proteins, pigments, vitamins, lipids, and carbohydrates. Particularly, starch can be used for bioethanol production in a well-established fermentative process. The aim of this work was to maximize and model biomass productivity and CO2 assimilation in continuous cultures of Chlorella vulgaris. The following culture parameters were studied: dilution rate, pH, temperature, light intensity, and nitrogen supply. The proposed model (r2 = 0.95) predicted a maximum biomass productivity of 0.7 g L−1 d−1 and CO2 assimilation of 1.3 g L−1 d−1. The experimental data agreed with these predictions, resulting in a maximum biomass productivity of 0.67 g L−1 d−1 (resulting in a CO2 assimilation of 1.23 g L−1 d−1). In addition, the starch content was determined, and the results were used as input into a second model, which aimed at predicting starch accumulation during CO2 abatement processes (r2 = 0.84). This second model predicted a daily and continuous production of biomass with a maximum starch content of 0.25 g g−1 d−1 (25% dcw), but under different culture conditions than those found for maximizing biomass productivity and CO2 assimilation. The maximum starch content experimentally determined was 0.2 g g−1 d−1 (20% dcw). Thus, to implement a biological system for CO2 abatement coupled to starch accumulation, it is necessary to find a compromise between these two processes. Hence, although yield in both processes would be reduced, a simultaneous process for CO2 mitigation and starch production would be feasible.