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Microalgae covers an extremely diverse type of unicellular microorganisms that use light and efficiently fix CO2 through the process of photosynthesis [...]

Large-scale microalgae cultivation for biofuel production is currently limited by the possibility of maintaining high microalgae yield and high lipid content, concomitantly. In this study, the physiological changes of Chlorella vulgaris NIES 227 during lipid accumulation under nutrient limitation was monitored in parallel with the photosynthetic capacity of the microalgae to fix carbon from the proxy of oxygen productivity. In the exponential growth phase, as the biomass composition did not vary significantly (approx. 53.6 ± 7.8% protein, 6.64 ± 3.73% total lipids, and 26.0 ± 9.2% total carbohydrates of the total biomass dry-weight), the growth capacity of the microalgae was preserved (with net O2 productivity remaining above (4.44 ± 0.93) × 10−7 g O2·µmol PAR−1). Under nutrient limitation, protein content decreased (minimum of approx. 18.6 ± 6.0%), and lipid content increased (lipid content up to 56.0 ± 0.8%). The physiological change of the microalgae was associated with a loss of photosynthetic activity, down to a minimum (1.27 ± 0.26) × 10−7 g O2·µmol PAR−1. The decrease in photosynthetic O2 productivity was evidenced to correlate to the cell internal-protein content (R2 = 0.632, p = 2.04 × 10−6, N = 25). This approach could serve to develop productivity models, with the aim of optimizing industrial processes.

Microalgae are an attractive source of biomass for fossil fuel alternatives and renewable energy sources. Regardless of their potential, the development of microalgal biofuels has been limited due to the associated economic and environmental costs. We followed and compared the biomass properties of six Chlorellaceae strains with a specific interest in lipid-based biofuels. The strains were cultivated under balanced nutrient limitation inducing a gradual limitation of nutrients that triggered reserve accumulation. The final biomass of each strain was characterized by its elemental and biochemical composition. Due to its high lipid content and overall composition, Chlorella vulgaris NIES 227 was identified as an ideal feedstock for biofuels with the best energy-content biomass. Its fatty acid profile also showed superior qualities for biodiesel production. Balanced nutrient limitation promoted not only the accumulation of storage compounds in all strains, but also resulted in a low content of heteroatom precursors and ashes for biofuel applications.

Arthrospira platensis (Spirulina) is the most cultivated microalga worldwide. Improving its cultivation in terms of biomass productivity, quality, or production cost could significantly impact the Spirulina industry. The objectives of this paper were defined as to contribute to this goal. Spirulina biomass productivity was investigated through medium choice. A modified Zarrouk’s medium was selected as it gave higher final dry weights and longer sustained growth than Hiri’s and Jourdan’s media. Then, in order to reduce Spirulina production cost, modified Zarrouk’s medium was rationalized by testing different dilutions. It was found that modified Zarrouk’s medium could be diluted up to five times without impacting the growth rates in a 28-days batch cultivation. Higher dry weights were even observed after 21 days of batch cultivation (1.21 g/L for 20%-modified Zarrouk’s medium in comparison to 0.84 g/L for modified Zarrouk’s medium). Iron uptake was then investigated as one of the major contributors to Spirulina nutritional quality. An increase in iron content was obtained by replacing iron sulfate by iron EDTA at a concentration of 10 mgFe/L (2.11 ± 0.13 mgFe/gbiomass for EDTA-FeNa, 3 H2O at 10 mgFe/L compared to 0.18 ± 0.13 for FeSO4,6H2O at 2 mgFe/L). Impact of light intensity on Spirulina biomass productivity was also investigated in a 2 L Photobioreactor (PBR). Specific growth rates were calculated for Photosynthetically Photon Flux Densities (PPFD) from 85 to 430 µmol/m2/s. At 430 µmol/m2/s, photoinhibition was not observed and the specific growth rate was maximum (0.12/day). Finally, a 40-day cultivation experiment was conducted in a 1000 L PBR giving a maximum daily areal productivity of 58.4 g/m2/day. A techno-economic analysis gave production cost two to 20 times higher for PBR (from 18.71 to 74.29 €/kg) than for open ponds (from 3.86 to 9.59 €/kg) depending on Spirulina productivity.