Chlorella for protein and biofuels: from strain selection to outdoor cultivation in a Green Wall Panel photobioreactor

BACKGROUND: Chlorella is one of the few microalgae employed for human consumption. It typically has a high protein content, but it can also accumulate high amounts of lipids or carbohydrates under stress conditions and, for this reason, it is of interest in the production of biofuels. High production costs and energy consumption are associated with its cultivation. This work describes a strategy to reduce costs and environmental impact of Chlorella biomass production for food, biofuels and other applications. RESULTS: The growth of four Chlorella strains, selected after a laboratory screening, was investigated outdoors in a low-cost 0.25 m² GWP-II photobioreactor. The capacity of the selected strains to grow at high temperature was tested. On the basis of these results, in the nitrogen starvation trials the culture was cooled only when the temperature exceeded 40°C to allow for significant energy savings, and performed in a seawater-based medium to reduce the freshwater footprint. Under nutrient sufficiency, strain CH2 was the most productive. In all the strains, nitrogen starvation strongly reduced productivity, depressed protein and induced accumulation of carbohydrate (about 50%) in strains F&M-M49 and IAM C-212, and lipid (40 - 45%) in strains PROD1 and CH2. Starved cultures achieved high storage product productivities: 0.12 g L⁻¹ d⁻¹ of lipids for CH2 and 0.19 g L⁻¹ d⁻¹ of carbohydrates for F&M-M49. When extrapolated to large-scale in central Italy, CH2 showed a potential productivity of 41 t ha⁻¹ y⁻¹ for biomass, 16 t ha⁻¹ y⁻¹ for protein and 11 t ha⁻¹ y⁻¹ for lipid under nutrient sufficiency, and 8 t ha⁻¹ y⁻¹ for lipid under nitrogen starvation. CONCLUSIONS: The environmental and economic sustainability of Chlorella production was enhanced by growing the organisms in a seawater-based medium, so as not to compete with crops for freshwater, and at high temperatures, so as to reduce energy consumption for cooling. All the four selected strains are good candidates for food or biofuels production in lands unsuitable for conventional agriculture. Chlorella strain CH2 has the potential for more than 80 tonnes of biomass, 32 tonnes of protein and 22 tonnes of lipid per year under favourable climates.