Bio-oil production by Stichococcus strains in laboratory scale photobioreactors

Biofuels production by microalgae appeared promising since the end of the last century. Microalgae may be burned/gasified as crude dry matter in combustors/gasifiers or may be processed to produce liquid fuels. Bio-oil extracted from microalgae may be adopted as crude fuels or may be transesterified to biodiesel. Key aspects for industrialization of microalgae-to-biofuel processes are: selection and improvements of algal strains through genetic and metabolic engineering; development of high-performance photobioreactor; improvement of oil extraction and transesterification processes; development of microalgae biorefineries. The present study reports results of an experimental campaign consisting of the autotrophic culture of Stichococcus strains in 0.8 L lab-scale bubble columns operated as batch photobioreactors. Air was sparged at the bottom through a porous ceramic distributor. Microalgae growth and lipid production were investigated at 23°C and 700 E/(m2s) light irradiance. Bold basal medium (BBM) supplemented with NaNO3 as nitrogen source was adopted. The culture was characterized in terms of pH and concentration of total nitrogen (TN), total organic carbon (TOC), total inorganic carbon (IC) and biomass. Lipids were extracted from dried microalgae and transesterificated with alkaline methanol. The lipid content/distribution of the bio-oil was assessed by HPLC analysis. The effects of the air volumetric flow rate (Qv) were investigated: the rate ranged between 0.4 and 4 vvm. Photobioreactors were operated under fed-batch conditions to support biomass growth. Best performances were observed when Stichococcus bacillaris culture was carried out at Qv=0.4 vvm (Fig.1). Typically, pH increased from the buffered value of BBM (6.8) up to 8. Accordingly, the IC content increased due to the favourable shift of the CO2/HCO3-/CO3-- equilibriums. Periodic addition of BBM stimulated the biomass growth up to about 10 OD. Notwithstanding the fast nitrogen consumption, the TN accumulated in the liquid phase. The TOC increased in the liquid phase up to 450 mg/L, suggesting protein secretion. The lipid content/distribution changed with the culture age and N-starvation time.