Comparison of growth of Tetraselmis in a tubular photobioreactor (Biocoil) and a raceway pond

Abstract

Microalgae cultivation systems can be divided broadly into open ponds and closed photobioreactors. This study investigated the growth and biomass productivity of the halophilic green alga Tetraselmis sp. MUR-233, grown outdoors in paddle wheel-driven open raceway ponds and in a tubular closed photobioreactor (Biocoil) at a salinity of 7 % NaCl (w/v) between mid-March and June 2010 (austral autumn/winter). Volumetric productivity in the Biocoil averaged 67 mg ash-free dry weight (AFDW) L⁻¹ day⁻¹ when the culture was grown without CO₂ addition. This productivity was 86 % greater, although less stable, than that achieved in the open raceway pond (36 mg L⁻¹ day⁻¹) grown at the same time in the autumn period. The Tetraselmis culture in the open raceway pond could be maintained in semi-continuous culture for the whole experimental period of 3 months without an additional CO₂ supply, whereas in the Biocoil, under the same conditions, reliable semi-continuous culture was only achievable for a period of 38 days. However, stable semi-continuous culture was achieved in the Biocoil by the addition of CO₂ at a controlled pH of ~7.5. With CO₂ addition, the volumetric biomass productivity in the Biocoil was 85 mg AFDW L⁻¹ day⁻¹ which was 5.5 times higher than the productivity achieved in the open raceway pond (15 mg AFDW L⁻¹ day⁻¹) with CO₂ addition and 8 times higher compared to the productivity in the open raceway pond without CO₂ addition (11 mg AFDW L⁻¹ day⁻¹), when cultures were grown in winter. The illuminated area productivities highlight an alternative story and showed that the open raceway pond had a three times higher productivity (3,000 mg AFDW m⁻² day⁻¹) compared to the Biocoil (850 mg AFDW m⁻² day⁻¹). Although significant differences were found between treatments and cultivation systems, the overall average lipid content for Tetraselmis sp. MUR-233 was 50 % in exponential phase during semi-continuous cultivation.