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. © 2013 Springer Science+Business Media Dordrecht.