ARTICLE Limits to Productivity of the Alga Pleurochrysis carterae (Haptophyta) Grown in Outdoor Raceway Ponds Navid Reza Moheimani, Michael A. Borowitzka Algae Research Group, School of Biological Sciences and Biotechnology, Murdoch University, Murdoch, WA 6150, Australia; telephone: þ61 8 9360 2333; fax: þ61 8 9360 6830; e-mail: m.borowitzka@murdoch.edu.au Received 28 March 2006; accepted 4 August 2006 Published online 31 August 2006 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/bit.21169 ABSTRACT: This study examined the effects of oxygen concentration, pond temperature and irradiance on pro- ductivity and CaCO 3 formation of the coccolith-forming alga, Pleurochrysis carterae CCMP647 grown in semi-con- tinuous culture in outdoor raceway ponds. During the day the oxygen content of the pond increases markedly and P. carterae photosynthesis is inhibited by these high O 2 con- centrations with the inhibition increasing with increasing temperature. The high irradiance outdoors presents less of a problem to photosynthesis and productivity as the algae can acclimate well to high irradiances over a period of several weeks. Pond depth also effects productivity and this effect varies with season. During autumn, productivities were highest at depths of 13 to 16 cm, and decreased when the depth was increased. During summer productivity was much lower at 13 cm pond depth and increased when the depth was increased to 16, 18 and 21 cm. Heating the ponds in the morning by approximately 3 to 58C improves pro- ductivity by 11%–21%, presumably because this allows the algae to photosynthesise faster in the conditions of low [O 2 ] which occur in the early morning. Biotechnol. Bioeng. 2007;96: 27–36. ß 2006 Wiley Periodicals, Inc. KEYWORDS: coccolithophore; photosynthesis; photoaccli- mation; photoinhibition; oxygen concentration; temperature Introduction The cultivation of photosynthetic microorganisms such as algae and cyanobacteria has been proposed as an option for CO 2 bioremediation (Herzog and Drake, 1996). Algae are attractive organisms for CO 2 bioremediation since they have a very high areal productivity when compared to other photosynthetic organisms such as trees (Benemann, 1997; Richmond, 1999; Tredici and Materassi, 1992). The coccolithophorid algae have the further advantage that they fix C not only into organic biomass but also produce CaCO 3 in the form of extracellular scales known as coccoliths (Brownlee and Taylor, 2004) and this CaCO 3 can be buried to ‘fossilise’ the C fixed. Furthermore, coccolithophorid algae also produce high amounts of lipids which have a potential application as renewable fuel. The coccolithophorid algae are common members of marine phytoplankton and are periodically known to form extensive blooms in the ocean under certain conditions (Tyrell and Merico, 2004) and the coccolithophorid, Emilianea huxleyi, is a significant global long-term sink of inorganic carbon (Westbroek et al., 1994). The long-term outdoor large-scale culture of coccolithophorid algae has recently been shown to be possible (Moheimani and Borowitzka, 2006), however, for commercially viable application of cultures of these algae for CO 2 bioremedia- tion or for the production of renewable fuels, sustainable high productivities must be achieved. The productivity of outdoor microalgae cultures in open ponds has been shown to be limited by a number of factors including: (1) physical factors such as light (quality and quantity), temperature, nutrients, O 2 and CO 2 ; (2) biotic factors including pathogens, predation and competition by other algae; and (3) operational factors such as: shear produced by mixing, dilution rate, depth and harvest frequency (Borowitzka, 1998). Management of parameters such as pond depth, cell density, turbulence and the dilution cycle, all of which affect the amount of light received by the cells, is possible in large outdoor culture ponds and optimisation of these parameters allows better use of sunlight, resulting in higher productivities (Richmond and Becker, 1986; Vonshak and Richmond, 1988). Correspondence to: M.A. Borowitzka ß 2006 Wiley Periodicals, Inc. Biotechnology and Bioengineering, Vol. 96, No. 1, January 1, 2007 27