Lumostatic strategy for microalgae cultivation utilizing image analysis and chlorophyll a content as design parameters Xue Chen, Qianru Yvonne Goh, Weifeng Tan, Iqbal Hossain, Wei Ning Chen, Raymond Lau ⇑ Nangyang Technological University, Singapore School of Chemical and Biomedical Engineering, 62 Nanyang Drive, Singapore 637459, Singapore article info Article history: Received 18 December 2010 Received in revised form 12 February 2011 Accepted 15 February 2011 Available online 19 February 2011 Keywords: Microalgae Draft-tube photobioreactor Lumostatic strategy Image analysis Chlorophyll a abstract Cultivation of microalgae Chlorella sp. was performed in draft-tube photobioreactors. Effect of light inten- sity on the microalgae growth performance was conducted under a light intensity range of 82–590 lmol/ m 2 s. A lumostatic strategy was proposed based on the light distribution profiles obtained by image anal- ysis and specific chlorophyll a content. The proposed lumostatic strategy allowed a maximum biomass dry weight of 5.78 g/L and a productivity of 1.29 g/L d, which were 25.7% and 74.3% higher than that achieved by the optimal constant light intensity, respectively. A comparison with other lumostatic strat- egies reported in the literature indicated that the proposed lumostatic strategy in the current study can be a promising approach in improving the growth of microalgae. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Microalgae have gained considerable attention and have been studied over the last decade by virtue of their high productivity, carbon sequestration potential and oil yield (Córdoba-Matson et al., 2010; Degrenne et al., 2011; Jena et al., 2011; Kliphuis et al., 2010). Microalgae are found to have higher photosynthetic efficiencies than most photoautotrophic organisms and have 10–50 times higher carbon removal efficiencies than other terres- trial plants (Li et al., 2008). Mass cultivation of microalgae has been employed in the production of valuable biochemicals such as asta- xanthine, b-carotene, pigments, vitamins, and polyunsaturated fatty acid (Chisti, 2007; Das et al., 2011; Kang et al., 2010; Spolaore et al., 2006). This technology has also been widely used in such environmental applications as carbon sequestration, waste water treatment and production of hydrogen and ethanol (Akkerman et al., 2002; Chiu et al., 2008; Richmond, 2003; Shi et al., 2007). In addition, microalgae typically have more than 30% oil yield and possess a great potential as a feedstock of renewable fuels such as biodiesel (Ahmad et al., 2011; Chinnasamy et al., 2010; Hsieh and Wu, 2009; Pittman et al., 2011; Singh et al., 2011). Light is an important factor in photosynthesis and is essential for microalgae photoautotrophic growth. When there is insuffi- cient light, the growth of microalgae is under photolimitation con- dition. In this state, an increase in the light intensity improves the microalgae growth until a saturation light intensity. The growth of microalgae becomes inhibited when the light intensity is increased beyond the saturation light intensity. The microalgae growth is then considered to be under photoinhibition condition (Adir et al., 2003; Ragni et al., 2008). Mass cultivation of microalgae is normally carried out under constant light intensity. However, at the beginning of the cultiva- tion period, the light intensity used may be so high that the low concentration microalgae cells are placed under a state of photoin- hibition. On the other hand, the growth of microalgae to a moder- ately dense broth can cause severe light attenuation due to mutual shading. In this case, the microalgae cells in the zone which is far away from the light source can be under a state of photolimitation. A lumostatic operation is thus developed to improve the micro- algae growth by optimizing the light energy supply. In a lumostatic operation, light is supplied in a continuous increasing manner in order to reduce both photoinhibition and photolimitation (Choi et al., 2003; Das et al., 2011; Das and Obbard, 2011; Kang et al., 2010; Lee et al., 2006a,b; Park and Lee, 2001; Suh and Lee, 2001; Wahal and Viamajala, 2010; Yoon et al., 2008a). Various parame- ters have been used to determine the ideal light intensity needed in the lumostatic operation. However, the design parameters all in- volve the use of indirect biological parameters, such as cell growth rate and light intensity flux. The accuracy and applicability of these lumostatic operations to other systems may be limited. Chloro- phyll is an essential component of the chloroplast that is responsi- ble for photosynthesis. Among various forms of chlorophyll, chlorophyll a is the principle pigment for the conversion of light energy to chemical energy. Chlorophyll a content is found to have 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.02.061 ⇑ Corresponding author. Tel.: +65 6316 8830; fax: +65 6794 7553. E-mail address: wmlau@ntu.edu.sg (R. Lau). Bioresource Technology 102 (2011) 6005–6012 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech