Short CommunicationTwo-stage heterotrophic and phototrophic culture strategy for algal biomass and lipid production
Highlights
► Develop a novel two-stage heterotrophic and phototrophic algae culture strategy. ► Investigate the growth kinetics of phototrophic and heterotrophic cultures. ► Reduce cost by using food waste and wastewater. ► Study performances of heterotrophic algal seed under phototrophic conditions. ► Control contamination with large volume inoculation of heterotrophic seeds.
Introduction
Algal lipid is considered as an ideal feedstock for transportation fuels (Pienkos and Darzins, 2009). However, prior to industrial scale application, a series of key challenges have to be resolved. For example, in northern climates, phototrophic biomass production is limited in the winter because of the cold temperature and lack of available sunlight. Even in summer months, open algae cultures have relatively low growth rates and biomass productivity (Chisti, 2007). Usually, light limitation is the major limiting factor, since light penetration is inversely proportional to the cell concentration (Chen, 1996). Although a relatively higher biomass productivity can be achieved in photobioreactors (PBRs), its high cost in facility and operation leads to a lower economical viability than open pond (Chisti, 2007). Additionally, open systems are continuously threatened by invading species, such as undesired algae and bacteria. Due to these problems, phototrophic algae are only commercially used to produce high value products (Spolaore et al., 2006). Large scale culture of phototrophic algae for biofuel production still has too high production costs, compared to the produced value (Pienkos and Darzins, 2009).
Compared to phototrophic growth, heterotrophic algae culture takes advantage of fast growth, high production rate, and convenient harvesting. A series of heterotrophic microalgae species were successfully used in industry-scale polyunsaturated fatty acids production (Chi et al., 2009). Recently, heterotrophic microalgae culture to produce biodiesel was reported and showed its promise, however, a high cost of organic carbon is one of limiting factors for this process (Chi et al., 2011).
To develop an efficient phototrophic process and overcome potential contamination issues, we have investigated a process that takes advantages of both heterotrophic culture’s high efficiency and phototrophic culture’s low cost. In this two-stage heterotrophic and phototrophic culture process, heterotrophic culture provided an efficient way for seed cells production, which can be used as inoculums in the subsequent phototrophic open pond cultivation for algal biomass and lipid production.
Section snippets
Organism and medium
Chlorella sorokiniana (UTEX 1602) was obtained from the Culture Collection of Alga at the University of Texas (Austin, TX, USA). Kuhl medium was used for phototrophic culture (Kuhl and Lorenzen, 1964). Heterotrophic culture was supplemented with different concentrations of glucose, as indicated in individual experiments.
Culture conditions
Flask cultures were conducted in 0.25-L Erlenmeyer flasks. Phototrophic cultures contained 0.2 L Kuhl medium and were bubbled with air supplemented with 0.9% CO2 at a rate of 0.08
Heterotrophic and phototrophic culture of C. sorokiniana for seed cells production
Flasks cultures were conducted to investigate the effect of initial glucose concentrations on the alga C. sorokiniana growth. As shown in Table 1, the final algal cell density increased when glucose went up from 5 to 20 g L−1, but the high concentration glucose 40 g L−1 showed inhibitory effects. At 20 g L−1 glucose, C. sorokiniana reached the highest growth rate, cell density and productivity of 1.48 d−1, 397 × 106 cells mL−1 and 182 × 106 cells mL−1 d−1, respectively. Compared with heterotrophic growth,
Conclusions
This study investigated a two-stage heterotrophic and phototrophic strategy for algal biomass and lipid production. Heterotrophic culture can be used as a better process to produce seed cells for the alga C. sorokiniana in large scale open systems, since it had much higher productivity but similar performance compared with its phototrophic counterpart. Organic waste and municipal wastewater can be utilized as good feedstock for this heterotrophic process. In addition, high inoculation rate of
Acknowledgements
Authors acknowledge the financial support from Boeing Company. Authors also thank Jim O’Fallon for his assistance with the algal biomass analysis.
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