Biofixation of carbon dioxide by Spirulina sp. and Scenedesmus obliquus cultivated in a three-stage serial tubular photobioreactor
Introduction
The growth in human population has stimulated the search for alternative food sources and new ecological technologies. Microalgae and cyanobacteria use solar light as their main source of energy, possess the potential for high productivity, are tolerant to alterations in environmental conditions and they can be cultivated in areas which are unstable for agriculture (Costa et al., 2000). The cultivation conditions of these organisms can be manipulated to induce the production of proteins, fatty-acids, vitamin A, minerals, pigments and other bio-compounds and their biomass can be used as a dietary supplement for humans and animals, including for aquaculture (Ono and Cuello, 2004).
Photosynthetic microorganisms use inorganic carbon for growth and hence can be used for the biofixation of carbon dioxide, the principal greenhouse gas. According to the French National Center for Scientific Research (CNRS), the level of atmospheric carbon dioxide has historically been between 180 and 260 ppm but during the last 100 years the atmospheric concentration of this gas has risen to between 260 and 380 ppm (Siegenthaler et al., 2005), mainly due to burning fossil-fuels associated with increased population and industrialization (Chang and Yang, 2003). Coupling the cultivation of photosynthetic microorganisms with the biofixation of carbon dioxide has the potential not only to reduce the costs of culture media for growing such organisms on an industrial scale but also to offset carbon emissions (Beneman and Hughes, 1997).
Photosynthetic microorganisms are normally cultivated in open raceway tanks using natural or artificial light but this requires large cultivation areas and suffers from various disadvantages, including difficulty in controlling cultivation conditions (Costa et al., 2006), evaporation of the cultivation medium and reduced light intensity with increased depth. An alternative is the use of tubular photobioreactors, but Luo et al. (2003) have pointed out that the configuration of such reactors is one of the main factors controlling the biomass productivity of photosynthetic cultures grown under these conditions.
Appropriately designed photobioreactors can reduce the cultivation area by distributing photosynthetic organisms vertically, vertical tubular photobioreactors also increasing the carbon dioxide residence time in the cultivation medium and, consequently, the carbon dioxide utilization efficiency (Ono and Cuello, 2004). Cultivations can also be carried out in serial photobioreactors in which unused effluent carbon dioxide from one photobioreactor is fed into another photobioreactor.
The objective of the work described in this paper was to cultivate the photosynthetic microorganisms Scenedesmus obliquus and Spirulina sp. in serial tubular photobioreactors and determine carbon dioxide fixation and their resistance to carbon dioxide limitation and excess.
Section snippets
Microorganisms and cultivation media
The Spirulina sp. (Cyanobacteria, Oscillatoriales) and S. obliquus (Chlorophyta, Chlorophyceae) (de Morais and Costa, 2007) were from stock cultures kept in our laboratory. We used carbon-free media for the maintenance and cultivation of both organisms, modified Zarrouk medium (de Morais and Costa, 2007, Zarrouk, 1966) for Spirulina sp. and MC medium (Watanabe, 1960) for S. obliquus. For the experiments, inoculum of both organisms were acclimatized to carbon dioxide by maintaining them for 7
Results
The maximum specific growth rate and maximum biomass concentration values for S. obliquus and Spirulina sp. growing in the three-stage serial tubular photobioreactors in the presence of three different concentrations of carbon dioxide are shown in Table 1 and the growth curves of biomass versus time are shown in Fig. 2, Fig. 3, Fig. 4.
In the absence of carbon dioxide Spirulina sp. runs biomass increased until day 5 of cultivation (Fig. 2a), with no significant difference (p > 0.6004) between the
Discussion
When fossil fuels are burned they produce about 12% of carbon dioxide (Lee et al., 2002) and it has been reported that cultivation of microalga could fix carbon dioxide equivalent to 500 MW of energy (Kadam, 2002). Our results show that both Spirulina sp. and S. obliquus can grow in media supplied with air containing this concentration of carbon dioxide, indicating that these organisms could be used to sequester the carbon dioxide produced by combustion gases from thermoelectric power stations.
Conclusions
This research shows the high carbon dioxide biofixation potential of the cyanobacteria Spirulina sp. as indicated by its higher kinetic parameters and mean and maximum daily fixation rates as compared to the microalga S. obliquus. For Spirulina sp. the highest biomass concentration was 3.40 g L−1 with 6% carbon dioxide and 3.50 g L−1 with 12% carbon dioxide, while the highest maximum specific growth rate was 0.44 d−1 and the highest maximum productivity was 0.22 g L−1 d−1, both in the presence of 6%
Acknowledgements
The authors thank the Centrais Elétricas Brasileiras S.A. (ELETROBRÁS) and the Companhia de Geração Térmica de Energia Elétrica (CGTEE) for financially supporting this work.
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