BACKGROUND: A fundamental step in assessing the viability of a CO 2 biofixation system based on microalgae is to identify the maximum CO 2 biofixation yield that can be achieved for this microorganism when it is cultivated under optimum operational growth conditions. Response surface methodology was applied to determine optimum culture conditions for CO 2 biofixation by a recently isolated freshwater cyanobacterium Synechocystis sp. The strain was cultivated in a 1 L bubble column photobioreactor, in semicontinuous mode.
The growth rate and CO 2 biofixation rate of a photosynthetic organism depend basically on the availability of light, all other factors being optimum. In dense cultures of cyanobacteria or micro-algae intended for biomass production, incident irradiance on the reactor surface is not the same as the intensity which is received by cells, as irradiance is attenuated by cell absorption and the self-shading effect. In a well-mixed, dense culture, only the average irradiance, I av , can be considered responsible for the photosynthetic response. In this study, the photosynthetic response of Synechocystis sp., estimated from its specific growth rate, was measured for each I av in batch cultures irradiated with different levels of external irradiance, I ext . The specific growth rate of Synechocystis sp. depends on I av , in accordance with the model proposed by Muller-Feuga (J Exp Mar Biol Ecol 236:1-13, 1999). A non-linear regression analysis estimated a maximum specific growth rate of 0.108 h −1 , at an I av of 930 μmol photons·m −2 ·s −1 . This reveals that Synechocystis sp. is a highly light-tolerant strain, suitable for outdoor cultures. Higher I av levels caused photoinhibition in batch cultures. Parameters obtained from the Muller-Feuga model show that the minimum irradiance needed to start growth mechanisms becomes less as light availability decreases, i.e. cells become more efficient in the use of light when it is scarce. This observation suggests that choosing for lowlight adaptation may be a good strategy to improve productivity in dense cultures, where light is a limiting factor.
One of the technologies available for coping with problems related to the rise in atmospheric concentrations of carbon dioxide is CO2 biofixation with microalgae or cyanobacteria. The selection of native strains that grow well at the specific site where the technology is to be used will increase the success possibilities of such a technology. Thus, with the aim of finding a suitable local variety for use in a CO2 biofixation system, three recently isolated freshwater strains, Scenedesmus sp., Chlorogonium sp. and Synechocystis sp. were studied. Chlorella sorokiniana was used as a control strain. All the strains were grown under the same culture conditions for seven days of batch culture, and various growth and CO2 biofixation parameters were determined. Synechocystis sp. showed the highest specific growth rate at 1.75 per day (l/d). Results for CO2 biofixation ranged between 0.650 and 0.953 g of carbon dioxide per litre per day (g CO2/l/d), but differences among native strains were noted, although they were not statistically significant. However, Synechocystis sp. was selected as the most suitable strain for CO2 biofixation, owing to its good capacity to use light in dense cultures, an essential requirement for sustainable commercial systems.
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