Photobioreactor scale-up for a shear-sensitive dinoflagellate microalga
Introduction
Dinoflagellates are one of the several divisions of microalgae. Dinoflagellates occur both in freshwater and in the seas. Although microalgae have an established history of commercial use and new uses are emerging [1], this is not the case for dinoflagellates. Dinoflagellates produce many structurally complex bioactive compounds of potential commercial interest [2], but development of marketable products from them has proved difficult. Small quantities of bioactives derived from dinoflagellates can be purchased, but generally at a prohibitively high price and in low purity. Unlike the other microalgae that are commonly grown in large-scale photobioreactors [3], [4], [5], [6], many dinoflagellates appear to be overly sensitive to the myriad of hydrodynamic forces that are encountered in the turbulent environment of a typical photobioreactor.
Photosynthetic marine dinoflagellates respond to hydrodynamic shear forces in diverse ways [7]. Turbulence sensitivity has been studied for several dinoflagellate species, but invariably from an ecological perspective that may be relevant in the natural habitat, but not in photobioreactor culture. Small scale studies that are potentially relevant to photobioreactor culture have been reported using the highly sensitive dinoflagellate Protoceratium reticulatum. This microorganism is damaged by shear rate values as low as 0.1 s−1 [8]. It is therefore a good model system for studies of photobioreactors that are intended for culturing highly shear sensitive microalgae. At operational scales of 2 L or less, the use of modified turbulence regimens and protective additives in the culture medium have been shown to reduce turbulence-associated damage to P. reticulatum [8], [9], [10]. How this successful operational capability at small scale might translate to a larger photobioreactor has not been previously addressed for dinoflagellates.
Scale-up of photobioreactors is complicated by a nonhomogenous distribution of light within the culture as a consequence of the self-shading by cells. For microalgae that are physically robust, increased intensity of agitation can be used to limit the time that a cell spends uninterrupted in the relatively dark interior of the bioreactor. This strategy is not workable with highly fragile dinoflagellates. Therefore, photobioreactor design and scale up methods that have proved successful for many relatively robust microalgae [3] cannot be directly translated to culturing the fragile dinoflagellates. This work reports on scale up of P. reticulatum culture from a previously reported 2 L stirred-tank photobioreactor [7], [9] to a 15 L photobioreactor.
Section snippets
Species and culture medium
Nonaxenic monocultures of the red-tide dinoflagellate Protoceratium reticulatum (GG1AM) were used. This yessotoxins (YTXs)-producer was obtained from the Culture Collection of Harmful Microalgae of IEO, Vigo, Spain. Inocula were grown in filter sterilized (0.22 μm Millipore filter) L1 medium prepared in Mediterranean Sea water [11]. The alga was grown at 18 ± 1 °C under a 12:12 h light–dark cycle. During the photoperiod, the average irradiance at the surface of the cultures was 100 μE m−2 s−1. Four
Results and discussion
Scale-up of mechanically mixed photobioreactors for culturing dinoflagellates was the focus of this work. The scale up study was informed by prior experience of a successful culture in a 2 L photobioreactor, the 2L-STB [7], [9], and the experimentally established limits on shear stress tolerance of P. reticulatum in shake flasks [8], [15]. First, the fluid-dynamics in the 2L-STB were characterized under different operational regimens that had previously proved successful for P. reticulatum
Concluding remarks
Principles for the successful scale-up of a stirred tank photobioreactor for growing the highly fragile dinoflagellate P. reticulatum were assessed. Successful scale-up from a 2 L operation to a 15 L photobioreactor was achieved by increasing the aspect ratio of the reactor to 4.5-fold of the value at the small scale. A relatively large axial flow impeller rotating at a slow speed was used for mixing. Scale up methods for photobioreactors remain largely undeveloped [32]. Although maintaining of
Acknowledgements
This research was supported by the Spanish Ministry of Science and Innovation (CTQ2008-06754-C04-02/PPQ) and the General Secretariat of Universities, Research and Technology of Andalusian Government (TEP-5375).
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