Towards the novel concept of microalgae production in surfactant-stabilized foams in a liquid foam-bed photobioreactor

Resum

Microalgae are a promising renewable feedstock for biofuels and chemicals with a variety of market applications, including cosmetics, health, human food and animal feed. However, the commercial applications of microalgae on an industrial scale are limited by the high production costs. Microalgae are currently produced in open and closed photobioreactors where the algal cells grow in liquid suspensions, but alternative cultivation systems should be taken into consideration in order to achieve the economic feasibility of additional microalgae-based production processes. In this sense, cultivation in surfactant-stabilized foams could become a suitable alternative for cost-effective production of microalgal biomass and derived products. This novel cultivation concept offers several advantages over the conventional cultivation systems, which ultimately result in reduced water and energy consumption. However, due to its novelty, many questions on its applicability remain unanswered. In this sense, this Thesis provides insight on several aspects of this novel microalgae cultivation concept. First, this Thesis focused on the selection of microalga-surfactant combinations with potential to be employed in a liquid foam-bed photobioreactor (LF-PBR). For this, sets of criteria to independently select suitable microalgal strains (Chapter 3) and surfactants (Chapter 4) were established. These criteria included the foaming properties of the microalgal suspensions and the surfactants, the resistance of the surfactant to biodegradation and its toxicity to microalgae. From a total of 6 algal strains and 10 surfactants investigated, cultivation of Chlorella sorokiniana in Pluronic F68-stabilized foams showed the largest potential according to all the aforementioned selection criteria. Besides, the biodegradability of several surfactants by bacteria naturally associated to microalgae was investigated in more detail (Chapter 5). As a result, it was concluded that biodegradable surfactants are not suitable for their application in a LF-PBR due to their inability to maintain a stable foam production. Furthermore, microalgae cultivation in surfactant-stabilized foams was expected to potentially impact the quality of the biomass and its posterior applications due to possible interactions of the surfactant with the cells and/or the particular characteristics imposed by the ‚foam environment‛ (i.e. high CO2 availability). To assess this, the effects of Pluronic F68 on the biochemical composition of C. sorokiniana were investigated in liquid and foam-based cultures (Chapter 7). For the later, a simple, easy-to-build and easy-to-operate lab-scale LF-PBR was designed and constructed (Chapter 6). Cultivation of C. sorokiniana in this LF-PBR showed much more rapid growth compared to liquid cultures, biomass with a distinct biochemical composition and an increased stress response. Our results indicate that, beyond the surfactant effect, the physicochemical conditions of the ‚foam environment‛ have an effect on the metabolism of C. sorokiniana. Besides, a first approach was made to assess the feasibility of using a fertilizer-based culture medium (Chapter 8), which could reduce the production costs further. The selection of an appropriate NPK fertilizer seems to be a promising tool to achieve sustainable production of microalgae and derived products in liquid foams. In addition, since surfactants can act as metabolites extractants and lytic agents, the biocompatibility of Pluronic F68 was investigated and demonstrated for C. sorokiniana (Chapter 9). Overall, the results obtained in this Thesis indicate a high potential of the LF-PBR for cost-effective microalgal biomass production with distinct metabolites profile and it gives a first insight on the main potentialities and challenges of this novel algal cultivation concept.