Microalgal photoautotrophic cultivation present limits related to the high operative and investment costs required to maintain adequate light supply rate (reactors with high S/V ratio) to ensure satisfactory productivity [1]. Heterotrophy represents a promising solution: some microalgal strains can grow without light, metabolizing organic compounds as carbon and energy source [1,2]. Moreover, the integration of heterotrophic microalgal production with wastewater or effluents treatment can increase economic sustainability of the microalgal biomass production, eliminating the cost of carbon feedstock required in heterotrophy [3]. Through gas fermentation, some acetogens bacteria convert CO2 and H2 into acetic acid, using the Wood-Ljungdahl pathway [4]. However, acetic acid produced through this way presents a low economic value. Using acetate as substrate to produce compounds with higher economic value could be an efficient solution [5]. We propose here an innovative approach to overcome both the limitations associated with photoautotrophic growth of microalgae and the economic sustainability of microbial acetate production by gas fermentation. This approach is based on a two-stage fermentation process: the first step consists in the conversion of CO2H2-based feedstock into acetate through gas fermentation, using the acetogenic strain Thermoanaerobacter kivui in a stirred tank reactor (STR); the fermentation effluent, acetate rich, is used in the second fermentation step as cultivation medium for Chlorella sorokiniana grown in heterotrophy using acetate as organic source. Chlorella growth was assessed starting from different acetate concentration - 1.1 g L-1, 2.2 g L-1, 3.3 g L-1 - in the medium, obtained by diluting and sterilizing (by microfiltration) the fermentation effluent of T. kivui. Biomass production and acetate removal were analyzed; the growth kinetics were modelled to determine the specific growth rate (?MAX), which was 0.075 h-1. Good values of biomass productivity were obtained; no growth inhibition was observed until 3.3 g L-1 acetate concentration. As next goals, microalgal proteins accumulation and quality will be analyzed to prove their economic value for food and feed. Finally, Chlorella growth will be assayed without any medium sterilization procedure, with a perspective of increasing the energy and economic sustainability of the whole process, considering oxygen exposure as lethal to acetogens [6]. References [1]J. Ruiz, R.H. Wijffels, M. Dominguez, M.J. Barbosa, Heterotrophic vs autotrophic production of microalgae: Bringing some light into the everlasting cost controversy, Algal Res. 64 (2022) 102698. https://doi.org/10.1016/j.algal.2022.102698. [2]D. Morales-Sánchez, O.A. Martinez-Rodriguez, J. Kyndt, A. Martinez, Heterotrophic growth of microalgae: metabolic aspects, World J Microbiol Biotechnol. 31 (2015) 1-9. https://doi.org/10.1007/s11274-014-1773-2. [3]F. di Caprio, G. Proietti Tocca, M. Stoller, F. Pagnanelli, P. Altimari, Control of bacterial contamination in microalgae cultures integrated with wastewater treatment by applying feast and famine conditions, J Environ Chem Eng. 10 (2022) 108262. https://doi.org/10.1016/j.jece.2022.108262. [4]S.W. Ragsdale, E. Pierce, Acetogenesis and the Wood-Ljungdahl pathway of CO2 fixation, Biochim Biophys Acta Proteins Proteom. 1784 (2008) 1873-1898. https://doi.org/10.1016/j.bbapap.2008.08.012. [5]J. Bae, Y. Song, H. Lee, J. Shin, S. Jin, S. Kang, B.K. Cho, Valorization of C1 gases to value-added chemicals using acetogenic biocatalysts, Chemical Engineering Journal. 428 (2022) 131325. https://doi.org/10.1016/j.cej.2021.131325. [6]V. Turon, E. Trably, A. Fayet, E. Fouilland, J.P. Steyer, Raw dark fermentation effluent to support heterotrophic microalgae growth: Microalgae successfully outcompete bacteria for acetate, Algal Res. 12 (2015) 119-125. https://doi.org/10.1016/j.algal.2015.08.011.
Biochemical CO2 conversion into value-added products using microalgae and acetogens : a two-step process.
Menin Barbara;
2022
Abstract
Microalgal photoautotrophic cultivation present limits related to the high operative and investment costs required to maintain adequate light supply rate (reactors with high S/V ratio) to ensure satisfactory productivity [1]. Heterotrophy represents a promising solution: some microalgal strains can grow without light, metabolizing organic compounds as carbon and energy source [1,2]. Moreover, the integration of heterotrophic microalgal production with wastewater or effluents treatment can increase economic sustainability of the microalgal biomass production, eliminating the cost of carbon feedstock required in heterotrophy [3]. Through gas fermentation, some acetogens bacteria convert CO2 and H2 into acetic acid, using the Wood-Ljungdahl pathway [4]. However, acetic acid produced through this way presents a low economic value. Using acetate as substrate to produce compounds with higher economic value could be an efficient solution [5]. We propose here an innovative approach to overcome both the limitations associated with photoautotrophic growth of microalgae and the economic sustainability of microbial acetate production by gas fermentation. This approach is based on a two-stage fermentation process: the first step consists in the conversion of CO2H2-based feedstock into acetate through gas fermentation, using the acetogenic strain Thermoanaerobacter kivui in a stirred tank reactor (STR); the fermentation effluent, acetate rich, is used in the second fermentation step as cultivation medium for Chlorella sorokiniana grown in heterotrophy using acetate as organic source. Chlorella growth was assessed starting from different acetate concentration - 1.1 g L-1, 2.2 g L-1, 3.3 g L-1 - in the medium, obtained by diluting and sterilizing (by microfiltration) the fermentation effluent of T. kivui. Biomass production and acetate removal were analyzed; the growth kinetics were modelled to determine the specific growth rate (?MAX), which was 0.075 h-1. Good values of biomass productivity were obtained; no growth inhibition was observed until 3.3 g L-1 acetate concentration. As next goals, microalgal proteins accumulation and quality will be analyzed to prove their economic value for food and feed. Finally, Chlorella growth will be assayed without any medium sterilization procedure, with a perspective of increasing the energy and economic sustainability of the whole process, considering oxygen exposure as lethal to acetogens [6]. References [1]J. Ruiz, R.H. Wijffels, M. Dominguez, M.J. Barbosa, Heterotrophic vs autotrophic production of microalgae: Bringing some light into the everlasting cost controversy, Algal Res. 64 (2022) 102698. https://doi.org/10.1016/j.algal.2022.102698. [2]D. Morales-Sánchez, O.A. Martinez-Rodriguez, J. Kyndt, A. Martinez, Heterotrophic growth of microalgae: metabolic aspects, World J Microbiol Biotechnol. 31 (2015) 1-9. https://doi.org/10.1007/s11274-014-1773-2. [3]F. di Caprio, G. Proietti Tocca, M. Stoller, F. Pagnanelli, P. Altimari, Control of bacterial contamination in microalgae cultures integrated with wastewater treatment by applying feast and famine conditions, J Environ Chem Eng. 10 (2022) 108262. https://doi.org/10.1016/j.jece.2022.108262. [4]S.W. Ragsdale, E. Pierce, Acetogenesis and the Wood-Ljungdahl pathway of CO2 fixation, Biochim Biophys Acta Proteins Proteom. 1784 (2008) 1873-1898. https://doi.org/10.1016/j.bbapap.2008.08.012. [5]J. Bae, Y. Song, H. Lee, J. Shin, S. Jin, S. Kang, B.K. Cho, Valorization of C1 gases to value-added chemicals using acetogenic biocatalysts, Chemical Engineering Journal. 428 (2022) 131325. https://doi.org/10.1016/j.cej.2021.131325. [6]V. Turon, E. Trably, A. Fayet, E. Fouilland, J.P. Steyer, Raw dark fermentation effluent to support heterotrophic microalgae growth: Microalgae successfully outcompete bacteria for acetate, Algal Res. 12 (2015) 119-125. https://doi.org/10.1016/j.algal.2015.08.011.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
