Doctoral thesis (Dissertations and theses)
Effects of carbon trophic mode on growth, pigments and fatty acids in green microalgae analyzed on the basis of well-defined growth kinetics of batch cultures
Thanh, Tung Le
2019
 

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Keywords :
microalgae; growth; pigments; fatty acids; chlorella; scenedesmus
Abstract :
[en] Microalgae are photosynthetic microorganisms that can be cultivated at industrial scale to produce valuable compounds present in their biomass. Among these, pigments and fatty acids are microalgal products having present or foreseen applications in the fields of nutraceutics, cosmetics or biofuels. For this purpose, green microalgae are already cultivated in an industrial scale. Indications or predictions of biomass and metabolite productivities generally rely on lab-scale batch cultures together with quantification of metabolites during growth of various species in different conditions. This work was undertaken with the aims of 1) improving the analysis and understanding of batch growth curves in different carbon trophic conditions and 2) compare photoautotrophy, heterotrophy and mixotrophy in terms of growth rate and biomass productivity and of pigment and fatty acid content in actively growing cells. For this purpose, we first compared heterotrophy to light-limited photoautotrophy in four species: Scenedesmus acutus, Scenedesmus vacuolatus, Chlorella sorokiniana and Chlamydomonas reinhardtii. A method was implemented to obtain reliable and well-resolved growth kinetics (based on optical density) using a commercial photobioreactor. Two main growth parameters are derived: the maximum specific growth rate (µmax) during the exponential phase, and the maximum biomass productivity (Vmax) observed during the deceleration phase. The variations of these parameters and of pigment and fatty acid contents were analyzed in relation to species, trophic mode (heterotrophy or light-limited photoautotrophy at different light intensities), growth phase and organic substrate (glucose or acetate). Application of weak light is shown to increase both chlorophyll and carotenoid contents in heterotrophically-grown microalgae. Weak light (defined as weak enough not to sustain photoautotrophic growth) also enhanced heterotrophic growth rate in one species, Scenedesmus acutus. In photoautotrophy, the pigment content of three of the four species was subject to large changes during growth as a result of photoacclimation. In heterotrophy, acetate promoted fatty acid content better than glucose, and led to a higher saturation degree of C18 fatty acids. Fatty acid content and composition also varied depending on species, growth conditions and growth phases. Mixotrophy on glucose under different light intensities was then analyzed in detail for Chlorella sorokiniana. Growth phase analysis in different conditions of aeration and CO2-supply, as well as analysis of respiration and photosynthetic processes, led to the conclusion that O2 and CO2 recycling between photosynthesis and respiration was the main reason for the fast mixotrophic growth in this species. Biomass-on-glucose yield improvement in mixotrophy is suggested to arise from the re-fixation of CO2 evolved during respiration. The cytochrome pathway of respiration was the main contributor to total respiration on glucose, in line with the high ATP demand during glucose assimilation and transformation. A practical implication of this section of our work is that mixotrophy is a way to avoid the need of strong aeration for glucose assimilation. As a final contribution, we compare pigment and fatty acid content and composition in all studied growth conditions for Chlorella sorokiniana, a species endowed with remarkable growth ability in photoautotrophy, heterotrophy or mixotrophy.
Research center :
Biological Sciences from Molecules to Systems - inBioS
Disciplines :
Biotechnology
Author, co-author :
Thanh, Tung Le ;  Université de Liège - ULiège > Form. doct. sc. (bioch., biol. mol. cel., bioinf. - paysage)
Language :
English
Title :
Effects of carbon trophic mode on growth, pigments and fatty acids in green microalgae analyzed on the basis of well-defined growth kinetics of batch cultures
Defense date :
2019
Institution :
ULiège - Université de Liège
Degree :
doctor in sciences
Promotor :
Franck, Fabrice ;  Université de Liège - ULiège > Département des sciences de la vie > Génétique et physiologie des microalgues
President :
Dommes, Jacques ;  Université de Liège - ULiège > Département des sciences de la vie > Biologie végétale translationnelle
Secretary :
Tocquin, Pierre  ;  Université de Liège - ULiège > Département des sciences de la vie > Care "PhytoSYSTEMS"
Jury member :
Remacle, Claire  ;  Université de Liège - ULiège > Integrative Biological Sciences (InBioS)
Hantson, Anne-Lise
Schoefs, Benoit
Available on ORBi :
since 17 October 2019

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