Cultivation of Microalgae in Phototrophic, Mixotrophic and Heterotrophic Conditions

Author: Yousef Ahmed Alkhamis

Alkhamis, Yousef Ahmed, 2015 Cultivation of Microalgae in Phototrophic, Mixotrophic and Heterotrophic Conditions, Flinders University, School of Biological Sciences

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Algal growth and production depend on nutrient supply and culture systems. This thesis compares the performance of algal growth in phototrophic, mixotrophic and heterotrophic conditions. Isochrysis galbana, a common species in aquaculture was used as a model species to identify the appropriate condition and nutrient requirement to optimise algal growth and bioproduct production. Experiment 1 identifies the source of organic carbon and environmental conditions in the culture of I. galbana. Algal growth was inhibited in heterotrophic culture when glucose, glycerol and sodium acetate were separately used as organic carbon. However, algal growth was enhanced by glycerol supplementation in mixotrophic culture compared to phototrophic culture. The optimal mixotrophic condition for algal growth is 50 mM glycerol, 35‰ salinity and a light regime at 100 photons mol m−2 s−1 with 12 h light and 12 h dark. Algal production in mixotrophy was higher than in either phototrophy or heterotrophy. Experiment 2 compares nitrogen and phosphorus requirements of I. galbana between phototrophic and mixotrophic conditions. Three nitrogen sources (nitrate, ammonium and urea) and one phosphorus source were tested at various concentrations. Better growth performance and nutrient conversion efficiency were achieved in the mixotrophic condition and urea was the preferred nitrogen source for algal growth. The requirements of nitrogen and phosphorus were not different between phototrophic and mixotrophic conditions but algal production and nutrient conversion efficiency in mixotrophy were greater than in phototrophy. Experiment 3 compares the pigment and proximate composition in I. galbana under phototrophic and mixotrophic conditions in an attempt to improve algal product quality. The contents of chlorophylls a and c and carotenoid in mixotrophic culture increased by 60% and the productions of protein, lipid and carbohydrate were also enhanced compared to those in phototrophic culture. This study indicates that mixotrophic culture promotes pigment and proximate production and the change of fatty acid profile depends on the addition of organic carbon in the culture medium. Experiment 4 examines the interactive effect of nitrogen and organic carbon concentrations on lipid and fatty acid production in I. galbana. Urea was used as the source of nitrogen at four levels (0, 12.5, 25 and 50 mg L-1) and each nitrogen level was tested against three levels of organic carbon (0, 25 and 50 mM glycerol). Relatively higher lipid content (>400 mg g-1) and lipid production (344.9 mg L-1) were obtained at 25 or 50 mg urea-N L-1 with 50 mM glycerol. This study indicates that manipulation of nitrogen and organic carbon can improve lipid and fatty acid production in I. galbana. Overall, this thesis research contributes to the knowledge of using mixotrophic culture to improve algal growth and production.

Keywords: Marine microalgae, Isochrysis galbana, Phototrophic, Heterotrophic, Mixotrophic, Micoralgal lipids, Fatty acids, DHA, EPA, Protein, Carbohydrate, Pigment, Chlorophyll, Organic carbon, Glycerol, Live food, Nitrogen, Phosphorus, Light intensity, Photoperiod, Salinity

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2015
School: School of Biological Sciences
Supervisor: Prof. Jian Qin