Public defence in ecology: Thi Quyen Nham

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Abstract

Microalgal solutions use microalgae, photosynthetic unicellular microorganisms (2-50 μm), to efficiently recover nutrients and CO₂ into a valuable algal biomass, containing lipids, fatty acids, proteins, amino acids, carbohydrates, vitamins, and more. Autotrophic mode, the most traditional microalgal cultivation method, relies entirely on photosynthesis, resulting in low biomass productivity in low light seasons, such as during Nordic autumns and winters. Mixotrophic mode, that supports both photosynthesis and heterotrophy using organic carbon (OC) for growth, yields higher biomass productivity and nutrient recovery.

Whey permeate and cheese whey, dairy processing wastewaters, have been tested as OC sources for mixotrophic algal production in small-scale controlled laboratory conditions. This thesis investigates the potential for using these wastewaters as sources of phosphorus and OC, in combination with landfill leachate or fluegas condensate water as a nitrogen source, for sustainable mixotrophic algal cultivations under outdoor Nordic conditions. Monocultures and polycultures of local green algae were cultivated in mixotrophic mode on glucose, whey permeate or cheese whey and autotrophic mode in pilot-scale raceway ponds during the spring and autumn in southern Sweden. The coexistence of algae and bacteria, the algal gene expression, and the metabolite profile of algal biomass in response to mixotrophic mode were studied to optimize the operations and suggest suitable applications of mixotrophic algal biomass.

The results demonstrated enhanced algal productions in mixotrophic mode under both high and low light conditions, with higher algal growth rates, productivity, and nutrient removal and recovery rates compared to autotrophic mode. Mixotrophic mode offers the potential for year-round algal cultivations in Nordic conditions. Additionally, cheese whey mitigated night algal biomass loss when added at sufficient concentrations.

The eukaryotic and prokaryotic composition of polycultures varied more in mixotrophic mode than autotrophic mode. In the polycultures, most enzymes involved in carbon metabolism were upregulated, while those related to photosynthesis were downregulated in mixotrophic mode on glucose compared to autotrophic mode. Mixotrophic mode did not affect the biochemical composition in polycultures, but resulted in higher carbohydrate, and lower protein and lipid content in monocultures. Metabolite profiles of polycultures and monocultures were species-specific and differed between cultivation modes, favoring carbohydrate accumulation in mixotrophic mode. Therefore, the selection of algal species and cultivation mode is crucial for specific applications of algal biomass and when targeting specific metabolites.

Keywords: mixotrophic microalgal cultivation, whey permeate, cheese whey, nutrient removal, nutrient recovery, algal biomass, green algae, metabolite profile, biochemical composition, wastewater, leachate, flue gas condensate