Carbon Dioxide Uptake by Microalgae Chlorella vulgaris and Porphyridium purpureum and Its Development towards Bioplastics Production

Increasing levels of atmospheric carbon dioxide (CO2 ) and plastics waste pollution are among the environmental problems that need to be addressed. Several potential candidates have been identified to mitigate these problems, including microalgae cultivation, which also offers parallel benefits such as the ability to remediate industrial wastes and produce value-added chemicals. This thesis focuses on assessing CO2 uptake in selected strains of microalgae towards developing a general understanding of microalgae carbon sequestration and development of microalgae feedstock for bioplastics production, as a simultaneous solution for addressing the two environmental issues mentioned. This research has performed an optimisation towards the assessment of dissolved inorganic carbon (DIC) in media in terms of analysis volume and storage duration. Assessment of DIC availability in two cultures of Chlorella vulgaris strains found that their behaviours are different in terms of carbon uptake, growth, nutrient consumption and biochemical production. Interestingly, it was found that the DIC saturation levels in both algae culture medium are higher than in a blank medium, suggesting a greater carbon sequestration capacity of the culture medium in the presence of microalgae. Later, the influence of nutrients towards carbon consumption was examined by increasing the amount of nitrate and phosphate supplied, at a constant carbon load. To achieve that, we described a dynamic method to quantify carbon uptake in a growth culture, based on the assessment of DIC. It was observed that carbon uptake rates did not increase with increased nitrate and phosphate. The rate was also independent of their growth phase. The increase of nutrients increased the growth, up until a point, with no significant change beyond. A similar treatment of red algae (Porphyridium purpureum) showed carbon uptake profiles of slightly lower value. Increasing nutrient does not change the carbohydrate proportion. Instead, it increases the protein percentage. Later, both intracellular polysaccharides and exopolysaccharides synthesised by the red algae were incorporated into bioplastics production. The best biofilm produced have similar mechanical properties to conventional low-density polyethene plastics, but with shorter elongation capability and less hydrophobicity. We also found that the use of extracted polysaccharides from the microalgae to produce microalgae bioplastics will result in higher tensile strength and elasticity, rather than incorporating the microalgae as whole cells.