| Abstract | The rise in anthropogenic carbon dioxide emissions has led to a need for efficient andscalable carbon capture systems. Microalgae offer a promising approach to carbon capture from
a variety of sources, including industrial gases and dissolved inorganic carbon. The resultingalgal biomass can serve as feedstock for biofuels and high-value products such as polyunsaturated fatty acids, carotenoids, and proteins, with applications in the personal care,healthcare, and nutraceutical markets. The aim of this thesis was to develop a bicarbonate based microalgal system for carbon capture from mushroom production and the concomitant generation of value-added products from the spent algal biomass and mushroom waste. Firstly,three freshwater microalgal species Parachlorella kessleri, Scenedesmus quadricauda, andVischeria sp., and two marine microalgal species Phaeodactylum tricornutum, and Porphyridium purpureum were evaluated at varying inorganic carbon concentrations to identify the most tolerant and suitable species. Vischeria sp. and Porphyridium purpureum were selected for further studies based on their growth profiles, carbon removal efficiencies,and concentrations of value-added products including fatty acids and pigments. Using these two species, a bicarbonate based integrated carbon capture and algal production system was developed for mushroom cultivation at small scale. This involved the capture of carbon dioxide generated from a mushroom growth chamber in a modified microalgal growth medium, followed by the cultivation of the selected microalgal species in this medium. The effects of pH on cell growth, carbon capture efficiencies, and biochemical profiles of both species were studied in (1) ‘mushroom-bicarbonate media’, containing dissolved inorganic carbon generated from the carbon dioxide captured during mushroom cultivation and (2) ‘synthetic media’containing commercial sodium bicarbonate. Vischeria sp. performed better in the synthetic media whereas Porphyridium purpureum performed significantly better in the mushroombi carbonate media. The inorganic carbon removal efficiency by Vischeria sp. from thesynthetic media (94.9%) was higher compared to mushroom-bicarbonate media (30.6%).Similarly, the estimated removal of inorganic carbon was 85.4% in mushroom-bicarbonate media, which was 2.1 times higher than in synthetic media (40.7%) by Porphyridium purpureum. These results suggest that Porphyridium purpureum exhibited higher adaptabilityand tolerance to high sodium concentrations in mushroom-bicarbonate media, while carbon utilisation was lower in synthetic media due to lower sodium content. Furthermore, the study highlighted the importance of culture pH in improving inorganic carbon utilisation. Finally, based on the results of the small-scale studies, an integrated process was evaluated in photobioreactors to demonstrate the scalability of the process. A small-scale study on Porphyridium purpureum performed significantly better in the mushroom-bicarbonate media with 87.3% inorganic carbon removal at pH 7 compared to Vischeria sp. (67.3%) at pH 9. The biochemical profiles of Vischeria sp. revealed that the carbohydrate content was similar, with 47.6% in synthetic media and 48.9% in mushroom-bicarbonate media. In Porphyridium purpureum, the lipid content in mushroom-bicarbonate media was similar compared to synthetic media, at 17.2% and 3.1%, respectively. The carbohydrate content was 47.7% in synthetic media and 51.7% in mushroom-bicarbonate media. Vischeria sp. in the synthetic media showed higher antioxidant properties compared to Porphyridium purpureum. Additionally, this study evaluated the antioxidant and photoprotective properties of extracts from spent algal biomass and mushroom exudate for potential applications in bio-based cosmetics. This suggests that the system could not only function as an effective model for evaluating the carbon dioxide sequestration capacities of microalgae, but also be integrated into a working mushroom farm to capture the carbon dioxide as well as generated value-added products from the spent algal biomass and mushroom waste. |
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