Nutritional effects in high density Chlorella vulgaris cultures.

Abstract

Photobioreactors (PBRs), capable of producing high density microalgal cultures are imperative for economical and effective utilization of microalgae and its products. The four major factors to be considered for the design of closed PBRs are light source and delivery, gas supply and exchange, nutrient supply, and removal of harmful secondary metabolites. The two latter issues, which are critical in obtaining high biomass concentrations, are addressed in this thesis. Specifically, this thesis addresses (i) the effects of the presence of conditioned medium on high-density cultures and (ii) the nutritional requirements of high-density microalgal cultures. Conditioned medium (CM) from high density cultures obtained from PBRs were tested for cell cycle and inhibitory activity. Low initial cell density cultures in presence of CM showed an accumulation of cells with two and four DNA equivalents, signifying a blockage of cell-division. Further, the culture pH increased by four units due to the presence of a high concentration of bicarbonate in the CM. Adding similar bicarbonate concentrations to fresh medium (FM) was found to increase the culture pH in an analogous fashion to CM cultures, causing blockage of cell division. The bicarbonate-free CM cultures at low (0.03%) CO$\sb2$ levels did not show any unusual cell cycle effects. CM cultures at high (5%) CO$\sb2$ levels supplemented with 100% FM components did not show any difference in growth and DNA content per cell distribution compared to 100% FM cultures. Further, experiments showed that there were no factors in the CM which were specific to cells in high density cultures. Taken together, these results lead to the conclusion that CM from high density cultures do not contain autoinhibitory or cell cycle modulating compounds that affect C. vulgaris in PBRs. N-8 medium, commonly used for culturing Chlorella, was evaluated for its capacity to support high density cultures on the basis of the elemental composition of Chlorella. It was found that the N-8 medium was depleted of iron, magnesium, sulfur and nitrogen at high cell densities. N-8 medium was redesigned to contain stoichiometrically balanced quantities of all the elements, to support a biomass concentration of 2% (v/v). Cultures in redesigned medium (named M-8 medium) showed significant enhancement in biomass production and chlorophyll content. The viability of the cultures increased two-three times and the medium demonstrated the capacity to sustain long-term high density viable microalgal cultures. Thus, high density C. vulgaris cultures can be achieved in PBRs with the use of proper medium and without performing ultrafiltration. This thesis underlines the significance of nutritional effects in high cell density cultures and the importance of medium development in the enhancement of PBR performance.