Development of luminescent photobioreactors for improved cultivation of microalgae

Abstract

This study investigated the effects of light quality, culture density, and carbon dioxide aeration on the biomass production, lipid accumulation, elemental contents, and photosynthetic pigment production of microalgae. The microalgae Chlorella vulgaris and Gloeothece membranacea were cultivated either in static (flask) or aerated and mixed (photobioreactor) cultivation modes. The highest biomass productivities in the static mode for both microalgae strains were achieved using violet luminescent filters which emitted the photosynthetic active radiation ranges. In the static mode, red wavelength ranges had a major influence on pigmentation of C.vulgaris, although they were the least efficient light condition for cultivation of G.membranacea. Red light was particularly inefficient to support growth in static cultures. Luminescent bubble column photobioreactors (PBRs) were constructed and used for the aerated mode in the blue, green, yellow, orange, and red wavelength-emitting ranges. Biomass production in both strains was enhanced in the red PBR. Lipid accumulation was significantly enhanced in high culture density groups, low light intensity, and 15% CO2 aeration conditions which all represented a form of stress to the culture. The highest biomass density of 2.12 and 2.52 g L-1 were achieved at high light intensity in the red PBR for C.vulgaris and G.membranacea respectively. The blue PBR was the least efficient light condition for biopigment production, whilst it improved lipid accumulation. Chlorophyll production in C.vulgaris was promoted by the green PBR, although it was less influenced by the light condition or culture density in G.membranacea cultures. Phycobiliproteins were the dominant pigments in G.membranacea and red light favoured synthesis of these pigments. Compared to C.vulgaris, the G.membranacea cells showed higher tolerance to 15% CO2 aeration by growing in this condition over a period of two weeks. At the 15% CO2 aeration condition G.membranacea accumulated the highest lipid content of 36.6% in the blue PBR. However, due to their lower carbon content and biomass productivity, G.membranacea was less efficient than C.vulgaris in sequestrating carbon dioxide. C.vulgaris sequestrated carbon dioxide up to 363 mg L-1 day-1 in the red PBR when aerated with 15% CO2. These results have significant implications for improved photobioreactor design for cultivation of microalgae under natural light. The proposed photobioreactor design can also tailor and improve the composition of lipids and photosynthetic compounds, using variation in light.