The Effect of Changes in Irradiance on the Growth, Biomass, Lipid Accumulation and pigment composition of Botryococcus braunii

Abstract

Light is one of the strongest factors affecting the growth and accumulation products within algae. While low irradiance brings about a reduction in biomass and productivity, exposure to high irradiance causes oxidative damage to the photosynthetic machinery. The green, colonial alga Botryococcus braunii (B. braunii) is regarded as a potential source of renewable fuel because of its ability to produce and secrete large amounts of hydrocarbons. B. braunii was cultured under a range of illumination conditions to assess the effect increasing irradiance had on growth, biomass and lipid accumulation and composition of the photosynthetic apparatus. Cells were grown in successive photobioreactor runs and exposed to irradiances of 300, 450 and 600 μmolphotons m−2 s−1 provided by an LED shroud. A fluorescent light shroud at 300 μmolphotons m−2 s−1 was also used to investigate the effect of light quality. The maximum growth rate was achieved at an intensity of 450 μmolphotons m−2 s−1. Production of botryococcene was found to be growth associated, while neutral lipid yield increased with irradiance. Extracts were compositionally identical for all illumination conditions investigated. B. braunii were shown to tolerate and acclimate to increasing light intensity by altering the composition of their photosynthetic apparatus. Although not statistically significant, growth at 450 and 600 μmolphotons m−2 s−1 resulted in a decrease in the chlorophyll content per cell. Increased accumulation of zeaxanthin and β-carotene was observed in cells cultured at 450 μmolphotons m−2 s−1 compared with those grown at 300 μmol m−2 s−1 and is consistent with their role in non-photochemical quenching and the dissipation of excess energy as heat and light (P > 0.05). Photoinhibition was observed during the third run at 450 μmolphotons m−2 s−1 and at all runs at 600 μmolphotons m−2 s−1. Excessive irradiance did not cause cell necrosis, but resulted in decreased viability of B. braunii cells due to the progressive accumulation of irreparable photodamage over successive photobioreactor runs. This was confirmed by microscopic analysis and a significant reduction in growth rate. Illumination type had no effect on the growth of B. braunii when cultured at 300 μmolphotons m−2 s−1, LED lighting did however prove to be a more reliable lighting source.