Extracellular hydrocarbon and intracellular lipid accumulation are related to nutrient-sufficient conditions in pH-controlled chemostat cultures of the microalga Botryococcus braunii SAG 30.81

Here we investigate the effect of environmental pH on biomass and hydrocarbon productivity in photoautotrophic cultures of Botryococcus braunii SAG 30.81 (race A). Successive steady-states of continuous cultures were used to study the effects of pH controlled by carbon dioxide feeds in a closed photobioreactor. At a fixed dilution rate of 0.2 d− 1, hydrocarbon productivity was found to be pH-dependent in the range 5.5 to 8.0, with an optimum at pH 6.5. pH variations from pH 6.0 to pH 8.0 had little effect on biomass productivity, nutrient requirement for biomass production, photosynthetic yield and pigment ratios, whereas acidic pH 5.5 conditions induced a slight decrease in biomass and hydrocarbon productivities as well as stressing the cells. The pH effect occurred even after a threefold increase in nitrate and phosphate concentrations in the feed culture medium, resulting in biomass productivity up to 8.9 g m− 2 d− 1 and hydrocarbon productivity up to 0.5 g m− 2 d− 1. Dissolved inorganic carbon data suggest that a maximum hydrocarbon content of 5.7% dry weight was reached at and above 1 mmol L− 1 dissolved CO2 and that hydrocarbon production was linearly related to CO2 up to 2.5 mmol L− 1, whatever the N and P enrichment of AF-6 culture medium.