Design of a continuous milking bioreactor for non-destructive hydrocarbon extraction from Botryococcus braunii

The production of low-value products such as biodiesel from microalgae is still too expensive. An alternative to commonly used species is the alga Botryococcus braunii, which excretes high quantities of hydrocarbons into the extracellular matrix. Because of its low growth rate, the common processing procedure (cultivation, harvesting, dewatering, cell disruption, lipid extraction) is not feasible for this alga, and moreover, at present, there is no existing process for the continuous extraction of hydrocarbons from B. braunii during cultivation without an inhibition of cell activity over a longer time period. Therefore, we attempted to develop an in situ extraction system which allows the continuous non-destructive extraction of the extracellular hydrocarbons during the cultivation. With the new extraction system, we were able to show that an average lipid yield of 19.15, 15.32, and 3.80 mg g(-1) dry weight (DW) day(-1) by the strains SCCAP 1761, CCAP 807/2, and SAG 807/1, respectively, could be achieved during a 5-day experiment. As only B. braunii SAG 807/1 showed a positive biomass yield, the optimization of the new system was conducted with this strain. We were able to show that a continuous extraction was possible for at least 6 weeks. The optimal effective extraction was in the range of 30 to 36 s day(-1) with a lipid yield between 1.22 and 2.20 mg g(-1) DW day(-1).