High‐density photoautotrophic algal cultures: Design, construction, and operation of a novel photobioreactor system

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Abstract
A photobioreactor system has been designed, constructed and implemented to achieve high photosynthetic rates in high‐density photoautotrophic algal cell suspensions. This unit is designed for efficient oxygen and biomass production rates, and it also can be used for the production of secreted products. A fiber‐optic based optical transmission system that is coupled to an internal light distribution system illuminates the culture volume uniformly, at light intensities of 1.7 mW/cm2 over a specific surface area of 3.2 cm2/cm3. Uniform light distribution is achieved throughout the reactor without interfering with the flow pattern required to keep the cells in suspension. An on‐line ultrafiltration unit exchanges spent with fresh medium, and its use results in very high cell densities, up to 109 cells/mL [3% (w/v)] for eukaryotic green alga chlorella vulgaris. DNA histograms obtained form flow cytometric analysis reveal that on‐line ultrafiltration influences the growth pattern. Prior to ultrafiltration the cells seem to have at a particular point in the cell cycle where they contain multiple chromosomal equivalents. Following ultrafiltration, these cells divide, and the new cells are committed to division so that cell growth resumes. The Prototype photobioreactor system was operated both in batch and in continuous mode for over 2 months. The measured oxygen production rate of 4‐6 mmol/L culture h under continuous operation is consistent with the predicted performance of the unit for the provided light intensity.