Stabilizing autotrophic cyanophycin production in continuous photobioreactors

Cyanophycin, an intracellular reserve molecule synthetized by cyanobacteria, is considered as a potential biobased raw material for the industrial sector. It is usually produced through the cultivation of photosynthetic microorganisms in batch systems, which are affected by the high variability of cyanophycin accumulation due to varying growth phases, which lower the overall productivity. In this work, Synechocystis sp. PCC 6803 was cultivated in a continuous photobioreactor to increase cyanophycin productivity and to assess the feasibility of large-scale application. A stable production of the compound was obtained at steady-state. The effect of residence time and inlet phosphorus concentration on the biomass and cyanophycin productivity was evaluated, to identify the optimal conditions for its accumulation. A maximum value of cyanophycin productivity of about 30 mg L−1 d−1 was obtained, doubling the one achieved in the batch system. A quantitative correlation between the phosphorus quota and the cyanophycin produced was eventually proposed, highlighting a threshold of 4 mg of phosphorus per g of biomass for cyanophycin accumulation.