Effect of permeate recycling and light intensity on growth kinetics of Synechocystis sp. PCC 6803

Permeate reuse for cultivating Synechocystis sp. PCC 6803 was studied in batch photosynthetic-growth experiments having a wide range of the ratio of membrane permeate to BG-11 medium. The 50% dilution of permeate gave the greatest biomass concentration and specific growth rate for Synechocystis, but up to 100% permeate had minimal impact on biomass growth; thus, a high ratio of permeate supported good growth of Synechocystis. With initial values of the spatially averaged light intensity (LI_SA) ranging from 49 to 479 μE m^{− 2} s^{− 1}, Synechocystis exhibited typical photo-inhibition growth kinetics in 50% permeate. The half-maximum-rate light irradiance (K_L) was 39 μE m^{− 2} s^{− 1}, and the half-inhibition-rate light irradiance (K_I) was 281 μE m^{− 2} s^{− 1}. The K_I value is larger than seen previously for Synechocystis in BG-11 medium, which suggests that culturing in permeate may have made Synechocystis less sensitive to photo-inhibition. The exponential growth rate of Synechocystis sp. PCC 6803 was not inhibited by recycled permeate through multiple rounds of growth in 50% permeate. These results demonstrate that permeate recycling is a feasible strategy for reusing nutrients and minimizing water loss.