Exploring the potential of high-density cultivation of cyanobacteria for the production of cyanophycin

Photoautotrophic cyanobacteria and microalgae offer significant potential for the renewable synthesis of high-value products. As yet, however, the productivity of phototrophic cultures is limited due to the low cell densities that are typically obtained in current pilot scale photobioreactors. Here, we explore the use of ultrahigh-density cultivation of cyanobacteria for the production of cyanophycin, a non-ribosomally synthesized biopolymer of high biotechnological interest. We demonstrate that ultrahigh-density cultivation using a two-tier vessel with membrane-mediated CO2 supply yields a cyanophycin content per cellular dry weight similar to previously reported values, while the volumetric productivity per culture volume is significantly increased. Already after 96 h of cultivation, the engineered production strain BW86 reached up to 1 g cyanophycin per liter culture, approximately a 4-fold increase over the previously reported maximal yield obtained after 12 days of cultivation. Under phosphate-limiting growth conditions, the wild-type strain Synechcocystis sp. PCC 6803 accumulates up to 0.6 g cyanophycin per L culture. Our results demonstrate that ultrahigh-density cultivation is a suitable strategy towards the development of viable phototrophic production processes for cyanophycin and possibly other products of interest.