Light-optimized growth of cyanobacterial cultures: Growth phases and productivity of biomass and secreted molecules in light-limited batch growth

Abstract

Cyanobacteria are photosynthetic microorganisms whose metabolism can be modified through genetic engineering for production of a wide variety of molecules directly from CO, light, and nutrients. Diverse molecules have been produced in small quantities by engineered cyanobacteria to demonstrate the feasibility of photosynthetic biorefineries. Consequently, there is interest in engineering these microorganisms to increase titer and productivity to meet industrial metrics. Unfortunately, differing experimental conditions and cultivation techniques confound comparisons of strains and metabolic engineering strategies. In this work, we discuss the factors governing photoautotrophic growth and demonstrate nutritionally replete conditions in which a model cyanobacterium can be grown to stationary phase with light as the sole limiting substrate. We introduce a mathematical framework for understanding the dynamics of growth and product secretion in light-limited cyanobacterial cultures. Using this framework, we demonstrate how cyanobacterial growth in differing experimental systems can be easily scaled by the volumetric photon delivery rate using the model organisms Synechococcus sp. strain PCC7002 and Synechococcus elongatus strain UTEX2973. We use this framework to predict scaled up growth and product secretion in 1L photobioreactors of two strains of Synechococcus PCC7002 engineered for production of l-lactate or L-lysine. The analytical framework developed in this work serves as a guide for future metabolic engineering studies of cyanobacteria to allow better comparison of experiments performed in different experimental systems and to further investigate the dynamics of growth and product secretion.

Pooled CRISPRi screening of the cyanobacterium Synechocystis sp PCC 6803 for enhanced industrial phenotypes

“…The CRISPRi library is also beneficial for finding stress tolerance phenotypes, which are generally difficult to engineer rationally. The commodity chemical L-lactate has been produced in several cyanobacteria but still at relatively low titers (up to 15 mM) 28 . The tolerance of Synechocystis to L-lactate is~0.1 M (9 g/L), at which specific growth rate is reduced by 50% (L100 condition, Supplementary Data 3).…”

Environmental impacts and limitations of third‐generation biobutanol: Life cycle assessment of n‐butanol produced by genetically engineered cyanobacteria

“…The importance of cell productivity in the final GHGe calculations was evident in all three modeled reactor types; increasing carbon partitioning to butanol from 50% to 90% reduced overall GHGe and energy demand by 30–50%. However, there is a large uncertainty as to whether such high partitioning factors can be sustained over long cultivations (Clark et al., ). Further, the “metabolic switch” is achieved in the laboratory with high‐cost inducer molecules (Shabestary et al., ), though significantly cheaper options have recently been reported (Ruegg et al., ).…”

“…Further, the “metabolic switch” is achieved in the laboratory with high‐cost inducer molecules (Shabestary et al., ), though significantly cheaper options have recently been reported (Ruegg et al., ). Regarding the CO 2 ‐uptake data, it is likely that actual CO 2 ‐uptake rates in a scaled process will be “higher” than those used here, as there are several genetically tractable Synechococcus cyanobacteria strains that assimilate CO 2 at least twofold faster than Synechocystis and exhibit high photosynthetic efficiency across range of light and salinity conditions (Clark et al., ; Jaiswal et al., ; Ungerer et al., ). Pilot‐scale cultivation of these strains has not been reported in the literature.…”

Pooled CRISPRi screening of the cyanobacteriumSynechocystissp. PCC 6803 for enhanced growth, tolerance, and chemical production

“…The CRISPRi library is also beneficial for finding stress tolerance phenotypes, which are generally difficult to rationally engineer 34 . The commodity chemical L-lactate has been produced in several cyanobacteria but still at relatively low titers (15 mM 35 . The tolerance of Synechocystis to L-lactate is approximately 0.1 M (9 g/L), at which specific growth rate is reduced by 50% (L100 condition, Supplemental Data 3).…”