Terpenoids are a large group of secondary metabolites with broad industrial applications. Engineering cyanobacteria is an attractive route for the sustainable production of commodity terpenoids. Currently, a major obstacle lies in the low productivity attained in engineered cyanobacterial strains. Traditional metabolic engineering to improve pathway kinetics has led to limited success in enhancing terpenoid productivity. In this study, we reveal thermodynamics as the main determinant for high limonene productivity in cyanobacteria. Through overexpressing the primary sigma factor, a higher photosynthetic rate was achieved in an engineered strain of S ynechococcus elongatus PCC 7942. Computational modeling and wet lab analyses showed an increased flux toward both native carbon sink glycogen synthesis and the non-native limonene synthesis from photosynthate output. On the other hand, comparative proteomics showed decreased expression of terpene pathway enzymes, revealing their limited role in determining terpene flux. Lastly, growth optimization by enhancing photosynthesis has led to a limonene titer of 19 mg/L in 7 days with a maximum productivity of 4.3 mg/L/day. This study highlights the importance of enhancing photosynthesis and substrate input for the high productivity of secondary metabolic pathways, providing a new strategy for future terpenoid engineering in phototrophs.

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The current paper deals with the effect of powder type and chemical admixtures on the rheological properties of mineral suspensions. The plastic viscosity of calcite, cement, and fly ash suspensions with or without superplasticizers (SP) and hydration retarders was characterized in a wide range of solid volume fractions. The results show that the plastic viscosity of suspensions increases with the decrease in particle size, and strongly decreases with the presence of superplasticizers. Besides, for reactive suspensions, hydration retarders decrease the plastic viscosity of the suspension, while competitive adsorption occurs when adding retarders to suspensions containing SP, leading to an increase in the plastic viscosity. Based on the experimental results, a relative plastic viscosity, i.e., the ratio between the total plastic viscosity and the theoretical viscosity contributed by the hard-sphere, is proposed to assess the effect of the contribution of colloidal forces. Moreover, the solid volume fraction of flocs in colloidal suspensions before percolation is identified by comparing the measured plastic viscosity with the Krieger-Dougherty model. Finally, a theoretical approach to determine the percolation packing fraction of minerals powders is further proposed.