Optimizing Isobutanol Production in Engineered Cyanobacteria Using High Salinity Stress and Establishment of a Resource Recycling Strategy

Background: Isobutanol is an attractive biofuel with advantages. Third-generation biorefineries that convert CO2 into bio-based fuels have drawn considerable attention due to their lower feedstock cost and more ecofriendly refining process. Although autotrophic cyanobacteria have been genetically modified for isobutanol biosynthesis, it is still lack of stable and convenient strategies to improve the production.Results: In this study, we first engineered Synechococcus elongatus for isobutanol biosynthesis by introducing five exogenous enzymes, reaching a production titer of 0.126 g/L at day 20. It was then discovered that high salinity stress can significantly enhance isobutanol production five-fold with a maximal in-flask titer of 0.637 g/L at day 20. A comprehensive analysis of the osmotic-induced cells demonstrated that a series of physiological functional self-adjustment in S. elongatus, including altering metabolic profiles to accumulate redox equivalents, strength in antioxidative ability and weaken in membrane integrity, contributed to the isobutanol production titer. A cultivation system was then developed by mixing wastewater with seawater to grow the engineered cyanobacteria, reaching a similar isobutanol production titer as cultivation in the medium. Conclusions: High salinity stress on engineered cyanobacteria is a practical and feasible biotechnology to optimize isobutanol production. This biotechnology provides a prospect for biofuel production cost-effectively, and simultaneously recycles chemical nutrients from wastewater and seawater.