Installing extra bicarbonate transporters in the cyanobacterium Synechocystis sp. PCC6803 enhances biomass production

• Overexpression of endogenous high-flux bicarbonate transporter in Synechocystis.
• Two-fold increase in carbon acquisition, growth, and biomass accumulation.
• Platform for photosynthetic production of fuels and chemicals in cyanobacteria.

As a means to improve carbon uptake in the cyanobacterium Synechocystis sp. strain PCC6803, we engineered strains to contain additional inducible copies of the endogenous bicarbonate transporter BicA, an essential component of the CO2-concentrating mechanism in cyanobacteria. When cultured under atmospheric CO2 pressure, the strain expressing extra BicA transporters (BicA+ strain) grew almost twice as fast and accumulated almost twice as much biomass as the control strain. When enriched with 0.5% or 5% CO2, the BicA+ strain grew slower than the control but still showed a superior biomass production. Introducing a point mutation in the large C-terminal cytosolic domain of the inserted BicA, at a site implicated in allosteric regulation of transport activity, resulted in a strain (BicA+(T485G) strain) that exhibited pronounced cell aggregation and failed to grow at 5% CO2. However, the bicarbonate uptake capacity of the induced BicA+(T485G) was twice higher than for the wild-type strain. Metabolic analyses, including phenotyping by synchrotron-radiation Fourier transform Infrared spectromicroscopy, scanning electron microscopy, and lectin staining, suggest that the excess assimilated carbon in BicA+ and BicA+(T485G) cells was directed into production of saccharide-rich exopolymeric substances. We propose that the increased capacity for CO2 uptake in the BicA+ strain can be capitalized on by re-directing carbon flux from exopolymeric substances to other end products such as fuels or high-value chemicals.