Bioalcohol Production  By Metabolically Engineered Cyanobacteria Using Photobioreactor

Authors: 
Horiuchi, J. I., Kyoto Institute of Technology
Hirokawa, Y., Kyushu University

The 3rd generation biorefinery research using microalgae is ongoing in many countries because fuel/chemicals production from carbon dioxide by photogenic microorganisms is essentially sustainable. Cyanobacteria is recognized as a proper platform for chemicals production due to their high growth rate, but it requires genetic modification because the original strain never produce usefulchemicals. In this presentation, I would like to introduce the current status of our research concerning the production of iso-propanol (IPA) and 1,3 propandiol (1.3PDO) by genetically engineered cyanobacteria using photobioreactor system with LED irradiation equipment, which can selectively supply various wavelength of light with low power consumption.

To achieve IPA production in cyanobacteria, we constructed a metabolically engineered Synechococcus elongatus strain TA1741 which produces IPA from acetyl-CoA by four steps. Successful cell growth was observed for blue, and blue and red LED, however, IPA was not detected for all cases. On the other hand, higher intracellular glycerol accumulation was observed, which suggests that the carbon dioxide fixed in a Calvin cycle is converted primarily into Glycogen under light condition. So we thought that, by changing from light to dark condition, accumulated glycogen may be converted to IPA. To verify this idea, Cells grown in a jar fermentor was harvested, re-suspended in new medium and purged by nitrogen gas. Then the tubes were maintained under anaerobic and dark conditions for 5-10days and analyzed. After five or 10 days cultivation, the production of IPA was confirmed for all cases showing that the genes concerning IPA biosynthesis were working in TA1741. Maximum IPA concentration was approx. 23 mg/L under anaerobic and dark conditions, however the concentration is rather low and complicated operation is required for practical IPA production.

Then, we examined another product, 1.3-Propandiol (1.3-PDO) production by genetically engineered cyanobacteria. In cyanobacteria, metabolic flux passing through acetyle-CoA is very narrow under light condition. On the other hand, PDO can be produced from DHAP which is main intermediate product in Calvin cycle. To achieve PDO production in cyanobacteria, five genes were introduced into the host strain, which produces 1.3 PDO from DHAP through glycerol. We call this strain TA2984. Using this strain, batch experiments were performed in the same way. Cell growth was successful and maximum PDO concentration was approximately 0.7 mM/L. It should be noted that the higher production of glycerol was observed. Their production was clearly associated with cell growth. This characteristics allows us to try continuous process. So we are now examining the possibility of continuous process using immobilized cyanobacteria. As a bioreactor for continuous process, an airlift fermenter with an internal draft tube was used. Successful growth was observed using an airlift bioreactor, maximum PDO concentration increased to approximately 1.8mM.