P166: Sustained Photosynthetic Conversion of CO2 to Ethylene in Recombinant Cyanobacterium Synechocystis 6803

Ethylene is the most widely produced petrochemical feedstock globally.  It is currently produced exclusively from fossil fuels and its production is the largest CO₂-emitting process in the chemical industry.  In this study, we report on a photobiological process for sustained production of ethylene from CO₂.  The efe gene encoding an ethylene-forming enzyme from Pseudomonas syringae pv. Phaseolicola was previously expressed in transgenic cyanobacterial strains, but was not stable. We modified the gene sequence to enhance it stability, and expressed in the cyanobacterium Synechocystis sp. PCC 6803, leading to continuous ethylene production. The same ethylene production rate was sustained across four successive sub-cultures without apparent loss in the ethylene-forming ability of the strain.  Up to 5.5% of the fixed carbon was directed to ethylene synthesis, surpassing the published carbon partition rate into TCA cycle. Nitrogen- and phosphorus-enriched sea water can support both growth of the strains and ethylene production.  Factors limiting ethylene production including efe expression levels, light intensity and nutrient status were identified and alleviated, resulting in a peak production rate of 5650 mL L^-1 h^-1 (7125mg L^-1 h^-1, 252mmol L^-1 h^-1 , or 171mg L^-1 day^-1), higher than has been reported for other algae biofuels and chemicals.  Evidence provided in this study suggests that Synechocystis expressing the modified efe gene has potential to be an efficient biological catalyst for the uptake and conversion of CO₂ into ethylene.