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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 cyanobacterial strains, but was not stable. We modified the gene sequence to enhance its stability, and expressed it in Synechocystis sp. PCC 6803, leading to continuous ethylene production. The same ethylene production rate was sustained across four successive sub-cultures without apparent loss of ethylene-forming ability. Up to 5.5% of the fixed carbon was directed to ethylene synthesis, surpassing the published carbon-partition rate into the TCA cycle. Nitrogen- and phosphorus …