A direct 'photosynthesis-to-fuels' approach envisions application of a single organism, absorbing sunlight, photosynthesizing, and converting the primary products of photosynthesis into ready-made fuel. The work reported here applied this concept for the photosynthetic generation of monoterpene ( β-phellandrene) hydrocarbons in the unicellular cyanobacteria Synechocystis sp. PCC 6803. Heterologous expression of a codon-optimized Lavandula angustifolia β-phellandrene synthase ( β-PHLS) gene in Synechocystis enabled photosynthetic generation of β-phellandrene in these microorganisms. β-phellandrene accumulation occurred constitutively and in tandem with biomass accumulation, generated from sunlight, CO, and HO. Results showed that β-phellandrene diffused through the plasma membrane and cell wall of the cyanobacteria and accumulated on the surface of the liquid culture. Spontaneous β-phellandrene separation from the biomass and its removal from the liquid phase alleviated product inhibition of cellular metabolism and enabled a continuous production process. The work showed that oxygenic photosynthesis can be directed to generate monoterpene hydrocarbons, while consuming CO, without a prior requirement for the harvesting, dewatering, and processing of the respective biomass. [ABSTRACT FROM AUTHOR]