Environmental impacts and limitations of third-generation biobutanol: Life cycle assessment of n-butanol produced by genetically engineered cyanobacteria

Photosynthetic cyanobacteria have attracted interest as production organisms for third-generation biofuels, where sunlight and CO2 are used by microbes directly to synthesize fuel molecules. A particularly suitable biofuel is n-butanol, and there have been several laboratory reports of genetically engineered photosynthetic cyanobacteria capable of synthesizing and secreting n-butanol. This work evaluates the environmental impacts and cumulative energy demand (CED) of cyanobacteria-produced n-butanol through a cradle-to-grave consequential life cycle assessment (LCA). A hypothetical production plant in northern Sweden (area 1 ha, producing 5-85 m(3) n-butanol per year) was considered, and a range of cultivation formats and cellular productivity scenarios assessed. Depending on the scenario, greenhouse gas emissions (GHGe) ranged from 16.9 to 58.6 gCO(2)eq/MJ(BuOH) and the CED from 3.8 to 13 MJ/MJ(BuOH). Only with the assumption of a nearby paper mill to supply waste sources for heat and CO2 was the sustainability requirement of at least 60% GHGe savings compared to fossil fuels reached, though placement in northern Sweden reduced energy needed for reactor cooling. A high CED in all scenarios shows that significant metabolic engineering is necessary, such as a carbon partitioning of >90% to n-butanol, as well as improved light utilization, to begin to displace fossil fuels or even first- and second-generation bioethanol.