To achieve sustainable growth of human society, fossil fuels must eventually be replaced with renewable resources. Ultimately, the energy and carbon in fuels and chemicals synthesized must come from the sun and CO2 directly. Biological systems hold the promise to catalyze the synthesis of such fuels or chemicals. This article discusses recent advances in developing biofuel production processes from CO2, which include photosynthetic processes using algae and cyanobacteria and the non-photosynthetic “electrofuel” processes using Ralstonia eutropha and other lithoautotrophic microorganisms. Each of these processes involves strengths and weaknesses. While none of these processes have achieved industrial success, the challenges involved may point the direction for further improvement within the limit of theoretical possibility. Finally, all biological processes produce cell mass rich in protein. Regenerating ammonium by deamination of hydrolyzed proteins may close the loop of the global nitrogen cycle, which is also one of the major challenges in large scale biological processes.
