In vitro Respiration and Artificial Photosynthesis: Yin-Yang of Biological Energy Transformations

Photosynthesis and cellular respiration support all biological activities of lives, representing the yin-yang (i.e., opposite yet complementary processes) of biological energy transformations on the Earth. Inspired by photosynthesis, artificial photosynthesis (AP) is being developed to store solar energy to chemical energy of compounds for human society. Here we propose a new concept, in vitro respiration (ivR), which converts the chemical energy stored in compounds (e.g., starch and methanol) to electricity or hydrogen through in vitro artificial catabolic pathways. ivR can be classified into two types, aerobic and anaerobic. Aerobic ivR generates electricity and could power electronics including portable devices. Meanwhile, anaerobic ivR produces hydrogen and could power fuel-cell-based machinery including new energy trucks. ivR and AP function as the yin-yang of artificial biological energy transformations. Furthermore, these two complementary processes can be utilized to constitute a closed-loop and carbon-neutral energy system called ECHO. ECHO demonstrates potential as an energy hub of the future bioeconomy, owing to three synergistic advantages: (1) the high energy efficiency of ivR, (2) high energy density of starch as an energy-storage compound, and (3) inherent biosafety of the whole energy flow. To address the energy demands of the coming bioeconomy, it is imperative to increase the energy transformation rate and reduce the cost of ivR and AP through cross-disciplinary collaborations integrating synthetic biology, electrochemistry, photochemistry, and process engineering.