Figure 4: A comparison of CO2-derived chemicals via chemical [3,7,9,127,128] and biological routes. Three observations are derived from this comparison: 1) some chemicals (e.g., salicylic acid, methane) can be produced via either a chemical or a biological route, 2) biological routes complement chemical approaches and expand the range of chemicals that can be derived from CO2, and 3) complex molecules (e.g., protein) can only be produced via a biological route because a microorganism can be regarded as an integrated chemical plant encompassing a multitude of biocatalysts.