Conceptual diagrams of multi-species microbial cell-factories. (a), a general overview of the principle consisting in the metabolic compartmentalization between a specialized photosynthetic module cross-feeding organic C, likely simple sugars (CaHbOc) and O2 to a production module for C-containing biofuels with some CO2 recycling. Although this is a widespread metabolic loop in natural communities, recent improvement by genetic engineering has represented valuable proof-of-concept suggesting that it worths to further develop this principle for biotechnological purposes.