Abstract: Syntrophic co-cultures containing at least two microorganisms, wherein (a) at least one of the microorganisms is a solventogen able to metabolize biomass to produce metabolic byproduct(s) therefrom, (b) at least one of the microorganisms is a microorganism different from the solventogen, wherein the microorganism different from the primary solventogen depends on the metabolites and/or the metabolism of the primary solventogen for survival and growth and is able to fix or metabolize the metabolic byproducts produced by the solventogen to produce metabolic byproduct(s) therefrom, and (c) the solventogen is able to metabolize the metabolic byproduct(s) produced by the microorganism different from the solventogen to produce further metabolic byproducts, such as liquid fuels and commodity chemicals, as well as methods for using these syntrophic co-cultures to produce such products via fermentation, are disclosed.