This disclosure describes recombinant Megasphaera microbes designed to include increased consumption of acetate, increased carbon flux to butyryl-CoA and/or hexanoyl-CoA, increased production of butyrate and/or hexanoate, or a combination thereof, than a comparable control. This disclosure also describes methods that generally include growing such recombinant microbes under conditions effective for the recombinant microbes to consume greater amounts of acetate, produce increased amounts of butyryl-CoA and/or hexanoyl-CoA, produce increased amounts of butyrate and/or hexanoate, or a combination thereof.

A system for carbon fixation is provided. The system comprises enzymes which catalyze reactions of a carbon fixation pathway, wherein at least one of the reactions of the carbon fixation pathway is a carboxylation reaction, wherein products of the reactions of the carbon fixation pathway comprise oxaloacetate and malonyl-CoA, wherein an enzyme which performs the carboxylation reaction is selected from the group consisting of phophoenolpyruvate (PEP) carboxlase, pyruvate carboxylase and acetyl-CoA carboxylase and wherein an export product of the carbon fixation pathway is glyoxylate. Additional carbon fixation pathways are also provided and methods of generating same.