A mixotrophic fermentation method comprising (i) providing (a) an isolated naturally acetogenic organism selected from wild type Clostridium ljungdahlii, Clostridium autoethanogenum, and Clostridium ragsdalei, the native form of the organism having a glucose metabolism rate of less than 0.01 g/hr/g cell mass, (b) an acetogenic organism selected from Clostridium ljungdahlii, Clostridium autoethanogenum, and Clostridium ragsdalei which has been genetically modified to express a thiolase, CoA-transferase subunit A, CoA-transferase subunit B, acetoacetate decarboxylase, and secondary alcohol dehydrogenase, or (c) an acetogenic organism selected from Clostridium ljungdahlii, Clostridium autoethanogenum, and Clostridium ragsdalei which has been genetically modified to reduce expression of a secondary alcohol dehydrogenase or to delete from the genome a secondary alcohol dehydrogenase, and which has been genetically modified to express a thiolase, CoA-transferase subunit A, CoA-transferase subunit B, and acetoacetate decarboxylase; (ii) providing a first feedstock and a second feedstock wherein said first feedstock comprises a sugar that is metabolized by the native form of the organism at a rate of less than 0.01 g/hr/g cell mass; and wherein said second feedstock comprises CO, CO 2, carbonate, bicarbonate, H 2, glycerol, methanol, formate, urea, or a combination thereof; and (iii) culturing said organism in a fermentation medium, whereby both feedstocks are metabolized and a fermentation broth is formed, which broth comprises at least one bioproduct selected from acetone, isopropanol, acetate, or a combination thereof; wherein the method yields a greater amount of the at least one bioproduct than the combined amounts of the at least one bioproduct produced by heterotrophic and autotrophic fermentation with the same organism under the same conditions; and wherein the carbon yield, based on the total amount of carbon in produced bioproducts divided by the total amount of carbon metabolized from said first feedstock, is at least 50%. (i) providing (a) an isolated naturally acetogenic organism selected from wild type Clostridium ljungdahlii, Clostridium autoethanogenum, and Clostridium ragsdalei, the native form of the organism having a glucose metabolism rate of less than 0.01 g/hr/g cell mass, (b) an acetogenic organism selected from Clostridium ljungdahlii, Clostridium autoethanogenum, and Clostridium ragsdalei which has been genetically modified to express a thiolase, CoA-transferase subunit A, CoA-transferase subunit B, acetoacetate decarboxylase, and secondary alcohol dehydrogenase, or (c) an acetogenic organism selected from Clostridium ljungdahlii, Clostridium autoethanogenum, and Clostridium ragsdalei which has been genetically modified to reduce expression of a secondary alcohol dehydrogenase or to delete from the genome a secondary alcohol dehydrogenase, and which has been genetically modified to express a thiolase, CoA-transferase subunit A, CoA-transferase subunit B, and acetoacetate decarboxylase; (a) an isolated naturally acetogenic organism selected from wild type Clostridium ljungdahlii, Clostridium autoethanogenum, and Clostridium ragsdalei, the native form of the organism having a glucose metabolism rate of less than 0.01 g/hr/g cell mass, (b) an acetogenic organism selected from Clostridium ljungdahlii, Clostridium autoethanogenum, and Clostridium ragsdalei which has been genetically modified to express a thiolase, CoA-transferase subunit A, CoA-transferase subunit B, acetoacetate decarboxylase, and secondary alcohol dehydrogenase, or (c) an acetogenic organism selected from Clostridium ljungdahlii, Clostridium autoethanogenum, and Clostridium ragsdalei which has been genetically modified to reduce expression of a secondary alcohol dehydrogenase or to delete from the genome a secondary alcohol dehydrogenase, and which has been genetically modified to express a thiolase, CoA-transferase subunit A, CoA-transferase subunit B, and acetoacetate decarboxylase; (ii) providing a first feedstock and a second feedstock wherein said first feedstock comprises a sugar that is metabolized by the native form of the organism at a rate of less than 0.01 g/hr/g cell mass; and wherein said second feedstock comprises CO, CO 2, carbonate, bicarbonate, H 2, glycerol, methanol, formate, urea, or a combination thereof; and (iii) culturing said organism in a fermentation medium, whereby both feedstocks are metabolized and a fermentation broth is formed, which broth comprises at least one bioproduct selected from acetone, isopropanol, acetate, or a combination thereof;