Supplemented mixotrophic fermentation method

1. A supplemented mixotrophic fermentation method comprising

i. providing a naturally acetogenic organism capable of metabolizing a sugar and H 2 for producing ethanol;

ii. providing a fermentation medium comprising a carbon source and a non-sugar reductant wherein said carbon source consists of a sugar that is metabolized by the native form of the organism at a maximum rate of less than 0.01 g/hr/g cell mass; and wherein said non-sugar reductant consists of H 2 at weight/weight ratio between said sugar and said H 2 in the range between 1.20 and 150; and

iii. culturing said organism in said fermentation medium, whereby both the carbon source and the non-sugar reductant are metabolized and a fermentation broth comprising ethanol is formed, and wherein:

a. the carbon yield based on the total amount of carbon in produced ethanol divided by the total amount of carbon metabolized from said carbon source is at least 67%;

b. the total reductant efficiency is at least 67%; or

c. CO 2 is emitted from a fermenter during culturing, and a weight/weight ratio between ethanol in said fermentation broth and the amount of emitted CO 2 is greater than 1.05.

2. The method of claim 1 , comprising adding to said fermentation medium a mixture of CO 2 and hydrogen at a molar ratio in the range of from 1:0.1 to 1:15.

3. The method of claim 1 , wherein said sugar comprises glucose or sucrose, wherein metabolizing the sugar produces CO 2 , and wherein the organism metabolizes the CO 2 produced.

4. The method of claim 1 , wherein said sugar consists of a mixture of glucose and sucrose, and wherein the organism metabolizes the CO 2 produced.

5. The method of claim 1 , wherein said fermentation medium comprises stillage of ethanol production.

6. The method of claim 1 , further comprising gasification of a corn processing co-product, whereby said non-sugar reductant is generated.

7. The method of claim 1 , co-located with ethanol production.