In contrast to alginate, an additional 1 mol of NADH is generated under fermentative conditions when mannitol is metabolized to 2 mol of ethanol (Fig. 3).
One mole of NADH is generated from the conversion of mannitol to fructose-6-phosphate.
When fructose-6-phosphate is converted to two pyruvates, two NADHs are generated and these NADHs are consumed in the conversion of 2 mol of pyruvate to 2 mol of ethanol.
Therefore, if 2 mol of mannitol and 1 mol of alginate monosaccharide are simultaneously utilized by cells under fermentative conditions, the amount of NADH is balanced (Fig. 3).
Interestingly, a typical sugar ratio of mannitol to alginate in brown algae is approximately 8:5.
Hence, these two sugar components in brown algae can be simultaneously used for ethanol fermentation.