Fermentation of sugars

Sugars can be fermented by a variety of different microorganisms via different pathways leading to typical end products (Gottschalk, 1985). Generally, C6 sugars are degraded by glycolysis or the Entner-Doudoroff pathway to pyruvate, while C5 sugars are converted via a combined pentose pathway and the glycolytic or the Entner-Doudoroff pathway to pyruvate. Conversion of sugars to pyruvate results in the reduction of nicotinamide adenine dinucleotide (NAD+) to form NADH. The further metabolism of pyruvate depends on the biochemical mechanism by which sugar-fermenting microorganisms dispose reducing equivalents. Facultative aerobic microorganisms perform a mixed acid fermentation, resulting in the formation of ethanol, lactate, succinate, formate and butanediol. These bacteria produce formate by pyruvate:formate lyase. Formate is split to H2 and CO2 by formate:hydrogen lyase. Alcoholic fermentation, lactic acid fermentation, homoacetogenic fermentation, propionic acid and butyric acid fermentation are examples of specific fermentations carried out by anaerobic microorganisms. The combined occurrence of all these fermentations by mixed microbial communities will yield a variety of products. Except for propionate, butyrate and long chain fatty acids, the reduced compounds are fermented further by specific microorganisms.

The utilization of hydrogen by methanogens affects the metabolism of fermentative microorganisms that have the ability to use protons as electron sinks. A typical example is the fermentation of glucose by Ruminococcus albus (Ianotti et al, 1973). In pure culture it forms acetate, CO2, hydrogen and ethanol, while in the coculture ethanol is not formed (Figure 2.3).

R. albus degrades glucose via a glycolytic pathway, leading to the formation of NADH and reduced Fd. The oxidation of reduced Fd is energetically easy to couple to hydrogen formation, while H2 formation from NADH is only possible at a low hydrogen partial pressure:

At a partial pressure of hydrogen of 1 pascal (Pa), created by methanogens, the AG0' of the two conversions is around -26 and -11 kilojoule per mol (kJ/mol), respectively. During sugar fermentation by R. albus in pure culture, hydrogen accumulates, and due to this, NADH oxidation to proton reduction is no longer possible. As an alternative, acetyl-CoA or acetaldehyde is used as electron sink to form ethanol. In coculture with a hydrogen scavenger,

Figure 2.3 Sugar fermentation by Ruminococcus albus in (A) pure and (B) mixed culture

Note: ATP = adenosine triphosphate

Figure 2.3 Sugar fermentation by Ruminococcus albus in (A) pure and (B) mixed culture

Note: ATP = adenosine triphosphate hydrogen is removed efficiently, and ethanol is not produced. Similar effects have also been observed with other sugar-fermenting microorganisms forming ethanol, lactate, succinate, butyrate or propionate (Stams, 1994; Schink and Stams, 2006). Some sugar-fermenting bacteria can only convert sugars to acetate, H2, CO2 and formate. These bacteria strictly depend for growth on hydrogen removal by methanogens (Krumholz and Bryant, 1986; Müller et al, 2008).

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