Anaerobic Metabolism

Anaerobic metabolism allows cells to grow in the absence of oxygen. 

Strict anaerobes are capable of only anaerobic metabolism.

Facultative anaerobes are capable of both aerobic & anaerobic metabolism.  (ex. E. coli)    

A.  Anaerobic Respiration - Involves an electron transport chain, but uses a compound other than oxygen as the final electron acceptor, allowing the cell to generate ATP by chemiosmosis.  Compounds that can be used as final electron acceptors include sulfate & nitrate.

1.    Nitrate users: These organisms, including E. coli, play a role in the nitrogen cycle (removing nitrogen from terrestrial & aquatic environments & returning it to the atmosphere).  Some microbes reduce nitrate to nitrite.  Some microbes reduce nitrite further to nitrogen gas.  We’ll see this in lab!

2.    Sulfate users: These organisms, called sulfur reducers, play a role in the sulfur cycle.  Sulfate is reduced to hydrogen sulfide gas.  Sulfate reducers typically grow in marine & river mud flats, giving these environments a rotten-egg odor & turning the mud black.  We’ll see this in lab!

B.    Fermentation - This type of anaerobic metabolism uses no electron transport chain.

            Some differences between fermentation & respiration (aerobic or anaerobic): 

1.       Fermentation generates fewer ATP per molecule of substrate. (ex. E. coli can produce 38 ATP/molecule of glucose by aerobic respiration, but only 3 ATP by fermentation.)

2.      Because many molecules of substrate must be metabolized to supply a cell's ATP requirements, the substrate must be in abundance in order for the microbe to grow.

3.      Sugars are usually the only substrate that can be used in fermentation.

1.    Lactic Acid Fermentation - This type of fermentation is carried out by lactic acid bacteria, the microbes that cause milk to sour (used to produce yogurt & buttermilk). Muscle tissue of animals also carries out lactic acid fermentation, when deprived of oxygen (during strenuous exercise - this is what makes your muscles sore).  Steps:

1.       Glycolysis - Glucose is metabolized to produce 2 molecules of pyruvic acid (a little ATP & NADH is also produced).

2.      Pyruvic Acid Oxidation - pyruvic acid is oxidized to lactic acid using NADH, thus using up the reducing power stored in glycolysis.

Parts of the Kreb's cycle & Pentose Phosphate pathway are used to help generate the 12 precursor metabolites; they cannot all be formed in fermentation.

       2.  Alcoholic Fermentation

This type of fermentation is typical of yeast, a type of fungi.  In this pathway, pyruvic acid is converted to carbon dioxide and ethanol.    

                     glucose à pyruvic acid à carbon dioxide + ethanol

V.  Nutritional Classes of Microorganisms

            Microbes are classified according to nutritional class, which depends on 2 factors: 

1.)    How it generates ATP & reducing power:  chemo- (from oxidation of inorganic compounds like sulfur nitrite, ammonia, iron) or photo- (sun)       

2.)   The source of carbon atoms it uses to make precursor metabolites:  auto- (obtain carbon from carbon dioxide) or hetero- (obtain carbon from organic compounds like carbs, proteins, lipids, etc.)

Can have combinations of all of these: 

chemoautotrophs, chemoheterotrophs, photoautotrophs, photoheterotrophs.