Metabolic Diversity

Microbial diversity at the biochemical level is more important from the industrial perspective. During the past decade, molecular biological and genetic approaches have come to dominate the study of microorganisms. Although fruitful, exclusive reliance on such approaches poses both practical and philosophical problems for microbiology. After an early phase of discounting the importance of physiology, people in industry are learning from the "biotechnology revolution" that gene cloning and biochemical manipulations do not solve all the product-development problems that may arise. Some of the more valued intellectual benefits anticipated in the field will emerge from understanding the physiology of peculiar and diverse microorganisms from unusual environments. As long as oxygen is present as the electron acceptor, denitrification, sulfate reduction, and methanogenesis are inhibited. De-nitrification begins after consumption of oxygen, and sulfate reduction begins after consumption of nitrate. Finally, methanogenesis occurs, which is partly a dissimilatory carbon dioxide reduction. The sequence of these reactions agrees with the sequence of their free-energy changes, which decrease from respiration to methanogenesis.

Table 2 lists some representative anaerobic bacteria that display different metabolic types based on their cat-abolic pathways. These different biochemistries explain, in part, the great diversity of anaerobic microorganisms.

Methanogenesis. Methanogenic bacteria are strictly anaerobic archaea with a unique form of energy metabolism involving the generation of methane. Biological methano-genesis is an important component of the carbon cycle in a variety of anaerobic habitats. It represents the terminal

Table 1. Representative Bacterial and Archaeal Species of Anaerobic Extremophiles



Optimal growth condition

Representative species



Acidophile Alkaliphile Syntrophile




Moderate salinity High salinity

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