Aerobes obligate facultative

Table 3.2 Alternative Electron Acceptors (redox couple) Used by Aerobic, Facultatively Anaerobic, and Obligately Anaerobic Bacteria at Neutral pH and the Associated Microbial Processes...

Microbes capable of carrying out fermentations are classified as either facultative or obligate anaerobes. Facultative anaerobes, such as the enterobacteria, utilize 02 if and when it is present, but if it is absent, they carry out fermentative metabolism. In contrast, obligate anaerobes are unable to synthesize the components of electron transport systems consequently, they cannot grow as aerobes. Moreover, many of the obligate anaerobes cannot even tolerate oxygen and perish in air these organisms are referred to as strict anaerobes. 

Mineralization of organic matter continues under anoxic conditions, due to the activity of various anaerobic bacteria (the main group being the Clostridia), but the overall rate is slower. As well as obligate anaerobes (e.g. Clostridium), these bacteria include aerotolerant anaerobes (e.g. Lactobacillus) and facultative anaerobes (i.e. bacteria that are usually aerobes but can function anaerobically, e.g. Escherichia). 

The other phyla contain but few genera which consist of facultative and obligate aerobic and anaerobic organisms [1]. 

Saturated soil conditions support microbial populations adapted to anaerobic environments. Aerobic microbial populations are restricted to zones where oxygen is available. Most of the aerobic organisms become quiescent, or die, and new inhabitants, largely facultative (these organisms can function under both aerobic and anaerobic environments) and obligate anaerobic bacteria, take over (Figure 3.8). 

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