Acetogen

Definition

Diverse group of obligate anaerobic bacteria which can derive energy only by anaerobic respiration (fermentation) and/or by ion gradient-driven phosphorylation are called acetogens and the process involved is called acetogenesis.

Characteristics

Acetogens carried out unique pathway called Wood-Ljungdahl pathway also known as reductive acetyl- CoA pathway which helps bacteria to grow in anaerobic conditions. 

They need strict anaerobic environment for growth. 

Acetogens use one carbon compound as substrate such as carbon dioxide, formate, methyl groups and carbon monoxide etc. and use H₂ as energy source.

Acetogens generates acetate (CH₃CHOO^-) as an end product of anaerobic respiration or fermentation. 

Acetate is the main end product of acetogenesis, but many other natural products such as butyrate, butanol, 2, 3-butanediol, and ethanol can also be produced by acetogens. 

Acetogens employ acetyl-CoA as a key enzyme during acetogenesis. 

Energy yield is low due to anaerobic respiration. 

Acetogens are ubiquitous in nature. 

Different genus of acetogens have different shape such as cocci (Diplococcus), Bacilli and Coccobacilli. 

Habitat

Acetogens possess broad range of habitat, generally present in anaerobic ecosystems such as sediments, soils, acidic habitats, and ecosystems with high salt concentrations or high temperatures. Acetogens are also present in the gastrointestinal tracts of mammalians and termites, deep subsurface samples, marine sediments, and freshwater. 

Metabolic role

Acetogens metabolic activity helps them to survive in different environmental conditions. Different host species possess different acetogens populations. Hydrogen present in their gut utilised by acetogens as energy source and acetate is produced as a major end product. Minor propionate, butyrate, and isovalerate were also generated. 

Hydrogen gas present in gut environment is an inhibitor of biodegradation process.    Acetogens use hydrogen, CO₂ and favours bio degradative capacity of host. 

Acetogens are important contributors of food chain and play a role of primary fermenters as they can use and convert alcohols, lactates and fatty acids, which are usually restricted to syntrophs.

Wood Ljungdahl Pathway

Source:  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4959221/

Importance 

Key component of anaerobic food web. 

Helps in CO₂ fixation.

Acetogens can out-compete methanogens and produce methane due to their wide substrate range utilization capacity. 

Methane producing ability makes them helpful in waste water treatment and generation of clean energy.

Acetogens are producer of acetate and alcohol which is beneficial for industries.

Acetogens are responsible for microbially influenced corrosion (MIC) of iron. e.g.  

Acetobacterium woodii. 

Acetogens can cause rapid degradation of vinyl chloride.

Example: 

     Clostridium asceticum

•                           Acetobacterium woodii