What is a Facultative anaerobes

Facultative anaerobes are microorganisms — typically bacteria or yeasts — that have the metabolic flexibility to grow and generate energy in both the presence and absence of oxygen. This adaptability makes them particularly important in wastewater treatment, sludge digestion, and various natural and engineered biological processes where oxygen availability may fluctuate.

Unlike strict aerobes (which require oxygen) or obligate anaerobes (which are harmed by oxygen), facultative anaerobes can switch between aerobic respiration when oxygen is available and anaerobic respiration or fermentation when it is not. This ability allows them to thrive in diverse environments, including biofilms, activated sludge, septic tanks, and anaerobic digesters.

Metabolic Flexibility

The key to the success of facultative anaerobes lies in their ability to alter their energy production pathways based on environmental oxygen levels:

  • In oxygen-rich environments, they preferentially use aerobic respiration, which yields the most energy (up to 38 ATP molecules per glucose molecule).
  • In oxygen-deprived environments, they switch to fermentation or anaerobic respiration, which yields significantly less energy but allows continued survival and reproduction.

Example Pathways:

  • Aerobic respiration: Glucose + O₂ → CO₂ + H₂O + Energy (ATP)
  • Fermentation: Glucose → Lactic acid or ethanol + CO₂ + Energy (ATP)
  • Anaerobic respiration: Glucose + nitrate/sulphate/other terminal electron acceptors → Energy (ATP) + reduced by-products

This metabolic adaptability enables facultative anaerobes to colonise a wide range of environments and outcompete organisms that rely solely on one respiratory mode.

Behaviour in Stratified Environments

In mixed or stratified systems such as sediment layers, sludge beds, or microbial colonies, facultative anaerobes often accumulate near the top, where oxygen levels are higher. This positioning allows them to take advantage of aerobic respiration, which is more efficient and yields more energy.

However, if oxygen becomes limited or depleted — for example, deeper within a biofilm or during high microbial activity — these organisms can continue functioning anaerobically. This trait is crucial in systems where oxygen penetration is limited but biochemical activity must continue.

Role in Wastewater Treatment

Facultative anaerobes are essential in various wastewater treatment processes, including:

1. Activated Sludge Systems

These bacteria contribute to the breakdown of organic material in aeration tanks. During aerated periods, they metabolise carbon using oxygen; during non-aerated zones (such as in flocs), they continue functioning anaerobically.

2. Septic Tanks and Lagoons

In primary treatment systems with intermittent or low oxygen availability, facultative anaerobes dominate microbial populations, breaking down solids through fermentation and anaerobic pathways.

3. Facultative Ponds (Stabilisation Ponds)

These treatment lagoons are designed to promote both aerobic and anaerobic activity. Facultative anaerobes play a central role in degrading organic matter throughout the water column.

4. Anaerobic Digesters

Although primarily designed for obligate anaerobes, facultative anaerobes can initiate early breakdown of organic matter and support intermediate metabolic steps, such as hydrolysis and acidogenesis.

Examples of Facultative Anaerobes

Some well-known facultative anaerobic microorganisms include:

  • Escherichia coli (E. coli) – Commonly found in the intestines and in wastewater.
  • Staphylococcus aureus – Known for its role in infections and its presence in wastewater streams.
  • Enterobacter species – Frequently used as indicators of faecal contamination.
  • Saccharomyces cerevisiae – A facultative yeast widely used in fermentation industries.

These organisms are of interest not only in treatment processes but also in microbiological monitoring and public health assessments.

Environmental and Engineering Considerations

When designing or operating treatment systems that rely on biological processes, understanding the presence and function of facultative anaerobes can influence:

  • Oxygen supply strategies: Optimising aeration to balance energy use with microbial efficiency.
  • Sludge retention times: Allowing sufficient time for complete biodegradation through mixed metabolic pathways.
  • Pathogen control: Some facultative anaerobes may also be opportunistic pathogens and require monitoring.
  • Biochemical oxygen demand (BOD) removal: These organisms play a key role in reducing BOD under both aerobic and anaerobic conditions.

Conclusion

Facultative anaerobes are highly adaptable microorganisms that play a vital role in both natural ecosystems and engineered water treatment systems. Their ability to switch between aerobic and anaerobic metabolism allows them to thrive under changing environmental conditions and makes them indispensable in the biological treatment of wastewater. For engineers, microbiologists, and environmental professionals, understanding facultative anaerobes is essential to designing efficient, resilient, and sustainable treatment systems capable of handling variable oxygen conditions.