What is diauxic?

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Understand diauxic: why do some graphs of Biochemical Methane Potential have steps? Discover how the microbial life cycle influences biogas production.

What is diauxic?

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Do you know why some biochemical methane potential graphs show a step?

Understanding diauxic and its Influence on Biogas Production

Typical Stepped Curve

This is a curious phenomenon that occurs in the anaerobic process, called diauxic. The first observation records were made by Jacques Monod in 1941, who identified diauxic growth during his experiments.

In fact, this behavior that can be observed in some curves of the result of the Biochemical Methane Potential test - BMP) of biogas production reflects the behavior of cell growth when there is multiplication of microorganisms in the anaerobic process.

During the anaerobic process, microorganisms go through an initial phase characterized as a physiological adaptation phase to nutrients and the environment they are in, called lag phase. According to Carneiro (2018), after adaptation, microorganisms initiate the fermentative and exponential growth process (considering glucose as the first source of energy, for example). This phase of exponential growth is called log phase.

In the third stage of the process, when glucose is depleted, cells enter a second lag phase, the stabilization phase that occurs between the first and second lag phases is called diauxic. At this moment, microorganisms go through another adaptation stage to consume a new substrate (degradation and obtaining energy from a new source). Then, the new growth stage is called post-diauxic (the second log phase), where cells start to metabolize ethanol (an example of a new substrate that will be degraded).

Cell Growth Phases

This adaptation behavior (lag phase), exponential growth (log phase), diauxic, post-diauxic (second lag phase), and stationary phase correspond to the beginning, middle, and end profile of the anaerobic process.

When the substrate used in the anaerobic process is heterogeneous and presents a wide variation in carbohydrate, protein, and lipid compositions, the diauxic behavior becomes more evident.

Example: Anaerobic Digestion of Substrate with Heterogeneous Composition

During BMP tests of heterogeneous substrates such as the organic fraction of municipal solid waste (OFMSW), diauxic occurs when microorganisms have "preference" for degrading (consuming) a specific element of the substrate composition, consuming it completely before starting to consume other elements of the substrate composition. For example, we can mention when glucose degradation will occur before cellulose degradation.

The typical stepped curve corresponds to the time required for inoculum adaptation (anaerobic microorganisms) to the substrate, consuming first the substrates with higher biodegradability rate and leaving the less biodegradable compounds for later.

In summary, diauxic is a term used to describe organisms that have two distinct modes of reproduction or life cycle, and these distinct modes are reflected in cellular growth (multiplication of the group of microorganisms) and are directly related to the activity of these microorganisms in substrate degradation and biomethanization. Thus, biogas production curves will exhibit stepped characteristics (diauxic).

Consulted Bibliographic References

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What are granules?

About anaerobic processes, under certain conditions, there is the formation of structures consisting of anaerobic microorganisms, the anaerobic granules.

These structures (clusters of different microorganisms) enable more efficient nutrient transfer and promote the survival of the microbial community.

These densely packed clusters of microorganisms contribute to the acceleration of the anaerobic digestion process, especially in UASB reactor sludges.

Figure 1 - Anaerobic granular sludge cultivation reactors.

The anaerobic granules are very small spheres and have a vast community of living beings. They act on the decomposition of organic matter and enable nutrient recycling.

Figure 2 - Reactor flasks with a mixture of anaerobic granules (black spots) and substrates (lighter content).

Following the concept of "small pearls with condensed content", the Energy and Biogas Portal publishes a series of posts "Knowledge Nuggets" - small posts to contribute to the dissemination of information about the biogas production process.

Always follow our specific content about the science behind the anaerobic process and biogas production.

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To explore other concepts, access our Glossary.

Also access Biogas in Brazil.

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