The Microbial Relay: How Bacteria and Fungi Drive Aerobic Organic Matter Degradation

Compost turning - from Scarab International https://scarabmfg.com

In composting—as in activated sludge, biosolids stabilization, and other biological treatment systems—the real work is performed by a dynamic microbial consortium. What looks like “rotting” is actually a structured succession of microbial guilds responding to substrate availability, temperature, moisture, and oxygen transfer.

Two functional groups dominate this process: bacteria and fungi. Their interaction is not static; it shifts through phases of competition, inhibition, and cooperation as the organic matrix changes. Understanding this succession is key to optimizing any aerobic stabilization process, whether in a backyard pile or a biosolids windrow.
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1. Functional Guilds: Rapid Responders vs. Structural Degraders

Microbial Group	Bacteria (Rapid Responders)	Fungi (Structural Degraders)
Primary Substrates	Soluble and readily biodegradable organics (simple sugars, amino acids, starches).	Recalcitrant carbon fractions (cellulose, hemicellulose, lignin).
Growth Dynamics	High growth rates; short generation times; dominate early stages.	Slow-growing; develop extensive hyphal networks; dominate late stages.
Key Contribution	Rapid oxidation of simple organics, driving thermophilic heat generation.	Enzymatic breakdown of lignocellulosic material and humification.

 

​This division mirrors what wastewater professionals see in RBCs, trickling filters, and aerobic digesters: bacteria handle the fast, soluble load; fungi and actinomycetes step in when the substrate becomes more complex.
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2. Temperature-Driven Succession: Three Distinct Phases
Microbial dominance in composting is governed largely by temperature, which itself is a byproduct of metabolic heat release.

Act I: Mesophilic Phase — Bacterial Bloom
Typical Range: ~20–40°C
Fresh nitrogen-rich inputs trigger a surge of mesophilic heterotrophic bacteria. Their rapid oxidation of soluble organics increases temperature and oxygen demand.

• Dynamics: Bacteria outcompete fungi due to faster growth rates and access to soluble substrates.
• Outcome: Heat accumulation pushes the system into thermophilic conditions, suppressing most fungal activity.
  
  

Act II: Thermophilic Phase — High-Rate Oxidation
Typical Range: ~45–70°C
As temperatures rise, mesophiles decline and thermophilic bacteria take over. This phase is analogous to high-rate aerobic digestion.

• Fungal Status: Most fungi are inhibited or sporulate; activity is concentrated in cooler outer zones.
• Notable Group: Actinobacteria, filamentous bacteria often mistaken for fungi, thrive here and contribute to lignocellulose breakdown and geosmin production (the “earthy” odor).
  
  

Act III: Curing Phase — Fungal Recolonization
Typical Range: Cooling to ambient
Once readily biodegradable organics are depleted, metabolic heat drops and the system cools.

• Dynamics: Fungi re-establish dominance, targeting lignin, the most recalcitrant fraction of plant biomass.
• Importance: Lignin degradation exposes cellulose and hemicellulose, enabling secondary bacterial activity and driving humus formation—the stable end product.
  
  

This curing phase is analogous to the endogenous respiration period in biosolids stabilization, where slow-growing organisms finish the job.

3. Carbon–Nitrogen Balance Through a Microbial Lens
The classic “Browns vs. Greens” rule maps directly to microbial ecology:

• Excess Nitrogen (Greens):
  High soluble organics → bacterial overgrowth → oxygen depletion → anaerobic pockets → odor generation (VFAs, reduced sulfur compounds).
• Excess Carbon (Browns):
  High lignocellulose → limited bacterial activity → insufficient heat generation → fungal-dominant, low-temperature composting.
  

Just as in wastewater treatment, substrate balance dictates microbial selection.

4. Operational Practices That Influence Microbial Performance

1. Limit late-stage turning:
   Excessive agitation disrupts fungal hyphae during the curing phase. Similar to biofilm systems, structural integrity matters for lignin degradation.
2. Particle size reduction:
   Shredding carbon-rich materials increases surface area, improving fungal colonization and oxygen penetration.
3. Moisture control:
   Bacteria require thin-film water for substrate transport; fungi tolerate lower moisture.
   Over-drying shifts the system toward fungal dominance and uneven decomposition.
   
   

Summary

• Bacteria drive the high-rate oxidation of soluble organics and thermophilic heat generation.
• Fungi are essential for degrading lignin and completing humification.
• Succession—not a single microbial group—is what stabilizes organic matter effectively.
• Operational controls (moisture, aeration, particle size, C:N ratio) determine which guild dominates and when.