Bacterial PHA Biosynthesis: Fermentation, Upstream & Downstream Recovery
Bacterial Polyhydroxyalkanoate (PHA/PHB) biosynthesis, Cupriavidus necator fermentation, nutrient starvation kinetics, intracellular granule accumulation, and cell disruption recovery.
01 · Why This Matters in Industry & GATE XE-F
Applied directly across petrochemical refining, compounding plants, mold-flow simulations, and automotive part manufacturing (e.g., Reliance Industries, Supreme Petrochem, IOCL, CIPET testing protocols).
Molecular Mechanism: Master conformational physics, transition temperatures, and reaction kinetics.
Process & Quality: Predict viscosity behavior, solve molding defects, and apply ASTM/ISO testing standards.
Bacterial PHA Biosynthesis: Fermentation, Upstream & Downstream Recovery
1. Why This Topic Matters
Polyhydroxyalkanoates (PHAs) are microbially synthesised, fully biodegradable biopolyesters — the closest bacterial equivalent to petroleum-based polyolefins. PHB (polyhydroxybutyrate), the simplest PHA, is produced by bacteria as an intracellular carbon/energy storage polymer. PHB and PHBV (polyhydroxybutyrate-co-valerate) are commercially produced by Danimer Scientific (USA), Tianan Biopolymer (China), and emergent Indian start-ups like Newlight Technologies and Napps Technology (Hyderabad). Understanding PHA fermentation biochemistry, fed-batch control, and solvent/non-solvent extraction is essential for sustainable plastics engineers.
2. Learning Objectives
- Write the PHA biosynthesis pathway from acetyl-CoA to PHB granules.
- Design a fed-batch fermentation strategy for maximum PHA accumulation.
- Calculate PHA cell content (wt%) from biomass and PHA mass data.
- Compare solvent extraction vs. enzymatic/non-solvent downstream recovery.
- Identify ASTM D6691 (biodegradability) and ISO 14855 (compostability) test standards.
3. Core Theory
3.1 PHA Biosynthesis Pathway
In Cupriavidus necator (formerly Ralstonia eutropha) — the model PHA producer:
| Step | Enzyme | Reaction |
|---|---|---|
| 1 | β-ketothiolase (phaA) | 2 Acetyl-CoA → Acetoacetyl-CoA |
| 2 | Acetoacetyl-CoA reductase (phaB) | Acetoacetyl-CoA + NADPH → (R)-3-Hydroxybutyryl-CoA |
| 3 | PHA synthase (phaC) | n (R)-3-HB-CoA → PHB granule + n CoA |
PHB accumulation is triggered by nutrient limitation (nitrogen, phosphorus, or oxygen) while carbon source (glucose, butyrate) remains in excess. Bacteria divert excess carbon into PHB rather than biomass.
3.2 Fed-Batch Fermentation Strategy
Phase 1 (Growth phase): Balanced nutrients + carbon source → maximise biomass X (g/L) Phase 2 (PHA accumulation phase): Deplete N source (NH₄⁺ < 0.1 g/L) while feeding carbon → PHA accumulates inside cells
Key performance metrics:
| Metric | Definition | Target Value |
|---|---|---|
| PHA cell content | PHA/(PHA + non-PHA biomass) × 100% | > 70% (wt/wt) |
| Volumetric productivity | g PHA/(L·h) | 2–5 g/L·h (commercial) |
| Carbon yield | g PHA / g glucose | 0.30–0.40 g/g |
3.3 Downstream Recovery
| Method | Process | Purity | Cost | Environmental |
|---|---|---|---|---|
| Chloroform extraction | Hot CHCl₃ dissolves PHB → non-solvent (MeOH) precipitation | >99% | High | Toxic solvents |
| Sodium hypochlorite | NaOCl digests non-PHB biomass — PHA granules survive | 85–95% | Low | Mild |
| Enzymatic (Protease/SDS) | Enzymes lyse cell wall — PHA granules released | 90–95% | Medium | Green |
| Supercritical CO₂ | scCO₂ plasticises PHB → extraction | >98% | High | Very green |
Trade-off: Chloroform extraction gives highest purity but highest environmental and regulatory cost. Non-solvent routes preferred for food/medical applications.
3.4 PHBV Copolymer — Property Tuning
PHB is brittle (elongation at break <5%). Incorporating 3-hydroxyvalerate (HV) units into the chain from propionic acid co-substrate gives PHBV:
| HV content (mol%) | Tm (°C) | Elongation at break | Application |
|---|---|---|---|
| 0% (PHB) | 177 | 2–5% | Rigid packaging (brittle) |
| 10% (PHBV) | 150 | 20–30% | Film, coatings |
| 20% (PHBV) | 135 | 50–80% | Flexible packaging |
4. Worked Example
Problem: A bioreactor run produces 12 g/L total dry biomass. PHB content (measured by GC after chloroform extraction) = 72 wt%. Calculate: (a) PHB concentration (g/L), (b) volumetric productivity over 18 h fermentation.
(a) PHB concentration:
(b) Volumetric productivity:
Interpretation: 0.48 g/L·h is below the commercial target of 2–5 g/L·h. Optimisation strategies: increase carbon feed rate, improve N-depletion timing, use higher-yielding strain (engineered C. necator with upregulated phaC synthase), or switch to fed-batch with pH-stat carbon feeding.
5. Indian Industry Context
Napps Technology (Hyderabad) produces PHA (PHB/PHBV) from sugar cane molasses and vegetable oil feedstocks. They have piloted a 10,000 L bioreactor system targeting Indian single-use plastic replacement applications. Their PHBV film achieved 100% biodegradation in 120 days in simulated marine environment (ASTM D6691).
CJ Bio India and Metabolix (now part of Yield10 Bioscience) have explored rice straw and agricultural residue as low-cost carbon feedstocks for PHA production in India — addressing the ₹8/kg PHA cost gap vs. ₹1.5/kg HDPE using waste valorisation.
6. Key Takeaways & Glossary
- PHA: Polyhydroxyalkanoate — bacterial intracellular biopolyester; synthesised under nutrient limitation.
- PHB: Polyhydroxybutyrate — simplest PHA; highly crystalline, brittle; Tm = 177°C.
- PHBV: PHB-co-HV — reduced crystallinity; improved flexibility; Tm 135–150°C depending on HV%.
- PHA synthase (phaC): The terminal polymerisation enzyme — key metabolic engineering target.
- PHA cell content: Target >70% for commercial viability; driven by N-limited accumulation phase.
- Carbon yield: g PHA / g glucose ≈ 0.33–0.40 — metabolic efficiency of PHB biosynthesis.
7. Standards Reference
- ASTM D6691 — Biodegradability of plastic materials in the marine environment
- ISO 14855 — Determination of the ultimate aerobic biodegradability (compostability)
- ASTM D5338 — Aerobic biodegradability under composting conditions
- ISO 17088 — Specifications for compostable plastics
8. Practice Questions
- A bioreactor yields 18 g/L biomass with 68% PHB content after 24 h. Calculate PHB concentration and volumetric productivity.
- Explain why PHB is brittle and how PHBV copolymerisation improves mechanical properties.
- Compare chloroform extraction vs. sodium hypochlorite for PHB recovery: which is preferred for food-contact applications?
9. Quiz
Q1. PHA accumulation in bacteria is triggered by: B) Nutrient limitation (N, P) with excess carbon source Q2. The enzyme responsible for final PHB polymerisation is: C) PHA synthase (phaC) Q3. Adding 3-hydroxyvalerate units to PHB improves: B) Flexibility and elongation at break Q4. Commercial target for PHA cell content: C) > 70 wt% Q5. Volumetric productivity target for commercial PHA: B) 2–5 g/L·h
Bacterial PHA Biosynthesis: Fermentation, Upstream & Downstream Recovery · Engineering Triad
Material Synthesis · Processing Hardware · Commercial Application
Poly(lactic acid) (PLA) & PBAT Blend
—[O—CH(CH₃)—CO]ₙ— (Enantiomeric L-Lactide / D-Lactide)
Multi-Layer Blown Film Extrusion Line with Internal Bubble Cooling
Co-Extrusion 3-Layer Die (Grooved Feed Extruders, L/D = 30:1)
Certified Industrially Compostable Carry Bags & Mulch Films
Single-use plastic replacement complying with PWM Rules 2022
Test Your Conceptual Understanding
In polymer science and processing thermodynamics, which factor most directly controls the critical transition temperature?
- Always evaluate molecular weight distribution (MWD) alongside zero-shear viscosity when calculating mold shear rates.
- Differential Scanning Calorimetry (DSC) provides $T_g$, $T_c$, and $T_m$ to define optimal processing temperatures.
- Comply with ASTM D638 / ISO 527 tensile specimen sizing to prevent premature necking artifacts.
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