Bacterial PHA Biosynthesis: Fermentation, Upstream & Downstream Recovery

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.

• 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.

3.1 PHA Biosynthesis Pathway

In Cupriavidus necator (formerly Ralstonia eutropha) — the model PHA producer:

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:

3.3 Downstream Recovery

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:

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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:

$$[PHB] = 12 \text{ g/L} \times 0.72 = \textbf{8.64 \text{ g/L}}$$

(b) Volumetric productivity:

$$r_{PHB} = \frac{8.64 \text{ g/L}}{18 \text{ h}} = \textbf{0.48 \text{ g/L·h}}$$

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.

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.

• 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.

1. ASTM D6691 — Biodegradability of plastic materials in the marine environment
2. ISO 14855 — Determination of the ultimate aerobic biodegradability (compostability)
3. ASTM D5338 — Aerobic biodegradability under composting conditions
4. ISO 17088 — Specifications for compostable plastics

1. A bioreactor yields 18 g/L biomass with 68% PHB content after 24 h. Calculate PHB concentration and volumetric productivity.
2. Explain why PHB is brittle and how PHBV copolymerisation improves mechanical properties.
3. Compare chloroform extraction vs. sodium hypochlorite for PHB recovery: which is preferred for food-contact applications?

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