Polyhydroxyalkanoates, or PHAs, are biodegradable plastics made by bacteria. The basic process involves feeding specific microorganisms a carbon-rich diet, starving them of key nutrients so they store energy as plastic granules inside their cells, then breaking those cells open to collect the polymer. While industrial production is still more expensive than conventional plastics, the steps themselves are well understood and scalable from a lab bench to a bioreactor.
What PHA Actually Is
PHA is a family of polyesters that bacteria produce naturally as an energy reserve, similar to the way your body stores fat. When a bacterium has plenty of carbon to eat but is running low on nitrogen or phosphorus, it converts that excess carbon into polymer granules packed inside its cell walls. More than 300 different microorganisms are known to do this, including species of Pseudomonas, Bacillus, and Cupriavidus necator (formerly known as Ralstonia eutropha), which is the most widely studied PHA producer.
The most common type of PHA is called PHB (polyhydroxybutyrate), a stiff, brittle plastic with properties similar to polypropylene. By adjusting the process, you can also produce copolymers like PHBV, which incorporate a second monomer to make the material more flexible and easier to process. Increasing the proportion of the second monomer from 0 to 25% lowers the melting point and crystallinity without significantly affecting thermal stability, widening the temperature window for molding and extrusion.
Choosing the Right Bacteria
The organism you select determines how much PHA you can produce and what type of polymer you get. Cupriavidus necator is the workhorse of PHA research because it accumulates large amounts of PHB under straightforward nutrient conditions. Pseudomonas species are another strong option, particularly for producing medium-chain-length PHAs with different material properties. In one study, a Pseudomonas strain designated P(16) achieved PHA yields between 73% and 92% of its dry cell weight, depending on the carbon source, which is exceptionally high.
Bacillus species are attractive because many are non-pathogenic and easier to work with from a safety standpoint. A Bacillus endophyticus strain optimized with statistical modeling reached about 50% PHA content using sucrose as its sole carbon source in a lab-scale bioreactor. That’s a solid yield, though lower than what top Pseudomonas and Cupriavidus strains can achieve.
Selecting a Carbon Source
Carbon is the raw ingredient bacteria convert into PHA, and it’s also the biggest cost driver. Pure sugars like glucose and fructose work well but are expensive at scale. The trend in PHA production is toward waste-derived carbon sources that cut costs and give agricultural or industrial byproducts a second life.
Effective low-cost substrates include rice bran, sugarcane molasses, soy molasses, date molasses, used cooking oil, and crude glycerol (a byproduct of biodiesel production). When the Pseudomonas P(16) strain was grown on soy molasses, it reached a 92% yield. Rice bran came in close at 91%, while date molasses produced 83%. The type of substrate matters significantly: different carbon sources change not only the yield but sometimes the composition of the polymer itself.
How Bacteria Build the Polymer
Inside the cell, PHA synthesis follows a three-step enzymatic pathway. First, the bacterium takes two molecules of acetyl-CoA, a common metabolic intermediate, and joins them together. Second, it reduces that product using cellular energy in the form of NADPH. Third, a polymerase enzyme links the resulting monomers into a growing polymer chain, building up granules that can eventually fill most of the cell’s interior. The availability of NADPH, the cell’s reducing currency, is considered the driving force behind how much PHA accumulates.
This pathway is the default for PHB production. To make copolymers like PHBV, you add a co-substrate such as propionic acid or valeric acid to the growth medium. The bacteria incorporate both types of monomer into the same polymer chain, and you control the final composition by adjusting the ratio of carbon sources in the feed.
Fermentation: The Three-Stage Approach
Getting bacteria to produce high levels of PHA requires more than just mixing cells with sugar. The established strategy uses a three-stage fermentation process in a bioreactor.
- Stage 1: Batch growth. Bacteria are inoculated into a nutrient-complete medium with carbon, nitrogen, phosphorus, and trace minerals. During this phase, cells multiply rapidly but accumulate little PHA because all nutrients are balanced.
- Stage 2: Fed-batch growth. Additional carbon is fed in pulses to keep the population growing and increase total biomass. Nitrogen is still available, so cells continue dividing.
- Stage 3: Nutrient limitation. Nitrogen (or sometimes phosphorus) is allowed to run out while carbon feeding continues. The bacteria can no longer divide but keep metabolizing carbon, channeling it into PHA storage granules. In one Cupriavidus necator fermentation, all the ammonium sulfate in the medium was consumed by 25 hours, triggering the PHB accumulation phase.
Temperature, pH, and dissolved oxygen also need monitoring. Most PHA-producing bacteria grow optimally between 30°C and 37°C, and aerobic conditions are essential. Pulsing the carbon feed during the accumulation phase, rather than adding it all at once, further increases the final PHA content.
Extracting PHA From Bacterial Cells
Once fermentation is complete, the PHA granules are trapped inside the bacterial cells. Getting them out is one of the trickiest parts of the process. The two main approaches are solvent extraction and chemical cell disruption, often used in combination.
In simple solvent extraction, dried cell pellets are stirred in chloroform for up to 48 hours at 37°C, then the dissolved PHA is precipitated by adding ice-cold ethanol. This method typically recovers about 82% of the polymer at around 85% purity. Adding a sodium hypochlorite pretreatment step before the solvent extraction boosts both numbers dramatically: yield jumps to 96% and purity reaches 99%. The hypochlorite breaks open cell membranes and removes lipid residues that would otherwise contaminate the final product.
Chloroform is the traditional solvent, but it’s toxic and environmentally unfriendly. Researchers have tested alternatives with the hypochlorite pretreatment approach. Ethylene carbonate achieved the highest yield at 98.6% when heated to 150°C. DMSO reached about 81% at just 50°C. Hexane performed poorly, recovering only around 2% regardless of temperature. The choice of solvent involves trade-offs between yield, purity, operating temperature, and environmental impact.
What PHA Costs Today
PHA currently costs between $4 and $6 per kilogram to produce, compared to $1 to $2 per kilogram for petroleum-based plastics like polyethylene or polypropylene. That price gap is the single biggest barrier to wider adoption. Carbon feedstock alone accounts for a large share of total production costs, which is why waste-derived carbon sources are so important for bringing the price down. Using agricultural byproducts like rice bran or molasses instead of refined sugars can significantly reduce raw material expenses without sacrificing yield.
How PHA Breaks Down in Nature
Unlike conventional plastics, PHA biodegrades in virtually every natural environment, though the timeline varies considerably depending on conditions. PHB powder fully degrades in soil in less than three months. PHB films lose about 97% of their mass in just 21 days when buried in soil at 28°C. Under industrial composting conditions at 58°C, PHB is completely composted in roughly 25 days.
Marine environments are slower. PHB films reach near-complete mineralization after about 100 days in seawater with marine sediment. PHBV films take roughly 210 days to reach 90% biodegradation in sand and seawater. Freshwater results vary widely: some PHA copolymer fibers degrade completely within 28 days, while one study predicted PHB would need around 282 days to reach 90% degradation under its specific freshwater conditions. Temperature, microbial activity in the environment, and the thickness and crystallinity of the material all influence how fast it disappears.
Safety Considerations
Most bacteria used for PHA production, particularly strains of Cupriavidus necator, Bacillus, and many Pseudomonas species, are classified as Biosafety Level 1 or 2 organisms. BSL-1 work requires standard lab coats, gloves, and eye protection. BSL-2 adds the requirement for a biological safety cabinet when performing procedures that could create aerosols, plus access to an autoclave for decontaminating waste. If you’re setting up a PHA production project in a university or research lab, the institutional biosafety committee will determine the appropriate level based on the specific strain you’re using and the procedures involved.
The extraction step introduces its own hazards. Chloroform is a volatile solvent that can cause liver damage with prolonged exposure and should only be used in a well-ventilated fume hood. Sodium hypochlorite is corrosive and produces chlorine gas if mixed with acids. Proper chemical handling protocols, including appropriate gloves, splash protection, and ventilation, are essential during the downstream processing stage.