Seawater treats PLA and PHA differently and so should you.
Industrial composting is engineered, piles held near 58 degrees with moisture controlled and microbial activity concentrated by design. PLA is built for those conditions. Its ester bonds need heat-driven hydrolysis before microbes could do their work. Because of this chemistry, it is not able to degrade in seawater. This is why it is said that compostability does not imply environmental degradation.
PHA runs on different logic. It is a material manufactured by the microbes themselves. Bacteria build PHA as their internal carbon reserves, similar to the way animals store fat. Since the polymer already exists in nature, microorganisms are able to feed on it, thereby degrading it naturally. PHA degradation microbes secrete an enzyme called as PHA depolymerases that cut the polymer into fragments small enough to absorb.
PHA doesn’t disappear by magic, it disappears by rate.
Across scattered field data, marine degradation averages 0.04 to 0.09 , meaning a standard PHA bottle takes 1.5 to 3.5 years to fully break down (Dilkes-Hoffman et al., 2019). Crucially, depth and water temperature dictate that timeline. When University of Tokyo researchers deployed PHA microbeads 757 meters deep via the Shinkai6500 submersible, five months yielded dense biofilms and pitted surfaces, but minimal mass loss. The same beads in warm Tokyo Bay surface waters reached ~85% degradation in just 25 days, outpacing cellulose (Hyodo et al., 2024).
Same polymer, wildly different outcomes. Calling PHA ‘ocean biodegradable’ without specifying depth, temperature, or wall thickness describes a material property, not a real-world timeline.
PHA Evaluation
When evaluating a PHA-based biodegradable plastic, it is important to focus on test results rather than marketing brochures.
First, ask which type of PHA is used and what else is included in the blend, including the percentage of PHA. Different PHAs such as P(3HB), PHBV, and PHBHHx can behave differently, and blends containing low amounts of PHA may perform similarly to conventional plastics in water.
Second, ask whether the material is marine certified or only compost certified. TÜV Austria’s OK biodegradable MARINE certification requires 90% of the carbon to be converted into CO₂ within six months under ASTM D6691 and ISO 23977, along with disintegration and toxicity testing; since ASTM D7081 was withdrawn without replacement, this remains the relevant marine certification (Normec OWS, n.d.).
Third, ask which marine zone was actually tested, because floating water is very different from the seabed. Pelagic environments are tested according to ISO 23977, the sediment–seawater interface according to ISO 19679, and sediment according to ISO 22404, which is particularly important because much plastic litter eventually sinks.
Finally, ask at what thickness and in what physical form the material was tested. Powder generally degrades faster than sheets, while a 20-micron film can behave very differently from a thick, moulded product such as a fork.
Where TerraPHA Fits
TerraPHA is the world’s first non-GMO biopolymer company to produce PHA using naturally occurring microorganisms. It uses precision fermentation processes to make sure the material is produced consistently and can be made on a larger scale.
Another important feature of TerraPHA’s technology is that it can use different renewable carbon sources as raw materials instead of depending on only one type of feedstock. This flexibility makes the production process more adaptable.
The PHA material is also designed to biodegrade, and this property comes from the actual chemical structure of the polymer, rather than being only a marketing claim. The packaging range covers the formats where marine performance earns its keep: carry bags, grocery bags, compostable bags, food wraps, food packaging, single-use cutlery, primary packaging material and agricultural mulching-film.
The Takeaway
“Is it biodegradable?” has no units, so it cannot really be answered. The answerable version is in what timeline, in which water, at what thickness, and verified by whom.
A bag that reaches a waste facility doesn’t need to be marine-degradable. PHA’s value lies in what happens on the days collection fails, and it fails often enough that more than eleven million tonnes of plastic reach the ocean each year (UNEP, n.d.).
Frequently Asked Questions
Is PHA just another name for PLA?
No, and the difference is structural. PLA is chemically synthesised from fermented plant sugar and needs industrial composting to break down. PHA is produced inside microbial cells as a natural energy store, so marine bacteria already carry the enzymes to digest it. In matched seawater testing, PLA reached about 1% biodegradation in 28 days against 31-35% for a PHA copolymer (Sashiwa et al., 2018).
How long PHA takes to break down in the sea?
For a thick-walled item such as a bottle. Roughly one and a half to three and a half years based on pooled field measurements (Dilkes-Hoffman et al., 2019). Thin films go considerably faster, and cold deep water slows everything down.
Does that mean PHA packaging can be thrown into the water?
No. Marine biodegradability is a safety net for packaging that escapes collection, not a disposal method. An item sitting in water for months can still entangle or be swallowed by marine life before it degrades. Collection and composting remain the intended route.
Reference
Dilkes-Hoffman, L. S., Lant, P. A., Laycock, B., & Pratt, S. (2019). The rate of biodegradation of PHA bioplastics in the marine environment: A meta-study. Marine Pollution Bulletin, 142, 15–24. https://pubmed.ncbi.nlm.nih.gov/31232288/
Hyodo, N., Gan, H., Ilangovan, M., Kimura, S., Kasuya, K., Isobe, N., & Iwata, T. (2024). Coastal and deep-sea biodegradation of polyhydroxyalkanoate microbeads. Scientific Reports, 14, 10302. https://doi.org/10.1038/s41598-024-60949-z
Normec OWS. (n.d.). Marine degradation. https://normecows.com/degradation-toxicity/marine-degradation/
Royer, S.-J., Greco, F., Kogler, M., & Deheyn, D. D. (2023). Not so biodegradable: Polylactic acid and cellulose/plastic blend textiles lack fast biodegradation in marine waters. PLOS ONE, 18(5), e0284681. https://doi.org/10.1371/journal.pone.0284681
Sashiwa, H., Fukuda, R., Okura, T., Sato, S., & Nakayama, A. (2018). Microbial degradation behavior in seawater of polyester blends containing poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx). Marine Drugs, 16(1), 34. https://doi.org/10.3390/md16010034
United Nations Environment Programme. (n.d.). Plastic pollution & marine litter. https://www.unep.org/topics/ocean-seas-and-coasts/ecosystem-degradation-pollution/plastic-pollution-marine-litter
