A scrutiny of the authenticity of ocean-degradable plastics
July 2026, Cornwall Peninsula, UK.
The water temperature was just above 17 degrees Celsius, and at a tidal flat near the shore, the experimental apparatus deployed nine weeks earlier was retrieved.
Researchers opened the rusted metal cages and removed the plastic film samples inside—these films were divided into two categories: one group consisted of bioplastics labeled as "biodegradable" (PLA/PBAT blend films and commercial compostable Mater-Bi films), and the other group was conventional low-density polyethylene (LDPE) used as a reference.
The result was unexpected, yet entirely reasonable.
Regardless of whether a "anti-chewing" cage was installed, most samples of the so-called "degradable" films lost over 95% of their mass within nine weeks, while the adjacent LDPE film remained almost completely intact.
This seems like good news: biodegradable plastics have indeed shown significant quality degradation.
But the next finding is truly intriguing: researchers carefully examined the degraded films using scanning electron microscopy and discovered numerous micrometer-scale pits and surface roughening. However, there was no significant difference in degradation levels between the films kept in "anti-biting" cages—shielded from marine invertebrate feeding—and those exposed to the open environment.
In statistical terms, the analysis of variance showed that exposure time accounted for approximately 94.6% of the degradation differences, while the presence or absence of large invertebrate grazing had little to no statistically significant effect.
This indicates that, at least in this experiment, large marine animals are not the key factor determining degradation rates. The romantic narrative of "marine organisms eating plastic," which has been highly anticipated, needs to be reevaluated in light of actual experimental data.
And even more intriguing findings were yet to come. In laboratory feeding experiments, researchers fed PLA/PBAT film fragments to marine isopods (small crustaceans resembling shrimp) and whelks.
As a result, 60% of the PLA/PBAT fragments and 20% of the LDPE fragments showed signs of amphipod biting.
Under the microscope, plastic micro particles were clearly detected in the intestines and feces of these organisms.
In other words, these so-called "biodegradable" bioplastics are broken down further by marine animals in the ocean before microorganisms can fully digest them, resulting in smaller plastic particles, some of which reach the size of microplastics.
This is one of the most authentic snapshots of PLA and PHA—the two materials that dominate the global bioplastic market.
Let's start with a more fundamental logical question.
1. Two Letters, Two Entirely Different Fates
When it comes to biodegradable plastics, PLA (polylactic acid) and PHA (polyhydroxyalkanoates) are two unavoidable giants.
PLA accounts for about a quarter of the global bioplastic capacity, with worldwide production approaching 600,000 tons by 2025. It is produced by fermenting sugars from biomass such as corn, sugarcane, and cassava into lactic acid, which is then polymerized. The biodegradable straw in your hand, fresh produce trays at supermarkets, and even some 3D printing materials are most likely made of PLA.
PHA, on the other hand, is a class of natural polyesters produced by bacteria under specific conditions. Over 150 different PHA monomers have been identified in scientific literature to date. Its applications range from packaging films and agricultural mulch films to medical implants. PHA enjoys an even greener reputation than PLA, as it not only can be biologically manufactured but also theoretically degrades more effectively in natural environments.
Yet the phrase "in theory" is precisely the eye of the entire controversy.
From a molecular perspective, the degradation of PLA primarily relies on hydrolysis: water molecules attack the ester bonds in the polymer chain, breaking it into progressively shorter fragments, which can then be consumed by microorganisms. The key factors in this process are temperature and humidity. Under the consistently high conditions of around 58°C and sufficient moisture found in industrial composting facilities, PLA can rapidly degrade within several months.
But what about the ocean?
The temperature in deep sea remains around 4°C year-round, while shallow sea temperatures vary seasonally but rarely reach the level required for rapid hydrolysis of PLA.
A deep-sea experiment published in 2025 in Polymer Degradation and Stability tells a harsh story:
Researchers simultaneously sank PLA film and a new type of PHA copolymer, LAHB (lactic acid-3-hydroxybutyrate copolymer), to the seabed of Sagami Bay, Japan, at a depth of 855 meters.
After 13 months, the LAHB film lost over 80% of its mass and was covered with colonies of deep-sea microorganisms and biofilm, while the adjacent PLA film showed almost no measurable weight loss, had barely any visible biofilm formation on its surface, and remained largely intact—almost like a stone in the deep-sea environment.
This is not an isolated case: a 2021 study published in Frontiers in Environmental Science exposed PLA and PHA films to a simulated marine environment (sea water at approximately 20°C) for six months. The results showed that, under these conditions, PHA demonstrated overall better degradation performance than PLA, although significant differences existed among the materials, making it difficult to draw a uniform conclusion regarding whether they are fully degraded.
Therefore, the "degradation" of PLA in real marine environments is more accurately described as a slow hydrolysis process in academic terms, rather than the intuitive notion of "disappearance," and its degradation may take decades or even longer.
PHA, although much better than PLA, is not always so "compliant."
A Hawaii Pearl Harbor experiment published on bioRxiv at the end of 2025 found that PHA showed early signs of degradation during a 22-week marine immersion test, while PLA remained largely unchanged.
More interestingly, the biofilm communities on PHA surfaces resemble the "natural" communities found on wood and mangrove seedlings, whereas those on PLA surfaces are more similar to those on conventional PET plastic.
PHA is more likely to form degradation-related biofilms and indeed has greater affinity than PLA, but the question is, is that enough to prevent it from becoming marine litter?
II. Standard Maze: What Does "Degradation" Really Mean? Who Decides?
If you look into ISO or ASTM standards for "marine degradation" testing (such as ASTM D6691), you'll notice an intriguing detail: these standards permit testing under laboratory conditions at approximately 30°C, primarily to ensure reproducibility rather than to simulate all real-world marine environments.
However, 30°C is the upper limit that surface seawater in temperate or cold coastal regions reaches only on a few days during the peak summer months, while temperatures in deep sea and during winter are far below this.
A systematic review published in the Journal of Environmental Sciences in 2024 bluntly stated that existing testing standards "fail to provide clear parameter targets for defining a substance as biodegradable" and grant users "excessive freedom in selecting process parameters."
What does this lead to? The same material can yield vastly different "degradation rates" under different laboratories and varying "standards."
A 2025 study published in the Journal of Polymers and the Environment tracked the performance of a "compostable" plastic bag made of Mater-Bi (a starch/PBAT blend) and a PHB (a type of PHA) film under simulated marine conditions.
This system is "flow-through," meaning seawater is continuously refreshed to closely simulate a natural intertidal environment.
As a result, during the nine-month observation period, Mater-Bi and PHB lost an average of about 25% to 47% of their area or mass in shallow water sediments.
Note that this level was achieved after nine months, and even in a relatively favorable environment for microbial activity such as sediment. It wasn't until the water temperature rose to about 20°C after 30 weeks that degradation became noticeable (approximately 0.87% per week).
But the question is, how many consumers realize that the "ocean-degradable" bags they purchase require being soaked in seawater at around 20°C for over half a year to break down less than half?
The standard provides a passing score under an "upper limit" scenario, while the real world presents a long-term test under a "lower limit" situation.
3. Reverse Thinking: If it cannot degrade, will PHA/PLA turn into new microplastics?
This is the central question of the entire article.
One of the hazards of traditional plastics is that they break down physically and degrade under ultraviolet exposure into microplastics (<5 mm), which then spread throughout the entire food chain.
Then, if PHA and PLA cannot degrade either, aren't they just another form of microplastic?
Returning to our initial experiment: isopods in the lab consumed PLA/PBAT films, and microplastics were detected in their feces and tissues—confirming that "bio-erosion" does indeed occur, as animals actively chew down bioplastics into smaller particles.
Another experimental evidence is that Korean researchers blended PLA and PHA to form films, which were then immersed in simulated seawater for 45 weeks. The results showed that the higher the PHA content, the greater the mass loss—films containing 40% PHA retained only about 30% of their original weight.
However, please note that the remaining approximately 30% still exists largely as fragments and oligomers, rather than being fully mineralized into carbon dioxide and water. In another marine sediment experiment lasting 424 days, the model predicted that PHA would require about 909 days to completely degrade.
What do these PHA/PLA microplastics do during the period of about two to three years (or even longer) before complete degradation?
When ingested by organisms, such as amphipods in experiments, they may be mistaken for food, and due to their "bio-based" labeling, they might be more likely to be consumed.
Acting as a pollutant carrier, a 2025 review on the negative impacts of bioplastics indicated that existing studies suggest PHA and PLA microplastics have the potential to adsorb persistent organic pollutants (POPs) and heavy metals from seawater, and may transfer through the food chain.
In the Pearl Harbor experiment, although PHA degraded, it significantly promoted the proliferation of sulfate-reducing microorganisms (SRM)—of course, this phenomenon was observed under specific experimental conditions and cannot be directly generalized to all marine environments worldwide.
A seemingly paradoxical conclusion is that bioplastics labeled as "degradable" may have a more "active" ecological impact than conventional plastics during their prolonged actual degradation process—they could be physically broken down more rapidly by organisms and also become more intimately involved in the energy flow of food webs.
From an environmental science perspective, regardless of whether the consequences of this involvement are more "harmless" or more "controversial," the fact remains: the process of transitioning into the microplastic stage is not necessarily significantly slower than that of conventional plastics.
4. The "Double-Edged Sword" of Chemical Structure
If PHA is indeed consumed by marine microorganisms, the end products are carbon dioxide, water, and biomass—substances that are inherently non-toxic.
But the problem is, what if it hasn't been completely eaten?
A 2024 study published in the journal *Macromolecular Research* found that blending amorphous PHA with PLLA (poly-L-lactic acid) indeed promoted the degradation of PLLA under simulated seawater conditions (approximately 30°C) over 12 weeks, but the mechanism was primarily "end-chain scission," meaning hydrolysis occurred from the ends of the polymer chains.
In the initial PLA/PBAT + Mater-Bi experiment, the microplastics generated during degradation were not digested after being ingested by organisms, but instead appeared in their feces and tissues. This indicates that these microplastics have the ability to penetrate the intestinal barrier and enter the organism's internal circulation.
Another overlooked issue is additives.
PLA and PHA often require the addition of plasticizers, antioxidants, pigments, and other additives during processing; as the polymer matrix slowly degrades in marine environments, these additives may gradually be released.
For marine organisms, what they face is not only "bio-based microplastics," but also chemical additives that adhere to them and are released together.
A 2025 academic review sharply pointed out that "the degradation products and additives of bioplastics may cause oxidative stress, tissue damage, neurotoxicity, and even endocrine disruption in marine organisms," and that these issues have been "severely overlooked in the existing body of research."
We seem to be caught in a "green paradox": we replace a fossil-based material with a bio-based one, claiming it's "degradable," and consumers then let their guard down, even discarding it more carelessly (after all, "it's degradable").
As a result, it entered an environment unsuitable for degradation (the ocean), transforming into microplastics at a rate similar to fossil plastics while simultaneously releasing unique chemical additives.
So-called "eco-friendly alternatives" may, in certain contexts, merely transform one form of long-term pollution into another uncertain, and even different type of ecological risk.
V. A Ray of Hope? The LAHB Story
At this point in the story, it's necessary to mention a "counterexample," as this is precisely the balance that scientific reporting should maintain.
The LAHB material mentioned earlier, which demonstrated remarkable performance at a depth of 855 meters, indeed represents a promising direction.
This microbially synthesized lactic acid-3HB copolyester degraded by over 80% within 13 months under the low temperature, high pressure, and nutrient-poor conditions of the deep sea, while PLA showed almost no change.
Its degradation mechanism is clear: specific deep-sea microorganisms (such as those in the genus Colwellia) secrete extracellular depolymerases that first break long chains into smaller fragments, which are then consumed by other microorganisms to complete mineralization.
The Taguchi research team from Shinshu University in Japan calls it an "advanced evolved version" of PLA—sharing PLA's excellent material properties while overcoming its shortcomings in marine degradation.
Even for an optimist like LAHB, there are still several "elephants in the room" that need to be confronted:
The cost of producing PHA through microbial fermentation is inherently higher than that of PLA. How economically viable is LAHB, a version engineered using genetically modified strains? How far are we from large-scale commercialization?
How much grain (glucose) is consumed throughout the entire lifecycle of LAHB production? What are the land, water, and fertilizer inputs required? Is its "carbon footprint" truly lower than that of PLA? If I consume more energy and grain to achieve biodegradability, how should this cost be calculated?
Even LAHB takes seven months to degrade by about 30% and over 13 months to break down more than 80%. During this 13-month period, it remains a "degradable waste" slowly releasing microplastics and additives.
LAHB proved one thing: the boundaries of science hold promise, but it also proved another: today's reality is stark and bleak.
VI. When "Biodegradable" Becomes a Marketing Symbol
At this point, I can't help but think of a question: The gap between the public's understanding of "biodegradable" and the scientific facts seems to be much larger than we imagined.
Many people subconsciously believe that "degradable = environmentally friendly = just throw it away at will".
But the reality is that "biobased" only indicates that the raw materials come from renewable sources (plants), but it does not mean that it will disappear quickly in any environment. On the other hand, "degradable" is a process description that heavily relies on "postnatal conditions" and is not an "inherent attribute".
In a chapter published by IntechOpen in 2025, the opening paragraph highlights a sharp contradiction: "Although bioplastics are highly praised for their biodegradability, the optimal conditions required for decomposition in marine and freshwater ecosystems, such as high temperatures and controlled humidity, are rarely achievable."
Therefore, most bioplastics, especially PLA, when transported by water flow, will exacerbate the problem of microplastic pollution.
This is not an overstatement.
When we throw "compostable" bags into regular trash bins, they end up in landfills or incinerators and do not decompose; when they are washed away by rain and flow into rivers and eventually into the sea, they are even more "deceitful" than ordinary plastic - because they carry the "environmental protection" guilt-free medal, yet they are doing the same thing.
To some extent, in certain promotional materials, there is a tendency to simplify "conditionally degradable" to "naturally environmentally friendly". This kind of communication method poses a risk of triggering the "greenwashing" controversy. It takes advantage of the public's ignorance of chemical terms, packaging a "conditional" solution as an "unconditional" moral choice.
And true sustainable development will never be achieved through a single solution such as "changing materials".
What it requires is a complete system: source reduction, reuse, recycling by classification, supporting infrastructure for industrial composting, and the demystification of the "degradable" myth.
Seven. Do we need to be pessimistic?
The answer is no.
In 2026, scientific research has already moved beyond the simplistic "is it environmentally friendly or not" debate and has entered the detailed quantitative stage of "under what conditions, over what period of time, and what kind of impact will it have".
We know that PLA is very difficult to degrade in water, but it can indeed be degraded in industrial composting.
We know that PHA can decompose relatively quickly in sediment, but it will also first go through a "awkward period" as a microplastic.
We know that there are microorganisms in the deep sea that can consume LAHB, but their "working efficiency" is not fast enough and the cost is not low enough.
So, if I were the decision-maker, what would I do?
Redefine the "degradable" label and require that it be clearly indicated on the packaging: "Suitable for industrial composting only, not suitable for random disposal in the natural environment."
Prudent promotion of PHA materials In scenarios where recycling is difficult and items are prone to loss (such as some agricultural plastic films and fishing gear, etc.), PHA may have greater application value. However, it still needs to be evaluated in combination with specific environments and recycling systems.
Investing a huge amount of money in building industrial composting and anaerobic digestion infrastructure to ensure that "compostable materials" have a proper place to go.
Strictly regulate "degradable" advertisements. Prohibit equating "biological-based" with "no environmental impact". Require companies to provide actual degradation data in the expected disposal environment, rather than standard laboratory data.
For ordinary people, the next time they see the "PLA" or "PHA" labels, they might as well ask a little more:
"Should this thing be composted or just thrown into the regular trash bin?"
The answer determines whether it is an "environmental solution" or "another source of microplastics".
