Understanding Plastic and Biopolymer Persistence in Nature: Degradation Mechanisms and Timelines

Addressing plastic pollution calls for innovative solutions, and we're excited to share valuable insights into how biopolymers behave in the environment. Fossil-based plastic materials undergo various changes when exposed to the outdoors. These changes affect how long the plastic sticks around and the impact it can have. Different natural processes influence whether plastic and biopolymers break down quickly or linger.

Distinct materials exhibit unique behaviors in this context. While some, like Polyethylene (PE) and Polylactic Acid (PLA), experience rapid fragmentation leading to the accumulation of microplastics and nanoplastics, others, like Polyhydroxyalkanoates (PHAs), follow a more gradual transformation.

"Understanding plastics fates and lifetimes in the environment after fragmentation to barely visible micro- and nano-plastics is important and challenging, with much more research to be done." - Bronwyn Laycock, Professor at the School of Chemical Engineering, Faculty of Engineering, Architecture and Information Technology, University of Queensland.

Two Pathways of Environmental Persistence

When considering environmental persistence of fossil-based or bio-based materials, two primary pathways come into play: abiotic processes causing random polymer chain cleavage, and surface-driven biotic-accelerated hydrolysis or oxidation. These pathways shape the persistence of the various states of plastic materials or suitable substitute materials: macroplastic, mesoplastic, microplastic, and nanoplastic. In other words, fossil-based plastics or suitable substitutes could break apart all by themselves because of sunlight, water, and so on. Other times, microorganisms speed up this breaking apart process. These two ways control how long plastic materials or suitable substitutes stay in the environment in different forms, like big chunks, medium-sized bits, or small particles.

Different Materials, Different Behaviors

It's worth noting that materials like PE and PLA tend to experience rapid fragmentation, resulting in the accumulation of microplastics and nanoplastics. In contrast, other types of materials like PHAs follow a distinct path. PHAs undergo surface-driven biotic-accelerated hydrolysis, transitioning from macroplastic and mesoplastic states to soluble products through depolymerization, eventually transforming into end products. Unlike PE and PLA, PHAs exhibit a more gradual transformation, thanks to the involvement of microorganisms. To simplify, microorganisms play a role in breaking down biodegradable polymers into smaller pieces that eventually disappear. It is expected that for most items, the ultimate lifetime of PHA films would be no longer than a few years.

This information holds significant relevance in the lead-up to the Fourth Meeting of the United Nations Intergovernmental Negotiating Committee on Plastic Pollution (INC-4) in Ottawa, Canada early next year. Understanding the environmental behavior of different types of plastics plays a vital role in preparing for meaningful discussions and decision-making during these negotiations.

Exploring PHA Persistence

Let's dive deeper into the fascinating realm of PHAs and their environmental residence time!

PHAs, a type of biodegradable polymer, can rapidly degradade in diverse conditions, making them a compelling option in the battle against plastic waste.

"The ability of PHAs to biodegrade in the natural environment is an important feature of that polymer. However, it is important to understand that even highly biodegradable materials like PHAs will take different amounts of time to mineralise depending on their form (e.g. thickness and shape), as well as how they are processed and the environment they end up in." - Bronwyn Laycock, Professor at the School of Chemical Engineering, Faculty of Engineering, Architecture and Information Technology, University of Queensland.

The Role of Microorganisms

The journey of PHAs in the environment involves a natural interaction of microorganisms, enzymes, and the properties of the material itself. It's a delicate process that leads to the gradual breakdown of PHAs over time. What makes PHAs special is their natural ability to break down in different environments, including marine settings. PHAs, like P3HB, actually come from carbon stored in microorganisms. This leads to a lot of tiny living things in nature being able to break down PHAs. This process is called enzymatic hydrolysis and it sets PHAs apart from other biodegradable materials like PLA, which break down through non-biological processes.

Transformation Over Time

The distinct behavior of PHAs also extends to how they change as they age in the environment. Enzymatic hydrolysis converts them into smaller fragments over time. Thin PHA films can change into end products quite quickly when microorganisms start to grow on them. However, for thicker items like bottles, it might take approximately 3.6 years to fully break down in marine environments. This microbial-driven breakdown underscores the importance of comprehensive research to better understand their behavior.

Biodegradability in Marine Environments

Crucially, the biodegradability of PHAs extends to marine environments, where these polymers embark on a distinctive breakdown journey. In the ocean, PHA engages in a fascinating interplay with cutin, a polyester found on plant surfaces. Cutin, comprising C16 and C18 hydroxy fatty acids, serves as an extracellular hydrophobic layer, sourced from both terrestrial and marine plants. Terrestrial cutin undergoes a transformative process upon entering the ocean, as indicated by studies detecting C16 and C18 hydroxy fatty acids in seabed sediments. This process not only showcases the dynamic interplay between organic materials and marine environments but also contributes to the formation of a sustainable carbon cycle. Cutin, whether synthesized locally in marine settings or transported from terrestrial environments, undergoes biodegradation in the marine environment, closing the loop in a cyclic and eco-friendly manner.

 Examining PHA's biodegradability in both marine and soil environments provides nuanced insights. In marine settings, PHA demonstrates swift degradation, marked by reductions in thickness and weight upon introduction to seawater. Microbial communities in the "plastisphere" play a significant role in this efficient breakdown. In contrast, PHA's interaction with soil unfolds at a more measured pace, reflecting variations in microbial communities and environmental factors.

These differences underscore the importance of a tailored understanding of PHA's biodegradability in specific environmental contexts. The complexities of microbial interactions and soil conditions emphasize the need for a nuanced approach to navigate PHA's fate in diverse ecosystems. In summary, delving into PHA's biodegradation journey in ocean and soil environments offers valuable insights for practical applications while emphasizing the necessity for context-specific strategies in addressing plastic pollution. A balanced comprehension of PHA's interaction with different ecosystems contributes to more informed and effective sustainable practices!

• The University of Queensland Study: "Hazardous state lifetimes of biodegradable plastics in natural environments"

  Read the latest study by the University of Queensland here.