Biodegradable plastic signifies false hope for threatened Mediterranean seagrass community

4_Seagrass_taken from Time Magazine

A blog post on seagrass by our IYOR intern, Talia Wong. Enjoy!:

4_Seagrass_taken from Time Magazine

At a 100,000 year old, Posidonia oceania meadow is one of the oldest things on Earth and it has been reported that they are also one of the most important carbon sinks in the area. Unfortunately, like Cymodocea nodosa, this species is also highly threatened. (Photo: Time Magazine)

Not a solution but creation of biodegradable plastic is an inventive step in trying to “reduce” our plastic footprint.

Plastic pollution, the bane of the 21st century. Today, an unprecedented amount of plastics accumulates and drifts in the world’s oceans. With countless disturbing images of turtles, seals and other marine creatures photographed caught in plastic trash and beaches filled with plastic bottles now etched into our minds, some countries are scrambling to be rid of this marine hazard by banning and replacing it with biodegradable plastic. So far, Vanuatu, is the only South Pacific nation in the world that officially bans the consumption of plastics.

While marine creatures are receiving much media attention due to their charismatic public appeal, little is known about the effects of plastics on the seagrass community.

Not the most exciting, cute or annoying species on the planet, majority of the world’s population have hardly heard of seagrass. Sometimes even classifying them as the marine macroalgae, “seaweed”.

Seagrass has been understudied and undervalued. It turns out that they exist in many parts of the world! Seagrasses are found in shallow coastal waters and estuaries across the world’s ocean floor (except Antarctica). These communities live either as a single specie community or coexist with other species of seagrass communities.

It is easy to dismiss a flourishing seagrass meadow as a useless and unimportant patch of grass. Sadly, this is the unavoidable and tragic reality of life when there is limited knowledge and exposure about the topic.

A vibrant and lush seagrass meadow not only maintains marine biodiversity by serving as a nursery for most juvenile reef fishes, it regulates coastal water quality, serves as coastal defence and storm buffers and plays a fundamental role in structuring communities – just to name a few.

Unfortunately, many species today face an uncertain future.

With the trickle-down effect of climate change such as ocean acidification, warmer waters, sea level rise (which will affect the amount of light reaching the seabed) and local stressors such as point source pollution, eutrophication and trawling activities to deal with, these clonal marine flowering plants are helpless against these pressures.

And, to make matters worse, plastic trash comprise a major component of marine litter. It has been reported that for every hectare along the Italian coat, 14 conventional bag fragments can be found lying near seagrass meadows.

Thankfully, our ability to create and innovate has not failed us. We can proudly proclaim that biodegradable plastic now exists although its use is not as ubiquitous (yet).

These bags are made from renewable raw materials like corn starch, cellulose or biosynthesized materials. Being denser, these bags will only remain at the sea surface for 10 to 25 minutes before sinking to the bottom of the ocean unlike conventional plastics which remain suspended near the surface for a longer time.  

Whilst on the ocean floor, these new bags supposedly degrade into water, carbon dioxide, methane and biomass when it interacts with microbial organisms. Such a scenario paints a rosy picture for humanity. It would mean one less anthropogenic hassle to deal with from the list of urgent issues to tackle. It would allow us to focus our attention on other pressing matters like limiting global warming under 2 degrees Celsius.  

However, a new study published in the journal of Science and Total Environment in July 2017 by biologists from the University of Pisa noted that these new generation bags are not as degradable as they were made out to be. While these bags have been validated and certified as compostable and biodegradable, they were tested in non-marine environment settings and are so not telling of their actual ability to disintegrate in the ocean.  

What made this outdoor study different was its use of natural marine sediment (silica sand) and constant flowing seawater to recreate the marine environment. The study examined the bag’s degradation rate under those conditions and brought in common Mediterranean seagrass species, Cymodocea nodosa and Zostera noltei to understand the bag-seagrass interaction.

Biodegradable bags (made of vegetable oil and corn starch) and cellulose filter paper (control) were cut up in small, standard sized pieces and buried in silica sand to mimic the seabed for the first experiment. And for the second experiment, standard sized biodegradable bags are placed in 3 types of setups consisting of 1 specie, 2 same species and 2 different species respectively.

After 6 months of sediment exposure, the bag pieces recorded negligible degradation while the control cellulose filter paper underwent substantial degradation. The bag showed initial signs of surface disintegration (cracks and holes) when studied under a microscope and displayed notable discolouration from white to pink and yellow. Nonetheless, the bag retained 85% of its initial weight whereas the filter paper lost 60% of its initial weight.

It was noted that the presence of the bags altered the seabed geochemistry. Temperature and oxygen concentration of the sediment were lower than before as the bags acted as a barrier against gas and heat exchange between the seawater and the sediment.

Consequently, the seabed acts as a sink for bioplastics. It becomes more apparent in temperate waters where water temperature is lower and less conducive for decomposition.

And, of course, we have forgotten about the seagrass community (yet again).

The presence of seagrass counteracts these changes.

The Mediterranean seagrass species reverse the geochemistry changes of the sediment. Its natural metabolic activities raise sediment temperature. And for species like Z. noltei, it releases oxygen into the ground to establish more favourable seabed conditions for root growth and to protect itself against any nearby reduced phytotoxic compounds.

In relation to plant growth and plant-plant interaction, the results of the study conclude that the bag increases competition for space and resources between specie(s), both in monospecific and mixed meadows. So, the presence of the biodegradable bag introduces yet another local stressor to the seagrass.

Consequently, the presence of these bags on the seabed acts just like highways cutting through terrestrial forests causing spatial fragmentation within the seagrass community. Like the terrestrial setting, fragmenting the seagrass ecosystem leads to habitat destruction and isolation leading to the formation of discontinuities within the original seagrass area. These discontinuities hinder genetic flow and reduce diversity and thus, weaken the resilience of the seagrass community to future disturbances.

Even as these biodegradable bags become the plastics of the near future, to end off on a positive note, they hold potential for future success.

In these outdoor stimulations, the controlled natural environment was sufficiently microbial active to degrade the cellulose and starch materials. This potentially suggests that marine environment itself already holds an important key to success. Furthermore, given the differential degradation rates of the filter paper and the bag pieces, the actual rate of bio-plastic deterioration really depends on the composition of the bag and its local context. Granted that the experiments conducted tried to emulate the real marine environment, the lack of wave abrasion and amount of ultraviolet radiation cannot be replicated.

For now, biodegradable plastics offers us a dim light amidst despair as it paves the way for better innovations. And as this new study shines more light onto the interface between bioplastics, sea grass and sediment bed, it creates more opportunities for further research into the long-term impacts of biodegradable plastics and the deterioration success of different types of bag compositions.

Regardless of this technological breakthrough, our plastic problem cannot be swept under the rug and forgotten.

Let us all keep in mind that plastics – biodegradable or not – are not natural entities that belong to the marine world and we should do our best to keep them out of it.

References

Balestri, Elena et al. “Biodegradable Plastic Bags On The Seafloor: A Future Threat For Seagrass Meadows?.” Science of The Total Environment 605-606 (2017): 755-763.

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