PLA Plastics Do Not Degrade in the Marine Environment Any Faster Than Plastics

A new study headed by the Institut de Ciències del Mar (ICM-CSIC) in Barcelona, in which the Instituto de Investigaciones Mariñas (IIM-CSIC) in Vigo has also participated, has disclosed the fact that PLA plastic, of biological and biodegradable origin, does not degrade in the marine environment any faster compared to plastics obtained from petroleum — polystyrene, polystyrene or polyethylene.

PLA plastic is utilized to produce plates, cutlery and single-use cups, among other things. Earlier studies had proved that this kind of plastic does not biodegrade less than 60 °C - conditions that do not happen in the ocean.

Even though experts believed that it would be highly sensitive to photodegradation — caused as a result of sunlight — and that the consequent degradation products would be more easily degraded with the help of marine bacteria, this is not the case.

The fact that plastic is biodegradable does not mean that it degrades under all conditions. For example, compostable plastic needs temperatures of more than 50 ºC to be biodegraded, and this does not occur in the ocean or in many other natural environments.

Cristina Romera-Castillo, Study Author and Researcher, Instituto de Investigaciones Mariñas

The study has been recently reported in the Marine Environmental Research journal.

The Degradation Process

To perform the study, scientists exposed various kinds of plastic to the temperature and solar radiation conditions of the ocean and examined the organic carbon liberated due to their degradation. They also measured the ability of marine bacteria to degrade this carbon.

The researchers discovered that biodegradable PLA plastic liberates no more carbon compared to petroleum-derived plastic. Also, marine bacteria seem to be equally or less efficient at degrading PLA photodegradation products, for instance, polystyrene.

Older, More Polluting

At the same time, laboratory experiments disclosed that old plastic tends to pollute much more compared to new plastic. The outcomes display that plastic thrown into the sea would be liberating around 57,000 tons of dissolved organic carbon annually. This is more than twice the amount recommended by earlier studies with the help of new plastic fragments.

This is because plastic loses the additives that protect it from degradation as a result of the impact of sunlight and erosion, leading to a greater release of chemical compounds into the water, either from the polymer itself or additives, which give the plastic its shape, color, flexibility, and other properties.

Cristina Romera-Castillo, Study Author and Researcher, Instituto de Investigaciones Mariñas

All these parameters have been reflected in another study that was recently published in the journal Frontiers of Marine Science. In this study, the authors warn about the effect this could have on both the carbon cycle and the marine ecosystem as the organic carbon liberated by plastic is the same carbon that backs marine bacteria. It is situated at the base of the marine food chain.

The good news is that, from what Romera-Castillo and her group have been able to prove, these bacteria have the ability to degrade a few compounds liberated by plastic. This helps reduce the impact they could possess on the ecosystem.

The compounds released by plastic could be resistant to degradation and accumulate in the ocean, but we have seen that at least some of them can be used by bacteria.

Marta Sebastián, Study Author and Researcher, Instituto de Investigaciones Mariñas

Indeed, for studies to be performed in the future, scientists will try to dig deeper into this last aspect to see if marine bacteria could be utilized to bioremediate or recover other environments that are contaminated by plastic.

This study has been performed within the framework of the PLASMAR projects of the ComFuturo Program of the FGCSIC and the COLPLAI project of the National Plan for Young Researchers.

Journal Reference:

Cristina, R-C., et al. (2022) Leaching and bioavailability of dissolved organic matter from petrol-based and biodegradable plastics. Marine Environmental Research. doi.org/10.1016/j.marenvres.2022.105607.

Source: https://www.icm.csic.es/en