Marine bacteria and biodegradable plastics: synergy is the key

Marine bacteria and biodegradable plastics: synergy is the key

I am a big fan of the late Hans Rosling. His book “Factfulness” is an excellent reminder that, if you look at facts and numbers, the world is actually becoming a better place than it was before: Poverty is on the decline, population increase rate is decreasing, there is less and less war.

This optimistic picture, however, does not extend to plastic pollution. We are unfortunately producing and improperly disposing more and more plastic each year. Plastics are cheap, durable, and although they were once considered an engineering wonder, these days they are potential thrash the moment they are produced. A large portion of plastic trash ends up in the marine ecosystem, with grave consequences. Biodegradable plastics are more accessible to enzymatic activity than conventional plastics, and can be used by microorganisms as a nutrient source. These were devised as an ecologically friendlier alternative to conventional plastics, especially in applications where the products are short-lived, blended with other materials or soiled too heavily after use to be recycled. Some examples are agricultural mulch films, compost bags, linings for paper cups and coffee capsules. These materials are certified to be biodegradable in certain environments, such as industrial or home composters. Their biodegradation in the marine ecosystem is however not well understood.

In our study, we enriched a marine community that can use the commercial aliphatic-aromatic co-polymer blend ecovio FT® as the only C source. This community turned the plastic into CO2 and biomass at ambient temperature in a very short amount of time. Already after three days, cracks were visible on the surface of the plastic films. After six days, we saw holes and craters, and the plastic film disintegrated. Within 15-20 days, maximum conversion to CO2 and biomass was reached. Even for a biodegradable polymer, this is quite an achievement.

Already after six days, large holes and craters were visible on the plastic film, surrounded by bacteria. Photo credit: Manfred Rohde, HZI Braunschweig.

To study the biodegradation mechanisms of this complex polymer by this complex community, we used an integrated multiomics approach: Metagenomics was employed to determine the structure and potential functions of the community and metatranscriptomics and metaproteomics were used to determine which genes and pathways were actively being transcribed and translated during plastic degradation.

The compositions of the biofilm community growing on the plastic surface and the free-living community were quite distinct from each other. A Marinobacter species dominated the biofilm, while Alphaproteobacteria, especially Pseudooceanicola were more prevalent in the free-living fraction. Similarly, the functions of the biofilm and free-living communities were quite distinct. No single bacterium possessed all enzymes and pathways required for complete degradation: The biofilm-dwelling Marinobacter depolymerized the film and fed on the carboxylic acid monomers, while the free-living Alphaproteobacteria degraded the aromatic monomers. Synergistically, the community consumed all parts of the plastic. Cultivation-dependent experiments do not always accurately reflect how degradation takes place in the environment, but we postulate that task-sharing among the microbial community for the complete degradation of complex polymers is more common than complete degradation by single organisms.  

In the grander scheme of things, what do our results tell us? First of all, there are marine microbes out there which, under laboratory conditions, can very quickly degrade this type of plastic. These bacteria encode for enzymes that can potentially be very biotechnologically relevant, especially in the current research landscape where biorecycling is more and more in focus. Last but not least, if we know how marine degradation works, we can maybe use this knowledge to design more degradable materials. One should never forget, however, that our results were obtained under laboratory conditions, and the degradation rates in the field will be very different. Plastic, biodegradable or not, should never be littered. Biodegradable plastics are but one of the many tools that one can use to reduce the accumulation of plastic trash, and the way things are right now, we need all the tools we can get.

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