As you may know, we are quite the wasteful species. Some countries are better than others at recycling plastics, but these polymeric materials without a doubt are too abundant to fully manage. Every year, more than 300 million tons of plastic are produced, which usually find their way to our oceans, streams, rivers or landfills. Over time, plastics mechanically break down into smaller and smaller pieces. This is when these manmade polymers become an issue, as they are absorbed into photosynthetic algae, ultimately increasing in abundance as they radiate up the food chain in a process called bioaccumulation. Most of the fish we have a taste for are located near the top of the food chain, where toxic compounds like phthalates, and bisphenol are most built up. In many ways, the health of our environments translates to our own health. This is an issue many individuals in the environmental disciplines strive to solve. Well, this week, they took a huge step towards this goal. A new strain of Aspergillus tubingensis has been isolated from a waste disposal site located in Islamabad, Pakistan, that just might aid humans from drowning in a sea of plastic.
Aspergillus tubingensis was originally described in the early 1930’s by Raoul Mosseray. The fungus itself is a borderline extremophile, with a high resistance to ultraviolet light, and a growth temperature optimum on the hot spectrum, compared to most other species (21–36 °C or 70–97 °F). Like many other molds, Aspergillus tubingensis doesn’t have a known sexual form, only producing genetically identical individuals through means of asexual reproduction. This may be a bit of a hurdle if this species is selected for ecological restoration projects to reduce certain types of plastic.
Now, it is important that you don’t get too overzealous over this finding. Unfortunately, this is not the holy grail of restoration ecology. Over the past 70 years, humans have synthesized numerous types of plastics and this particular species can only break down polyester polyurethane, hereafter PU. PU is used in many applications and can be found in plastic foams, bumpers, gaskets, synthetic leathers, paints, cushions, adhesives, and refrigerator insulation. Although PU is a widely used plastic, there are still eight common plastic polymers that this species cannot break down. Luckily, there are more than 5 million species of fungi, and it is likely that many still have not even been discovered yet.
This is not the first species of plastic metabolizing fungi that has been discovered either. In 2011, Yale students isolated a species of fungi from the Amazon that could grow anaerobically on polyester polyurethane (PUR). Unlike Aspergillus tubingensis, Pestalotiopsis microspora is a sexual fungus, forming a fruiting body. Sexual fungi are more useful in this application because they can be cultivated on, for instance, PUR media with the spores of the most bioactive individuals being collected. By combining spores from the most ‘plastic hungry’ individuals, we can actively create more voracious strains of polymer decomposing fungi that can aid us in this modern, plastic dominated world. To make asexual fungi more efficient at breaking down plastic polymers, we must use more advanced, expensive genetic editing technology, that may ultimately take longer.
Again, this is not an immediate solution to our plastic problem. Even if we mass produce the spores of this Aspergillus tubingensis strain to inoculate landfills around the world, the PU won’t be degraded overnight. The recent study that initially isolated the species, closely analyzed the biodegradation process. They found that the process was completed in three steps; adhesion of fungus to the polymer surface, extension of hyphal filaments over the surface, and finally, secretion of PU degrading enzymes. The efficiency of these three steps is highly dependent on pH, temperature, and plastic composition- all of which are extremely variable. If all of Earth's plastic was the same, and all landfills fell in the environmental optimum for this species, degrading PU would be kind of easy. Of course, this is not the case.
It should be no surprise that an organism that can utilize a manmade polymer comes from the fungal kingdom. I mean, one of the key roles fungi perform in ecosystems around the world is the breakdown of long polymer chains, which decomposes and recycles recalcitrant carbon pools that otherwise would just accumulate atop the forest floor. The long chained organic polymer lignin, the compound that gives trees their rigidity, is a choice carbon supply for many saprotrophic forest fungi. We must utilize the fungal realm if we want to ensure the health of our environment and ourselves. Evolutionary processes may just be on our side, as natural selection should favor metabolic pathways that can be supported an enormous resource pool we subsequently created.