Introduction
Today, we face the global challenge of plastics pollution in nearly all environments. The pollution has meanwhile reached levels that will ultimately have impact on our food chain and well-being within the next decades. A recent study implied that about 399000 tons of plastics are present in the oceans alone, of which 69000 tons are microplastics1. Thus, urgent actions need to be implemented for removal of plastics from the environment and by reducing the steady input into the environment. Whereas it is perhaps more likely that large pieces can be removed mechanically from ocean surfaces or terrestrial sites, smaller particles (microplastics) will remain there unless microbial or chemical degradation (i.e., weathering) will occur2–4. Plastic waste is a valuable raw material, therefore recycling is a promising alternative to incineration, either as a basis of synthesis of polymers or as a carbon source for fermentation5.
Petroleum-based plastics are in general extremely stable and durable; hence it is widely accepted that plastics do not degrade well in nature6, nor can be directly used in fermentation. The degradation processes described so far are slow, and it was shown that a PET bottle remains up to 48 years in the ocean until it is decomposed by microbial degradation 7. Within this setting, it is reasonable to speculate that prior to microbial and enzymatic degradation, mechanical treatment (waves, wind, friction) and photodegradation by UV light (especially for aromatic ring-containing polymers such as PET and PS) break down the debris into microplastics, thereby increasing the surface area, which mediates microbial degradation. For more details on microplastics-associated bacteria and fungi, we refer to excellent reviews of the field of plastics ecology8–10. However, the colonization of microplastics does not necessarily indicate that the polymer is degraded, because additives are in general more bioavailable than the polymers. Therefore, measuring weight loss as an indicator for degradation might result in a false interpretation of the data 11, and in the conclusion that we already have many plastics-active enzymes from different microbial sources. A detailed search in the PubMed database revealed that today roughly 1500 publications address the topic of plastics degradation. However, less than 50 described the isolation and biochemical characterization of plastics-active enzymes (Tables 1, S1-S3 ). Nevertheless, while this obvious challenge can be met by better analytical techniques, the by far greater risk for misinterpretation of data comes from the unfiltered and non-critical use of the predicted plastic degrading microorganisms and consortia by not verified bioinformatic tools and pipelines.
For instance one recent study developed Hidden Markov models for some plastic degrading enzymes and predicted a global distribution even though no such enzymes have been biochemically characterized12. Others have developed phylogenetic trees and global distribution patterns by simply using automated literature searches without critical analyses of the data13. These very recent studies in high-ranking journals are perhaps only the tip of the iceberg, but clearly demonstrate that there is an urgent need for standardized and verified enzyme databases in this rapidly developing field. The non-critical and unfiltered use of many of the potential plastic degrading gene sequences ultimately leads to incorrect conclusions on the availability of plastic degrading enzymes and their role in nature. These studies do not only mislead researches, they furthermore suggest to environmentalists, policy and law makers and even to the broader public audience that we would have solutions for the global plastics problem, which we however do not have. Within this framework, the proposed PAZy database will be a reliable and very useful tool giving an overview on truly functional enzymes.
Notably today, only for polyethylene terephthalate (PET), polyurethane (PUR), and polyamide (PA), a rather small number of degrading enzymes are known, but none for other major polymers such as PVC, PE, PP, and PS, and most of the PUR-based polymers. The known plastics-degrading enzymes are hydrolases, often annotated as lipases, esterases, cutinases, amidases, or proteases (E.C. 3.1.x). However, we have still a limited understanding of the mechanism of enzymatic degradation. It is not clear to which extent bacteria have evolved specific enzymes that bind to the polymers and cleave the bonds similar to the processes that occur when cellulose or other biopolymers are degraded. It is supposed that plastics-degrading enzymes are exoenzymes, and it can be speculated that plastics-binding domains or proteins might contribute to degradation, similar to the role of cellulose binding domains or expansins in the degradation of cellulosic materials.
To advance the research field, we have collected information of the currently known and verified plastics-active enzymes in the Plastics-Active Enzymes Database (PAZy). It will serve as a comprehensive resource for the identification of further novel plastics-active enzymes, pathways, or microorganisms for plastics removal in industry and the environment. It will further help to advance improved circular use of the different plastic types. Finally, PAZy will in general be a valuable repository and tool in this emerging field of plastics research.