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.