Enrichment of bioplastic degraders in mesophilic compost from widespread degradation potential in the environment

Polylactic acid (PLA) is a polymer that is known to exhibit compostability at thermophilic temperatures, and this activity is thought to be connected to the presence of PLA hydrolyzability in environmental microbes. We recently developed a set of compost that can biodegrade PLA at mesophilic conditions, and one possible reason underlying our success could be due to the enrichment of PLA hydrolyzability. Here, we investigated the potential selection of bioactivities related to PLA breakdown in our trained compost and surveyed the occurrences of those activities in the environment for comparison. Ten different environments were sampled, including PLA, larval gut of black soldier flies, and organic material from our trained compost, as well as terrestrial soil, estuarine sediment, brackish water, shell biofilm, coastal stranded polystyrene, bottle, and bottle cap. We found a small fraction of cultivable bacteria in many samples that harbored PLA degradability. For our trained compost, PLA-degrading isolates were twice as efficient as those from other environments, even though the frequency at which they were detected was not significantly higher. These findings suggested that PLA breakdown ability is commonly present at a low percentage in most environments, and that our trained compost has been able to select for more effective isolates. As this enhancement is likely insufficient to explain the increase in PLA compostability in our trained compost compared to standard mesophilic composts, we propose that additional microbial activities are needed to act synergistically and overcome the requirement for elevated temperature in PLA composting. IMPORTANCEThis study represents our work in investigating the biodegradation activity of the most common bioplastic, PLA, in the environment and in a special compost we recently developed that exhibited the novel ability of being able to achieve PLA composting at ambient temperatures. Our work is a rare survey that compares PLA hydrolytic activity across different environments, helping unmask the underlying prevalence of environmental PLA hydrolysis activity, as well as whether our special compost is especially enriched for such activity, would facilitate the design of PLA biodegradation implementation strategies. We found that PLA hydrolytic activity was generally present in environmental microbes at low frequencies, and that our special compost selected for those that were more efficient. However, full PLA compostability under mesophilic conditions likely depends on embedded, synergistic microbial functions beyond hydrolysis alone, motivating future work to disentangle complementary activities that collectively enable complete breakdown.