Enrichment and Characterization of Microbes Degrading Biodegradable Plastic Mulch Films.

Plastic mulches are commonly used in agriculture because they provide conservation of moisture, reduced weed growth and increased soil temperature. Polyethylene black plastic is most commonly used by farmers. However, polyethylene is not readily biodegradable and its long term fate in soil is unknown.  Polyethylene mulches are generally not recyclable and their disposal through burning or landfilling can have harmful impacts on the environment. A shift to biodegradable plastic mulches (BDMs) is thus required as an environmentally safe and sustainable choice. While promising as an alternative, there remains considerable uncertainty regarding the fate of these BDMs in soil.  This work aimed to identify the role of soil microorganisms in degrading four commercially available BDMs. To accomplish this, plastic-degrading microbes were enriched from soils collected from the East Tennessee Research and Educational Center (Knoxville, Tennessee) on BDM films. Enrichments were done in liquid minimal media with pieces of BDM films as the carbon source.  Carbon dioxide evolution was measured weekly from culture flasks. After 3 and 6 weeks of incubation, plastics were analyzed for mass loss, microscopy used to visualize microbes adhering to films, and DNA extracted from films for characterization of the microbial communities. Plastic-degrading consortia were further plated to isolate individual microbes for characterization.  Plastics lost mass over time and there was an increase in CO₂ indicating biodegradation of the plastics was occurring.  We also observed visible bacterial and fungal colonies growing on mulches. Microscopic images of the films show increased surface roughness and thinning of films as degradation occurs. Fluorescence microscopy showed live cells interacting with polymer fibers. Taken together, these results confirm that the enrichment conditions selected for a consortium of soil microorganisms capable of degrading BDMs.  Characterization of BDM-degrading microbes will provide more insight into the degradation process of these films and may inform management strategies for efficient degradation of BDM films in agricultural soils.