Large plastic sheets (mulch films) are used in agriculture to cover the soil to retain moisture and inhibit weed growth. A goal of the plastics and agricultural industries is the design of biodegradable plastic mulch films that can be tilled into the soil, where degradation is on the order of years. Biodegradable plastics are designed to disintegrate, with the help of microorganisms, into carbon dioxide, water, and methane. However, the process inherently requires plastic fragments ranging from macroscopic to molecular level to be formed en route to mineralization. The goal of this project is the assessment of effects of these plastic fragments in soil ecosystems which is largely unknown.
The PIs aims to: 1) produce and characterize pure polymer films using polymers relevant to those used in agricultural mulches, describe their degradation products, and optimize methods for measuring and quantifying degradation products in soil and aqueous media. 2) use aflatoxin, a common and economically important natural toxin found in agricultural soils, in a proof-of-principle study to ask whether natural toxins have potential to adsorb and accumulate on fragments of biodegradable plastics in soil. The fungus that produces aflatoxin, Aspergillus flavus, is a frequent and avid colonizer of starch-based biodegradable plastics, and, 3) query whether the fungus is attracted to the starch polymers or the biodegradable plastic polymers, or both. The PIs will assess global gene expression of A. flavus during growth on biodegradable plastics because, besides aflatoxin, it produces other secondary metabolites that are potentially toxic in the soil food web. The ability to analyze in situ polymer degradation will allow realistic modeling of the fate of biodegradable plastic mulch films in soil ecosystems. This study would represent the most thorough comparison of the degradation characteristics of commercially-relevant polymers in real soil under realistic conditions yet reported the literature. Understanding the interaction of Aspergillus flavus and aflatoxin with biodegradable polymers will aid in generating biodegradable plastic formulations used in agriculture and food packaging that are less likely to become contaminated with aflatoxin. Assessing genes expressed by Aspergillus flavus during growth on biodegradable polymers will reveal whether, and which of, the many hydrolytic enzymes produced by Aspergillus flavus are involved in polymer breakdown. In addition, altered expression of secondary metabolite genes would support further experiments to assess the potential for aflatoxin and other microbial metabolites to adsorb to biodegradable plastic mulch film fragments - sequestered from their natural roles, and concentrated onto particles that may enter the food web. Since the fall of 2011, ten WWU Biology and seven WWU Engineering undergraduate students have gained 1-2 years' research experience on the biodegradable plastics project. Of those, five students were coauthors on peer-reviewed publications. All of these students have gone on to research jobs in their field of interest, graduate programs, or medical school. Two undergraduate researchers trained on this grant would have the advantage to focus full-time on their projects because they will be paid stipends. They will receive training in interdisciplinary research methods, attend joint laboratory meetings, and present their data at a university-level conference (Scholars Week). The results from this work will be relevant to agroecosystem and human health. The ability to analyze in situ polymer degradation will allow realistic modeling of the fate of biodegradable plastic mulch films in soil ecosystems and pave the way for studies of fate and transport of polymer degradation products in soils.
