Collaborative Research: Linking microplastic decomposition rates in soils to their microbe-mineral associations using carbon stable isotopes and microspectroscopy

Plastics use has skyrocketed globally since the mid-1950s due to a combination of their utility and low price. A large fraction is for single-use applications. Consequently, more than 25 million metric tons of plastic are annually discarded into terrestrial environments. Bio-based plastics produced from readily renewable carbon sources (i.e., corn) are increasingly being used as substitutes for legacy plastics sourced from fossil fuels. Bio-based plastics are advantageous because their carbon is converted from atmospheric CO2 instead of petroleum. Furthermore, some of these plastics are designed to biodegrade in bioactive environments. All plastics are broken down in the environment by chemical and physical processes into smaller microplastics (less than 5 mm in size) that may become accessible to microorganisms and utilized for their life function or survival. The fate of microplastic residues depends on their degradation in the environment. This research tracks the degradation of microplastic particles of polylactic acid (PLA, a bio-based plastic) and polyethylene terephthalate (PET, a petroleum-based plastic) in soils, where their slow decomposition can lead to plastic accumulation. The research exploits unique carbon isotopic ‘tags’ naturally inherent or artificially introduced to plastics to quantify decomposition with imaging and microbial community analysis to identify how degradation is occurring. The main goal of the work is to monitor the transferal of the isotopic tags to microbial biomass, and eventually, the carbon dioxide and/or methane gas microbes produce or “exhale”. The research is important because it will expand society’s limited understanding of how plastics impact soil health and function, and natural earth processes (i.e., carbon cycling) given plastics’ potential to alter the natural emission of climate warming gases like carbon dioxide and methane in soil systems. Bringing the science of microplastics to a diverse community is a priority of the research team. The project involves and supports secondary, undergraduate, and graduate level students that will be co-mentored by a multidisciplinary faculty team. Students will be involved in research objectives and trained in communicating science to the public. Importantly, students will gain experience across three increasingly related fields for solving the plastic pollution crisis: geochemistry, analytical chemistry, and polymer/green chemistry. Microplastic decomposition occurs through synergistic abiotic weathering of the plastic and key enzymatic and/or microbial interactions. Due to their acclimation to anthropogenic waste, it is hypothesized that the soil microbiome will assimilate and mineralize microplastics, and that natural soil processes like physical mixing and chemical hydrolysis will promote the integration of soil plastics within aggregates and affect the overall assimilation and mineralization of soil organic carbon and plastics by the soil microbiome. The hypotheses will be tested in controlled soil microcosms by utilizing naturally abundant and isotopically labeled (synthesized) polymers that are experimentally degraded and exposed to soils and their native microbiome. Isotopic labels offer an approach to identify assimilation and/or mineralization since they will separate these and other competing processes and/or those that may be impractical to measure in a short period. The incorporation of the plastics’ isotope label will be monitored via phospholipid fatty acid biomarkers and final mineralization gases (i.e., carbon dioxide and methane) using isotope ratio mass spectrometry. Spectroscopy based approaches (i.e., synchrotron-based scanning transmission X-ray microscopy and near-edge X-ray absorption fine-structure spectroscopy) will account for the plastics’ reactivity and association with soil aggregates. The combination of stable isotopic, spectral, and isotopic mass balance approaches will establish a fundamental understanding of plastic decomposition, and include a modeling of their assimilation and mineralization, transformation to lower weight products, and final conversion to carbon dioxide and methane in soils. This research will further basic science understanding of physical, chemical, and biological processes in soils and address a topic of great current practical interest in environmental geochemistry. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.