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

自20世纪50年代中期以来，由于塑料的实用性和低廉的价格，塑料的使用量在全球范围内飙升。其中很大一部分是单机版应用。因此，每年有超过2500万吨塑料被丢弃到陆地环境中。从易再生的碳源(即玉米)生产的生物基塑料越来越多地被用作化石燃料来源的传统塑料的替代品。生物塑料具有优势，因为它们的碳是从大气中的二氧化碳而不是石油中转化而来的。此外，其中一些塑料是为在生物活性环境中生物降解而设计的。所有塑料在环境中通过化学和物理过程分解成更小的微塑料(尺寸小于5毫米)，微生物可以接触到这些塑料，并为其生命、功能或生存而利用。微塑料残留物的命运取决于它们在环境中的降解。这项研究跟踪了聚乳酸(一种生物基塑料)和聚对苯二甲酸乙二酯(PET，一种石油基塑料)的微塑料颗粒在土壤中的降解情况，在土壤中，它们的缓慢分解会导致塑料积累。这项研究利用天然或人工引入塑料中的独特碳同位素“标签”，通过成像和微生物群落分析来量化分解，以确定降解是如何发生的。这项工作的主要目标是监测同位素标签向微生物生物量的转移，并最终监测微生物产生或“呼出”的二氧化碳和/或甲烷气体。这项研究很重要，因为它将扩大社会对塑料如何影响土壤健康和功能以及自然地球过程(即碳循环)的有限理解，因为塑料有可能改变土壤系统中二氧化碳和甲烷等气候变暖气体的自然排放。将微塑料的科学带到一个多样化的社区是研究团队的优先事项。该项目涉及并支持中学、本科生和研究生水平的学生，他们将由一个多学科的教员团队共同指导。学生将参与研究目标，并接受向公众传播科学的培训。重要的是，学生将在解决塑料污染危机方面获得三个日益相关的领域的经验：地球化学、分析化学和聚合物/绿色化学。微塑料分解是通过塑料的协同非生物风化以及关键的酶和/或微生物相互作用进行的。由于土壤微生物对人为废物的适应，推测土壤微生物群落将同化和矿化微塑料，而土壤的物理混合和化学水解等自然过程将促进土壤塑料在团聚体中的整合，并影响土壤微生物对土壤有机碳和塑料的整体同化和矿化。这些假说将通过利用自然丰富的和同位素标记的(合成的)聚合物在受控的土壤微观宇宙中进行验证，这些聚合物经过实验降解并暴露在土壤及其本地微生物群中。同位素标记提供了一种识别同化和/或矿化的方法，因为它们将把这些和其他相互竞争的过程和/或那些在短期内可能无法测量的过程分开。塑料的同位素标签的加入将通过磷脂脂肪酸生物标记物和使用同位素比率质谱仪的最终矿化气体(即二氧化碳和甲烷)进行监测。基于光谱学的方法(即基于同步加速器的扫描透射式X射线显微镜和近边X射线吸收精细结构光谱)将考虑塑料的反应性和与土壤团聚体的关联。稳定同位素、光谱和同位素质量平衡方法的组合将建立对塑料分解的基本理解，包括对其同化和矿化、转化为较轻产品以及最终转化为土壤中的二氧化碳和甲烷的模拟。这项研究将进一步加深对土壤中物理、化学和生物过程的基础科学理解，并解决当前环境地球化学领域极具实际意义的一个主题。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.