Meret Aeppli

Publications

For a list of my reviewing activities, please head to Publons.

Lacroix, EM, Aeppli, M; Boye, K; Brodie, E; Fendorf, S; Keiluweit, M; Naughton, HR, Noel Vincent No\"el, V; Sihi, D. Consider the Anoxic Microsite: Acknowledging and Appreciating Spatiotemporal Redox Heterogeneity in Soils and Sediments.ACS Earth Space Chemistry, 2023, xxx, doi:10.1021/acsearthspacechem.3c00032.

Aeppli, M; Schladow, G; Lezama Pacheco, J S; Fendorf, S. Iron Reduction in Profundal Sediments of Ultraoligotrophic Lake Tahoe under Oxygen-Limited Conditions. Environmental Science & Technology, 2023, 57 (3), 1529-1537, doi:10.1021/acs.est.2c05714.

Aeppli, M; Thompson, A; Dewey, C; Fendorf, S. Redox Properties of Solid Phase Electron Acceptors Affect Anaerobic Microbial Respiration under Oxygen-Limited Conditions in Floodplain Soils. Environmental Science & Technology, 2022, 56 (23), 17462-17470, doi:10.1021/acs.est.2c05797.

Lopez, AM; Nicolini, CM; Aeppli, M; Luby, SP; Fendorf, S; Forsyth, JE. Assessing Analytical Methods for the Rapid Detection of Lead Adulteration in the Global Spice Market. Environmental Science & Technology, 2022, 56 (23), 16996-17006, doi:10.1021/acs.est.2c03241.

Aeppli, M; Babey, T; Engel, M; Fendorf, S, Bargar, JR; Boye, K. Export of Organic Carbon From Reduced Fine-Grained Zones Governs Biogeochemical Reactivity in Simulated Aquifer. Environmental Science & Technology, 2022, 56 (4), 2738-2746, doi:10.1021/acs.est.1c04664.

Aeppli, M; Giroud, S; Vranic, S; Voegelin, A; Hofstetter, TB; Sander, M. Thermodynamic Controls on Rates of Iron Oxide Reduction by Extracellular Electron Shuttles. Proceedings of the National Academy of Sciences of the United States of America, 2022, 119, e2115629119, doi:10.1073/pnas.2115629119.

Biswakarma, J; Rushworth, D; Srivastava, G; Singh, G; Kang, K; Das, S; Anantharaman, SB; Aeppli, M; Popp, AL; Bhuyan, DJ. Organizational Level Responses to the COVID-19 Outbreak: Challenges, Strategies and Framework for Academic Institutions. Frontiers in Communication, 2021, 6:573585, doi:10.3389/fcomm.2021.573585.

Aeppli, M; Vranic, S; Kaegi, R; Kretzschmar, R; Brown, AR; Voegelin, A; Hofstetter, TB; Sander, M. Decreases in Iron Oxide Reducibility during Microbial Reductive Dissolution and Transformation of Ferrihydrite. Environmental Science & Technology, 2019, 53 (15), 8736–8746, doi:10.1021/acs.est.9b01299.

Aeppli, M; Kaegi, R; Kretzschmar, R; Voegelin, A; Hofstetter, TB; Sander, M. Electrochemical Analysis of Changes in Iron Oxide Reducibility during Abiotic Ferrihydrite Transformation into Goethite and Magnetite. Environmental Science & Technology, 2019, 53 (7), 3568-3578, doi:10.1021/acs.est.8b07190.

Aeppli, M; Voegelin, A; Gorski, CA; Hofstetter, TB; Sander, M. Mediated Electrochemical Reduction of Iron (Oxyhydr-)Oxides under Defined Thermodynamic Boundary Conditions. Environmental Science & Technology, 2018, 52 (2), 560-570, doi:10.1021/acs.est.7b04411.

Armanious, A; Aeppli, M; Jacak, R; Refardt, D; Sigstam, T; Kohn, T; Sander, M. Viruses at Solid-Water Interfaces: A Systematic Assessment of Interactions Driving Adsorption. Environmental Science & Technology, 2016, 50 (2), 732-743, doi:10.1021/acs.est.5b04644.

Franchini, AG; Henneberger, R; Aeppli, M; Zeyer, J. Methane Dynamics in an Alpine Fen: A Field-Based Study on Methanogenic and Methanotrophic Microbial Communities. FEMS Microbiology Ecology, 2015, 91 (3), doi:10.1093/femsec/fiu032.

Armanious, A; Aeppli, M; Sander, M. Dissolved Organic Matter Adsorption to Model Surfaces: Adlayer Formation, Properties and Dynamics at the Nanoscale. Environmental Science & Technology, 2014, 48 (16), 9420-9429, doi:10.1021/es5026917.

Current Projects

Redox properties of soil organic carbon

Soil organic carbon is an important component of the carbon cycle. Carbon can be released from the soil into the atmosphere if soil organic carbon is microbially degraded and mineralized. The oxidation state of carbon is linked to its physicochemical properties and susceptibility toward oxidation and thus, could provide insights into controls on soil organic carbon mineralization. This project aims to develop experimental approaches to characterize the oxidation state of carbon in soil samples. A promising technology that has previously been used on water samples is the photoelectrochemical oxygen demand (peCOD) analyzer (Mantech). The analyzer determines the peCOD of organic compounds by fully oxidizing them using a photo-activated titanium dioxide catalyst. Procedures to apply this technology to soil samples are currently underway.

Effect of soil aggregation on microbial respiration pathways

Soil aggregates are groups of soil particles that bind to each other (see photo for an example). Inside such aggregates, oxygen can become depleted if microbial consumption outpaces its supply through diffusion. Under such conditions, microbial respiration shifts from aerobic to anaerobic pathways and the mineralization of organic carbon slows down. Together with Emily Lacroix at Stanford University, we are testing if this is the case in aggregates collected from soils that developed on different bedrock at the Stanford Dish Area. We are assessing potential differences in soil redox state and active microbial genes between the inside and outside of soil aggregates.

Redox reactivity of iron minerals in the Calhoun Critical Zone Observatory

In a collaboration with Aaron Thompson at the University of Georgia, we are looking into the characteristics of iron minerals in soils from the Calhoun Critical Zone Observatory. We are interested in how iron mineral properties, in particular the reactivity toward electrochemical and microbial reduction, varies across soil horizons and usage (cultivated, hardwood and pine soils). The photo shows the differences in colors among soil horizons and usage which are caused by variations in iron mineralogy.

Redox buffer behavior of clay minerals

Iron in clay minerals can be involved in redox reactions over an unusually large range of environmental conditions. In collaboration with researchers from the Swiss Federal Institute of Aquatic Science and Technology, Newcastle University , University Poitiers, and ETH Zurich, we are combining electrochemical analyses of iron redox reactivity with the evaluation of iron binding and redox state by spectroscopic and microscopic analyses. We perform these analyses on synthetic clay minerals with well-defined iron coordination that we subject to redox cycling, as shown in the illustration above. More details on the project are available here.

Past Projects

Biogeochemistry of Lake Tahoe sediments

Lake Tahoe is a deep lake with very low nutrient concentrations located in the Sierra Nevada mountain range at the border between California and Nevada. The sediments of Lake Tahoe contain large amounts of ferric iron minerals. These minerals are at risk to be reductively dissolved as oxygen supply to the sediments decreases due to increased lake stratification in a warmer climate. Reductive dissolution of iron minerals releases ferrous iron as well as nutrients that are associated with the minerals into the water column, impacting lake water quality. In collaboration with researchers at the Tahoe Environmental Research Center, we determined which biogeochemical processes are active in the lake sediments today and how these will change in the future. To this end, we performed whole-core incubation experiments at three different oxygen levels. We found that a decrease in dissolved oxygen concentration in the top 2 cm of the sediment resulted in an extension of the microbial iron reduction zone from below 4.5 to below 1.5 cm depth. Our findings further suggest that microorganisms preferentially utilized reactive iron and/or iron minerals became less reactive due to mineral transformation and surface passivation. Although the estimated release of iron mineral-associated phosphorus in this work is not expected to change Lake Tahoe’s trophic state, it will likely contribute to increased phytoplankton productivity if mixed into surface waters. This work was published in Environmental Science & Technology as Iron Reduction in Profundal Sediments of Ultraoligotrophic Lake Tahoe under Oxygen-Limited Conditions.

Controls of mineral redox reactivity on carbon turnover in floodplain soils

Floodplains are active sites of carbon turnover. Under oxygen-depleted conditions, solid phase electron acceptors including iron minerals are commonly used in anaerobic microbial respiration. Yet, remains unclear to what degree the redox properties of solid phases limit respiration rates and hence organic matter degradation. Here, we assess such limitations in soils that we collected across a gradient in redox conditions from the floodplain of Slate River in the Rocky Mountains, Colorado. We incubated soils in the absence of oxygen and measured carbon dioxide production and iron reduction as indicators for microbial respiration. In addition, we characterized the redox properties of solid phase electron acceptors using mediated electrochemistry. We found that carbon dioxide production rates increased with increasing redox reactivity of solid phase electron acceptors across a wide range of in-situ redox conditions, identity and concentrations of electron donors and acceptors, and iron mineralogy. These findings suggest that the redox properties of solid phase electron acceptors are important drivers of organic carbon transformations in oxygen-depleted environments. This work was published in Environmental Science & Technology as Redox Properties of Solid Phase Electron Acceptors Affect Anaerobic Microbial Respiration under Oxygen-Limited Conditions in Floodplain Soils.

Development of rapid detection methods for lead in spices

Despite lead being a potent neurotoxin, spices such as turmeric are sometimes adulterated with lead-laced chemical compounds. In a project led by Jenna Forsyth and in collaboration with other researchers at the Stanford Woods Institute for the Environment and Stanford Earth, we compared multiple rapid detection tests for lead in spices. One of these methods uses a purple dye (diphenylcarbazide) to detect lead in turmeric roots, as shown in the photo above. We propose detection tests for lead in spices that are simple and safe to use in an effort to lessen human exposure. This work was published in Environmental Science & Technology as Assessing Analytical Methods for the Rapid Detection of Lead Adulteration in the Global Spice Market.

Transport of organic carbon across subsurface interfaces

Groundwater quality in alluvial systems is controlled by biogeochemical and transport processes. In a project with members of the Scientific Focus Area: Groundwater Quality team, we studied how exports across subsurface interfaces affect biogeochemical processes in a simulated aquifer system. We used a dual-domain column system with ferrihydrite-coated sand and embedded natural, reduced sediment lenses from floodplains of Slate River (Crested Butte, Colorado) and Wind River (Riverton, Wyoming). We found that organic substrates and live microorganisms were exported from fine-grained, reduced sediments and induced in-situ microbial oxidation of organic carbon and reduction of ferrihydrite in the sand. Our work is the first to show that anaerobic activity in coarse-grained aquifers is governed not just by the export of aqueous products but also particulate phases, including microbial cells, from reduced sediments. This work was published in Environmental Science & Technology as Export of Organic Carbon from Reduced Fine-Grained Zones Governs Biogeochemical Reactivity in a Simulated Aquifer.

Thermodynamic controls on rates of iron oxide reduction by extracellular electron shuttles

Iron-reducing microorganisms need to transfer electrons that are released during the oxidation of organic carbon to extracellular, solid phase ferric iron. To overcome this challenge, microorganisms use organic molecules, so-called extracellular electron shuttles (EES), to shuttle electrons from the microbial cells to the mineral surfaces and enhance respiration rates. Due to large variations in rate enhancements among organisms, EES, and iron minerals, it has proven challenging to determine the contribution of this widespread terminal electron accepting process to total anaerobic respiration. In this work, we show that broadly varying reduction rates determined in this and past work for different iron oxides and EES under varying solution chemistry can be reconciled into rate-free energy relationships when considering the free energy of the less exergonic first of the two electron transfers from the fully, two electron-reduced EES to ferric iron oxide. These relationships allow for a generalized assessment of EES-mediated respiration to organic matter decomposition in anoxic environments. This work was published in Proceedings of the National Academy of Sciences of the United States of America as Thermodynamic controls on rates of iron oxide reduction by extracellular electron shuttles.

Decreasing redox reactivity of iron oxides during microbial reduction of oxides

Iron-reducing microorganisms transfer electrons to ferric iron during anaerobic respiration. This process produces ferrous iron that induces phase transformation into thermodynamically more stable oxides that are harder for microorganisms to reduce. In incubations with the iron-reducing bacterium Shewanella oneidensis and ferrihydrite, we showed that the redox reactivity of the iron oxide decreased as ferrihydrite transformed into goethite and magnetite. The cessation of microbial respiration in the incubations coincided with a drop in redox reactivity due to magnetite formation, suggesting that microbial respiration rates were limited by the redox reactivity of ferric iron. This work was published in Environmental Science & Technology as Decreases in Iron Oxide Reducibility during Microbial Reductive Dissolution and Transformation of Ferrihydrite.

Changes in the redox reactivity of iron oxides during oxide transformations

Amorphous iron oxides, like ferrihydrite, transform into thermodynamically more stable iron oxides over time. Using mediated electrochemistry, we showed that the transformation of ferrihydrite into goethite and magnetite is accompanied by a decrease in the reactivity of the iron oxide toward accepting electrons. Quantitative information on the decrease in oxide reactivity during ferrihydrite transformation is key to estimating how transformation affects redox reactions involving iron oxides, such as anaerobic microbial respiration to ferric iron. This work was published Environmental Science & Technology as Electrochemical Analysis of Changes in Iron Oxide Reducibility during Abiotic Ferrihydrite Transformation into Goethite and Magnetite.

Development of electrochemical approach to quantify electron transfer to iron oxides

We developed a mediated electrochemical approach to characterize the electron accepting properties of ferric iron oxides. Ferric iron oxides are redox-active minerals that are widespread in nature and electron transfer to ferric iron is a key step in many biogeochemical processes that control the cycling of nutrients and trace elements, as well as the dynamics of organic and inorganic pollutants. Our approach allows quantifying rates and extents of electron transfer to iron oxides at defined applied reduction potential and pH. The applied reduction potential and pH determine the thermodynamic driving force for electron to the ferric iron oxide. We conducted electrochemical measurements at varying applied reduction potential and solution pH and related the resulting rates of electron transfer to the iron oxides goethite and hematite to the thermodynamics of the electron transfer reaction. Describing electron transfer properties on the basis of thermodynamics allows us to compare these properties among iron oxides and solution conditions. This work was published in Environmental Science & Technology as Mediated Electrochemical Reduction of Iron (Oxyhydr-)Oxides under Defined Thermodynamic Boundary Conditions.