From The Swiss Federal Institute of Technology in Zürich [ETH Zürich] [Eidgenössische Technische Hochschule Zürich] (CH): “It all comes down to the first electron” 

From The Swiss Federal Institute of Technology in Zürich [ETH Zürich] [Eidgenössische Technische Hochschule Zürich] (CH)

1.12.22
Andri Bryner

All living organisms that respire have to get rid of electrons. In oxygen-​free environments, microorganisms deploy special molecules which act as extracellular electron shuttles to transfer the electrons from cells to minerals. A group of researchers has now discovered what determines the electron transfer efficiency of these “cabs”.

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Concentric iron oxide accumulations around plant roots in a floodplain soil. (Photograph: Andreas Voegelin, Eawag – Swiss Federal Institute of Aquatic Science and Technology [Eidgenössische Anstalt für Wasserversorgung, Abwasserreinigung und Gewässerschutz](CH))

Every living thing requires energy. This is also true of microorganisms. This energy is frequently generated in the cells by respiration, that is by the combustion of organic compounds, in other words: food. During this process, electrons are released which the microorganisms then need to get rid of. In the absence of oxygen, microorganisms can use other methods to do so, including transporting the electrons to minerals outside the cells.

Reduction rates vary considerably.

In oxygen-​free soils or sediments, iron oxides play a major role as acceptors of the released electrons. But how do the electrons get from respiration in the cells to the iron oxides which are found outside the cells? For this process microorganisms can use special molecules that receive two electrons at the cell surface, and then transport them to the iron oxides like a taxi. There the two electrons alight from the taxi, and reduce trivalent iron in oxides to its divalent form. The taxi is then free to transport more electrons.

These extracellular electron shuttles (EES) have been known about for a long time. Until now, however, it has never been clear why their efficiency is so dependent on their structure and the environmental conditions – and why the speed of the iron oxide reduction varies by several orders of magnitude. All attempts to explain the massive efficiency differences on the basis of known parameters such as pH or temperature have failed until now.

The electrons have to be considered individually

A study by Eawag and ETH Zürich researchers, just published in the journal PNAS, shows how efficiency differences in the EES can be explained by a single, unmistakable relationship. “In our relationship, we didn’t look at the average energy of the two transported electrons as has been done up to now, but rather at the individual energy level of each electron,” reports Meret Aeppli, lead author of the study. Eawag-​environmental chemist Thomas Hofstetter adds: “It turns out that the transfer of the first electron from the EES to the iron oxide is often decidedly less energetically efficient than the transfer of the second.” The researchers have shown that the energy difference between the first electron transferred from the EES to the iron oxide determines the iron reduction rate. Using this concept, it is possible to explain the efficiency differences between various EES, even across a sizeable pH range, as well as between two different iron oxides. Michael Sander from the ETH Zürich explains the process with an analogy: “Under many conditions, the first electron is actually very reluctant to leave the EES taxi, but it is pushed out from the back seat, so to speak, by the second electron.”

Electron transfer made visible using UV light

To arrive at their findings, the authors of the study not only devised their own experiments and to collected the resulting data, but also integrated the results of previous studies. For the experiments in the Eawag and the ETH laboratories, the researchers used natural and synthetic EES molecules and investigated two widely available iron(III) oxides. The rate of electron transfer from the EES to the iron oxide, and thus the efficiency of the electron transport, can be made visible with UV light. This light is absorbed differently by the EES depending on whether they are underway with or without the two electrons.

Tiny but crucial

The study describes just one small step in microbial respiration, but while it may be small, it is critical to many processes. Now that anaerobic respiration of mineral phases using EES is understood at a generic level, comparisons can be made more easily between studies and systems. This paper is therefore a must-​read for anyone working with anaerobically-​respiring microorganisms and their carbon exchange. While this step may appear to be a small one, it is nevertheless highly relevant for the understanding of global biogeochemical processes – for example the anaerobic breakdown of organic substances in thawing arctic soilsside, a process in which enormous quantities of climate-​critical CO2 are released.

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Iron oxide in a stream sediment (Image: Meret Aeppli / ETH Zürich)

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ETH Zurich campus

The Swiss Federal Institute of Technology in Zürich [ETH Zürich] [Eidgenössische Technische Hochschule Zürich] (CH) is a public research university in the city of Zürich, Switzerland. Founded by the Swiss Federal Government in 1854 with the stated mission to educate engineers and scientists, the school focuses exclusively on science, technology, engineering and mathematics. Like its sister institution The Swiss Federal Institute of Technology in Lausanne [EPFL-École Polytechnique Fédérale de Lausanne](CH) , it is part of The Swiss Federal Institutes of Technology Domain (ETH Domain)) , part of the The Swiss Federal Department of Economic Affairs, Education and Research [EAER][Eidgenössisches Departement für Wirtschaft, Bildung und Forschung] [Département fédéral de l’économie, de la formation et de la recherche] (CH).

The university is an attractive destination for international students thanks to low tuition fees of 809 CHF per semester, PhD and graduate salaries that are amongst the world’s highest, and a world-class reputation in academia and industry. There are currently 22,200 students from over 120 countries, of which 4,180 are pursuing doctoral degrees. In the 2021 edition of the QS World University Rankings ETH Zürich is ranked 6th in the world and 8th by the Times Higher Education World Rankings 2020. In the 2020 QS World University Rankings by subject it is ranked 4th in the world for engineering and technology (2nd in Europe) and 1st for earth & marine science.

As of November 2019, 21 Nobel laureates, 2 Fields Medalists, 2 Pritzker Prize winners, and 1 Turing Award winner have been affiliated with the Institute, including Albert Einstein. Other notable alumni include John von Neumann and Santiago Calatrava. It is a founding member of the IDEA League and the International Alliance of Research Universities (IARU) and a member of the CESAER network.

ETH Zürich was founded on 7 February 1854 by the Swiss Confederation and began giving its first lectures on 16 October 1855 as a polytechnic institute (eidgenössische polytechnische Schule) at various sites throughout the city of Zurich. It was initially composed of six faculties: architecture, civil engineering, mechanical engineering, chemistry, forestry, and an integrated department for the fields of mathematics, natural sciences, literature, and social and political sciences.

It is locally still known as Polytechnikum, or simply as Poly, derived from the original name eidgenössische polytechnische Schule, which translates to “federal polytechnic school”.

ETH Zürich is a federal institute (i.e., under direct administration by the Swiss government), whereas The University of Zürich [Universität Zürich ] (CH) is a cantonal institution. The decision for a new federal university was heavily disputed at the time; the liberals pressed for a “federal university”, while the conservative forces wanted all universities to remain under cantonal control, worried that the liberals would gain more political power than they already had. In the beginning, both universities were co-located in the buildings of the University of Zürich.

From 1905 to 1908, under the presidency of Jérôme Franel, the course program of ETH Zürich was restructured to that of a real university and ETH Zürich was granted the right to award doctorates. In 1909 the first doctorates were awarded. In 1911, it was given its current name, Eidgenössische Technische Hochschule. In 1924, another reorganization structured the university in 12 departments. However, it now has 16 departments.

ETH Zürich, EPFL (Swiss Federal Institute of Technology in Lausanne) [École polytechnique fédérale de Lausanne](CH), and four associated research institutes form The Domain of the Swiss Federal Institutes of Technology (ETH Domain) [ETH-Bereich; Domaine des Écoles polytechniques fédérales] (CH) with the aim of collaborating on scientific projects.

Reputation and ranking

ETH Zürich is ranked among the top universities in the world. Typically, popular rankings place the institution as the best university in continental Europe and ETH Zürich is consistently ranked among the top 1-5 universities in Europe, and among the top 3-10 best universities of the world.

Historically, ETH Zürich has achieved its reputation particularly in the fields of chemistry, mathematics and physics. There are 32 Nobel laureates who are associated with ETH Zürich, the most recent of whom is Richard F. Heck, awarded the Nobel Prize in chemistry in 2010. Albert Einstein is perhaps its most famous alumnus.

In 2018, the QS World University Rankings placed ETH Zürich at 7th overall in the world. In 2015, ETH Zürich was ranked 5th in the world in Engineering, Science and Technology, just behind the Massachusetts Institute of Technology(US), Stanford University(US) and University of Cambridge(UK). In 2015, ETH Zürich also ranked 6th in the world in Natural Sciences, and in 2016 ranked 1st in the world for Earth & Marine Sciences for the second consecutive year.

In 2016, Times Higher Education World University Rankings ranked ETH Zürich 9th overall in the world and 8th in the world in the field of Engineering & Technology, just behind the Massachusetts Institute of Technology(US), Stanford University(US), California Institute of Technology(US), Princeton University(US), University of Cambridge(UK), Imperial College London(UK) and University of Oxford(UK) .

In a comparison of Swiss universities by swissUP Ranking and in rankings published by CHE comparing the universities of German-speaking countries, ETH Zürich traditionally is ranked first in natural sciences, computer science and engineering sciences.

In the survey CHE ExcellenceRanking on the quality of Western European graduate school programs in the fields of biology, chemistry, physics and mathematics, ETH Zürich was assessed as one of the three institutions to have excellent programs in all the considered fields, the other two being Imperial College London(UK) and the University of Cambridge(UK), respectively.