Examination of electron transport shuttles in microorganisms

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

Reduction rates vary greatly

In soils or sediments devoid of oxygen, iron oxides play a major role as acceptors of liberated electrons. But how do electrons move from respiration in cells to iron oxides found outside cells? In this process, microorganisms can use special molecules that receive two electrons on the cell surface, and then transfer them to iron oxides like a taxi. There the two electrons release from the cab, reducing the trivalent iron in the oxides to its divalent form. The taxi is thus free to transport more electrons.

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

Electrons must be viewed individually

A study by researchers Eawag and ETH Zurich, just published in the journal PNAS It shows how efficiency differences in EES can be explained by one unambiguous relationship. “In our relationship, we looked not at the average energy of the two transferred electrons as has been done so far, but rather at the individual energy level of each electron,” says Mert Epley, lead author of the study.

“It turns out that transferring the first electron from the eastern region of space to iron oxide is often less energy efficient than transferring the second electron,” says Thomas Hofstätter, an environmental chemist at IWAG. The researchers showed that the energy difference between the first electron transferred from the EES to the iron oxide determines the rate of iron reduction.

Using this concept, it is possible to explain the efficiency differences between different EES, even across a large pH range, as well as between different iron oxides. Michael Sander of ETH Zurich explains the process by analogy: “Under many circumstances, the first electron is actually very reluctant to leave the EES cab, but is pushed out from the back seat, so to speak, by the second electron.”

Making electron transmission visible using UV light

To arrive at their findings, the study authors not only created their own experiments and collected the resulting data, but also combined the results of previous studies. For the experiments at Eawag and ETH Laboratories, the researchers used natural and synthetic EES particles and examined two widely available iron (III) oxides. The electron transfer rate from EES to iron oxide, and thus the electron transfer efficiency, can be made visible using UV light. EES absorbs this light differently depending on whether it is conductive with or without the two electrons.

small but decisive

The study describes only one small step in microbial respiration, but while it may be small, it is important for many processes. Now that the anaerobic respiration of metallic phases using EES is understood at a general level, comparisons can be made more easily between studies and systems. Therefore, this paper is a must read for anyone working with anaerobic microorganisms and carbon exchange. While this step may seem small, it is nonetheless very relevant to understanding global biogeochemical processes – for example the anaerobic decomposition of organic matter in Arctic soil thawing, a process in which huge amounts of climate-critical carbon dioxide are used up.2 was issued.


Altering microbial biomass alters the role of iron oxides in organic carbon mineralization in anoxic rice soils.


more information:
Meret Aeppli et al, Thermodynamic controls on rates of iron oxide reduction by extracellular electronic shuttles, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2115629119

Provided by the Swiss Federal Institute of Aquatic Science and Technology

the quote: Examining Electron Transfer Shuttles in Microorganisms (2022, January 12) Retrieved on January 12, 2022 from https://phys.org/news/2022-01-electron-shuttles-microorganisms.html

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