Reviewing Redox

A rusting surface. Iron oxidizers play an important role in cycling iron on Earth and contribute to the corrosion of metal objects like bridges and pipes. Image Credit: Laitr Keiows

A rusting surface. Iron oxidizers play an important role in cycling iron on Earth and contribute to the corrosion of metal objects like bridges and pipes. Image Credit: Laitr Keiows, Wikimedia Commons

Chemical reactions are the stuff of life on Earth. They happen all the time, and in every living cell on our planet. Astrobiologists study this chemistry in order to determine the basic mechanisms behind life, and whether or not these mechanisms could operate on other worlds.

Among these chemical reactions are redox reactions (also called oxidation-reduction reactions). These reactions occur when electrons are transferred between different species of molecule. In a way, they are similar to acid-base reactions, where positively and negatively charged ions are shuffled around.

The transfer of electrons can make things interesting for microorganisms because it signifies energy. On Earth, there are microbes that take advantage of redox reactions to gain the energy they need to live. This ability can allow them to thrive independently from the energy produced by the Sun.

Oxidation is the removal of one or more electrons from a substrate. Protons (H+) are often removed with the electrons. Reduction of a substrate refers to its gain of one or more electrons. Each time a substance is oxidized, another is simultaneously reduced. Credit: Midlands Technical College

Oxidation is the removal of one or more electrons from a substrate. Protons (H+) are often removed with the electrons. Reduction of a substrate refers to its gain of one or more electrons. Each time a substance is oxidized, another is simultaneously reduced. Credit: Midlands Technical College

For astrobiologists, these microbes are an important example of how life might be able to survive in environments where sunlight isn’t necessarily an option on Earth and beyond. One such habitat is in the deep subsurface, where rocks and minerals can be plentiful… but sunlight is completely absent.

The thing is, redox reactions don’t have to involve biology. They also occur naturally in nature (referred to as abiotic).

Diagram of the iron cycle and its involvement in heavy metal immobilization. Credit: USGS

Diagram of the iron cycle and its involvement in heavy metal immobilization. Credit: USGS

In the case of iron (Fe), many of the redox reactions that were once thought to be entirely abiotic are now known to be mediated by microorganisms. This overlap can make it difficult to determine whether abiotic or biotic (microbially mediated) reactions are the most dominant in an environment.

A new review published in the journal Nature Reviews: Microbiology outlines our current knowledge of the various chemical reactions that occur in the iron cycle on Earth. Iron redox reactions are broken down into those that are abiotic, and those that are known to involve microorganisms.

The new paper could be a valuable resource for understanding how microbes are involved in cycling iron, and which of these reactions could provide energy for microbial communities to thrive.

 


Organic Chemistry 51B. Lecture 13. Reduction and Oxidation, Part 1. Credit: UC Irvine (YouTube)