Feb

27

Electricity In Life: Research In Electron Transport Systems

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this is a nanowire! learn something new every day with bwog science :)

Yesterday, Bwog Daily Editor Ramisa Murshed and Science Editor Alex Tang attended a highly interdisciplinary physics colloquium talk given by Dr. Moh El-Naggar, where he discussed the relevance and mechanisms behind electron transport in living organisms. Read more if you’re interested in physics, chemistry, and/or biology!

Dr. Moh El-Naggar of the University of Southern California made an appearance at Columbia’s Physics Colloquium to talk about the fascinating mechanisms behind electron transport (in microbe mitochondria) in his lecture, “Life Electric: What Can Microbes Teach Us About Electron Transport, Energy and Sustainability.” Dr. El-Naggar’s work lies within the intersection between physics, biology, and chemistry, and aims to grasp a multifaceted knowledge of electron transport.

Dr. El-Naggar began by stating the purpose of his lecture: to tell a story primarily about the electron transport chain and microbes. His story begins with a comparison between hard materials and life: the digital revolution was enabled by fundamental advances in our understanding of how electric charges change hard materials, but many may overlook the existence of a similar understanding in biological entities. Because his lecture is part of the Physics Colloquium, he recognized that the primary audience would be more physics-oriented, so he began to introduce some biological concepts, starting with mitochondria. After mentioning the now-famous maxim that “the mitochondria is the powerhouse of the cell,” Dr. El-Naggar explained that mitochondria are fundamental electron transport machines that extract electrons from fuels, like food, to produce ATP (energy), which is a mechanism that is common in all respiratory organisms (including us humans!).

Life, Dr. El-Naggar explained, is made possible through these electron transport machines. Microbes specifically are remarkably fast electron transport machines; they convert energy faster than other entities. For example, in the time that it takes the sun to convert approximately 0.0002 Watt/kg, a bacterium can convert approximately 0.1 to 100 Watt/kg. Dr. El-Naggar then began to discuss a certain group of microbes, metal-reducing bacteria, that don’t have to use oxygen as the final electron acceptor for the energy conversion process to occur. These bacteria, as long as they are redox-active, are capable of using external metallic surfaces to transport electrons. This extracellular electron transport serves as an interface between the biotic and abiotic world, or the living and non-living worlds.

By growing microbes on metallic electrodes, the microbe can donate electrons to the metallic surfaces, acting as the catalyst for whatever work must be done by the electrodes. By exploiting these microbial electrochemical technologies in a laboratory setting, one can give microbes fuel, pump electrodes into these microbes, or design a metabolic pathway inside of the organism that creates a desired end product. Additionally, some microbial fuel cells are commercial, and microbial electrochemical technologies can be used for sustainability. Dr. El-Naggar cited a specific example, Aquam (www.aquam.tech), an organization that uses these technologies to treat wastewater or oxidize organics that are in wastewater.

Essentially, all of these processes boil down to the ability of microbes to get electrons from the inside of the cell to the outside of the cell. Dr. El-Naggar began to enter a more physics-oriented realm when discussing ways in which electrons travel from the inside to the outside, beginning with microbial electron conduits, or tunnels that allow the electron to travel from inside the cell to outside. A cytochrome conduit is a complex of two proteins and has ten iron centers per protein. A protein on the inside of the cell essentially “plugs in” to a protein on the outside of the cell into something that looks like a socket. The conduit gives a viable pathway for electrons to tunnel through the cellular membrane. The specific cytochrome conduit of Dr. El-Naggar’s interest is the porin-cytochrome conduit, the extracellular electron transfer pathway in the bacterium Shewanella.

Amazingly, microbes have evolved the ability to donate electrons to a metal receptor even when the microbe isn’t directly in contact with the surface. For this to be possible, some organisms have developed shuttles that can contain the electron and move from the microbe to the metallic surface. Other organisms developed nanowires, essentially long filaments that extend from the microbe directly to the metallic surface. Through nanowires, the electron is given a path to extend from the organism to the electron receptor.

The rest of the talk, and in my opinion, the most interesting part, was devoted mainly to nanowires. The El-Naggar Lab primarily focused on developing experiments that mirrored biological environments, which tend to be warm and wet. Rather than studying nanowires on a hard, dry surface, common in physics, the lab developed fluorescence-focused methods to study nanowires in vivo. The main experimental method to study nanowires was a clever technique called cryo-tomography, which is a visualization of thin slices across a three-dimensional object. Through this method, the lab was able to map out the structures of nanowires (which ranged from structures resembling wavy strings of pearls to straighter tubes), as well as the proteins that dot the surface of the nanowire, which has a diameter of about 20 nanometers.

To end his talk, Dr. El-Naggar discussed current directions in his research, including a fascinating collaboration with NASA’s Astrobiology Institute. Together, the team aims to insert metal electrodes deep underground (around a mile or so), and see if they can capture new microorganisms that don’t require oxygen to survive. In other words, microorganisms that can use other materials as an electron receptor during cellular respiration will be drawn towards the metal electrode. This “hunt for deep electric microbes,” as Dr. El-Naggar described it, has already discovered about half a dozen new organisms! The mechanisms that Dr. El-Naggar described throughout the talk, therefore, could lead to newer definitions of living organisms beyond the traditional oxygen-based cellular respiration and photosynthesis models.

image via nanowerk

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