Energy. We all have it and we all need it on multiple levels. Within our body, energy is stored in a molecular currency that is conserved among all living organisms, adenosine triphosphate, ATP. Of course, you should know that as humans, we require oxygen to live. During our metabolism, hydrogen atoms are divided into their two opposite parts, the proton and electron. The electrons are shuttled through several enzyme complexes while the protons are pumped out of the mitochondrial matrix creating a much greater proton concentration outside than inside. This imbalance is what drives Nature’s smallest rotary motor, ATP synthase. But what about the electrons? Your body has no need for them, energetically speaking so something needs to accept them for the big show to continue. In our case, the acceptor is oxygen. Oxygen accepts the electrons, and the protons that come along to reconstitute the full hydrogen atom, to form water, H2O. A lot of organisms need oxygen for the same reason. However, just as many organisms have no such requirement while some others are afraid of oxygen.
This leads to the question of what accepts the electrons within organisms that don’t have oxygen present? Anaerobic (“no oxygen”) respiration can utilize many different molecules to accept electrons depending upon the genetic capacity of the organism in question. The more genes within a genome that encode enzymes that can coerce compounds to accept electrons, the more options an organism has in regards of what environment they can survive and thrive. If you have kept up with this blog, you know about a group of bacteria that have evolved a variety of strategies to survive in some of the most undesirable environments on (or in) Earth; Geobacter.
Geobacter have been identified in many anaerobic environments including, soil, sediments, wetlands, and even rice paddies. Geobacter are the predominant species in these environments where there is no oxygen and few other choices for electron acceptors. They are very efficient with their energy usage as well as creative in the ways in which they “relieve” themselves of unwanted electrons. In the absence of oxygen, Geobacter have two major methods of removing the reducing power of electrons. The method of choice used depends upon the type of compounds within their environment capable of accepting electrons; soluble or insoluble. Soluble, water dissolving, compounds include many common organic materials such as amino acids and carbohydrates. Uptake of these molecules is possible and necessary. However, not all soluble compounds are easily tolerated by Geobacter including heavy metals. How can Geobacter utilize these electron acceptors if they can’t bring them inside their cell membranes? The answer is by taking the electrons outside the cell through a labyrinth of electron shuttling proteins called cytochromes. Cytochromes, especially the predominant Geobacter type cytochrome c, use prosthetic cofactors like hemes or copper ions to ferry electrons out of the cell to waiting acceptors.
This is where it gets interesting…
What if only insoluble electron acceptors are present? There’s an op for that! Operon that is. Actually, several operons that are active when sources of soluble electron acceptors are very low. Geobacter can synthesize extracellular appendages that can navigate over several cell lengths to find insoluble acceptor compounds including the predominant iron Fe3+ within the subsurface. These appendages called pili are found in many other bacteria. However, there is something a little more special about Geopili, they can conduct electricity. The protein subunits that compose the geopilus have a shorter peptide sequence than the one found in a majority of other pili systems. Also, a few of the cytochromes c proteins that shuttle electrons to the outer membrane of the cell can actual be deposited along the pilus to deliver electrons to waiting acceptors far away from the actual cell itself.
Welcome to Week 4 of My Tiny Highlight (MyTH) series. This week I will focus on not a species. Instead, I will focus on a genera; Geobacter. Like the previous highlights, Geobacter are proteobacteria that has become relevant only more recently. Geobacter were first discovered and isolated in the late 1980s by UMass professor Derek Lovley. In a short amount of time, Geobacter has become a model organism in highly active research areas. These include bioremediation and microbial fuel cells. Several different Geobacter spp. are routinely found in soil and sediment samples from contaminated sites. For many bacteria, oxygen is not required to survive. During the course of evolution, many bacteria, including Geobacter, can undergo anaerobic respiration, or create energy without the need for oxygen. The first Geobacter genome was published in 2003 to much fanfare in the journal Science. One reason for this was the discovery that Geobacter are motile, having several chemotaxis proteins. Also found was an unprecedented number of cytochrome (111!) genes which are usually used for electron transfer via attached heme groups to the protein. The number of bound hemes vary between 1 and 27. Very impressive. In order to survive, these bacteria can use a host of molecules as an “electron sink” so their metabolism can continue. Geobacter have two main strategies for this; if the “sink” is soluble, they can utilize a host of cytochrome c proteins on their outer membrane exposed to their environment. If these “sinks” are insoluble like metals for example, they can essentially extend appendages from their membrane to the “sink”.
This is where it gets interesting…
These appendages called pili have extracellular cytochrome c proteins along their length. So, electrons are transported from inside the cell through the pili and cytochromes to the available electron sink. Essentially, they are able to conduct electricity as a means of respiration. Here are two animations showing the differences:
I would start with a ringer; someone high profile that does a lot for humanity. Since a majority of Americans can not name a living scientist, this is difficult. I would have to start with Bill Nye. He is a beloved TV personality and advocate for action on climate change and science literacy. Heck, he even has ‘Science’ in his name (The Science Guy).
Following Lucy would be Derek Lovley. With a great PR department at UMass, Lovley has created a major buzz in the microbial and alternative energy world. Lovley’s work with Geobacter has discovered the potential to create a microbial fuel cell due to extracellular electron transfer. In other words, bacteria conducting electricity as a by-product of cell metabolism.
George Church would be a great addition to any fantasy science team. He recently made headlines due to awful translation. He had pioneered genetic research and to add a cool factor, has appeared on The Colbert Report. He most recent book was ‘translated” onto DNA. 20 million copies of his book were encoded onto DNA and is kept in a small vial.
I think 5 team members is a good start. I may pick up some more scientists off the waiver wire later, but I like my chances to make it to the championship. We’ll see…