{"id":1138,"date":"2023-12-19T16:24:40","date_gmt":"2023-12-19T16:24:40","guid":{"rendered":"https:\/\/www.homeworld.bio\/?p=1138"},"modified":"2024-03-15T10:16:17","modified_gmt":"2024-03-15T10:16:17","slug":"the-mini-fridge-paradox","status":"unlisted","type":"post","link":"https:\/\/www.homeworld.bio\/blog\/the-mini-fridge-paradox\/","title":{"rendered":"The mini fridge paradox"},"content":{"rendered":"\n

All life on earth, blue whales, rabbits, ants, bacteria, even TSA agents, is electrically powered. The powerhouse of the cell is just a warehouse where countless high-power electrons discharge energy through thousands of nano<\/em>metre-scale generators. We keep these generators spinning1<\/a> by molecularly incinerating the food that we eat to release more high-power electrons.<\/p>\n\n\n\n

Harnessing electrons is so universal to life that some scientists even posit<\/a> electron flow as a prerequisite to life\u2019s origin. Living systems eat energetic electrons trapped in molecules and then literally burn them to extract power. While oxygen makes for great fires, life is so addicted to this process that it will make do with whatever \u201celectron acceptor\u201d it can find. If you\u2019ve ever built a Winogradsky column<\/a>, then you\u2019ve seen this principle in action. But life isn\u2019t picky about where the energized electrons come from either. Sunlight<\/a>, sugar<\/a>, hydrogen gas<\/a>, actual electrodes<\/a>, maybe even radioactive decay<\/a> – it all works.<\/p>\n\n\n\n

It seems as though life will \u201ccolonize\u201d any suitable source of nutritious electrons, and we call this seeming \u201cmicrobial infallibility<\/a>\u201d. So I ask: given that mini fridges exist, where is all the heat-powered life?<\/strong><\/p>\n\n\n\n

On Earth, most electricity is made by turning an axle. Wind, water, and steam from coal fires and nuclear fission are all cajoled into rotating a generator and inducing a current. In space, we tend to want to limit the number of moving parts and so rotating generators aren\u2019t really the mode<\/em>. Instead, we either sun-roast electrons off semiconductors (solar panels), or use batteries powered by red-hot plutonium. These \u201cradioisotope thermoelectric generators<\/a>\u201d exploit a heat differential between the radioactive metal and the inhospitable cold of space to induce a modest, but reliable electrical current without any moving parts. I wonder if there are organisms that do something similar (but maybe without the plutonium).<\/p>\n\n\n\n

This \u201cthermoelectric effect<\/a>\u201d is not just relegated to multi-million dollar space missions, it\u2019s also found in hotel rooms around the world. Beyond its mystical ability to triple the price of any snack it contains, the hotel mini fridge also manipulates electrons in a way that is hitherto unknown<\/em> to biology. By passing electrical current through a plate of two conductors, the \u201cPeltier effect<\/a>\u201d leads to the build up of heat on one side, and the cooling of the other – a quiet way to cool a box of treats. And the crazy thing is that if you instead provide the heat and the cold to the plates, you\u2019ll receive a current out of its wires. A change in heat can directly give you voltage.<\/p>\n\n\n\n

Much to the chagrin of my undergrad self, Anthonie Muller first proposed that life could be driven by \u201cthermosynthesis<\/a>\u201d over a decade before my birth2<\/a>. Questioning if the boring sunlight and sugars we\u2019ve known about for centuries were the only vital forces on Earth. Muller proposed multiple models for how a biological \u201cheat engine\u201d could be built over the ~8 nano<\/em>metre outer membrane of a cell. While fascinating, these ideas all rely on a gradient forming over a tiny<\/em> distance – just 30 water molecules end-to-end. I\u2019m no entropist, but that doesn\u2019t feel workable to me. But if we take the long view – I think there\u2019s something here.<\/p>\n\n\n\n

A thin Peltier module can be ~0.5 milli<\/em>metres thick, so we\u2019re looking for naturally occurring half-centimetre biological wires. Enter the star of our speculative science fiction: Desulfobulbaceae<\/em> – better known as Cable Bacteria<\/a>. As their moniker suggests, Cable Bacteria grow in chain-shaped colonies that easily exceed our threshold, reaching lengths of 15 mm. But unlike other \u201cfilamentous\u201d microbes, Cable Bacteria chains are electrically conductive. In fact, they rely on this conductivity to short-circuit aquatic sediments to steal oxygen from layers that they wouldn\u2019t normally have access to. Cable Bacteria keep their lights on by shunting their electrons from, microbially speaking, low prestige sulfidic sediments into snooty, oxygen-rich layers by physically bridging these layers with their Cable. So my question is, what happens if one side of one of these Cables gets warm?<\/p>\n\n\n\n

Let\u2019s say we have a little microscopic forest of Cable colonies at the bottom of a geologically active alpine lake, happily redistributing the oxidative wealth. The lake water is cold, keeping the highest, oxygen-rich layers chilled. But the sediment has built up on the geologically warmed lakebed, so the sulfidic base is routinely warmer. Could our Cables experience an electrical bump from this temperature differential? I have no clue<\/em>. The best I understand it, a Peltier module needs two different conductors, do the Cable\u2019s different conductive cytochrome proteins satisfy this requirement? Maybe<\/em>? And in the best case, let\u2019s say there\u2019s a 50 \u00b0C temperature differential over 1 cm, how many electron volts<\/em> could we hope to draw? Could that power, or at least supplement bacterial life? I wouldn\u2019t know<\/em>. If you have an idea on how to calculate these ballparks, I\u2019d love to hear your thoughts in the comments!<\/p>\n\n\n\n

It turns out that dictating the flow of electrons is a lucrative endeavor: often, electrical engineers restrict a current\u2019s flow to a particular direction by using a one-way door called a \u201cdiode\u201d; the brain that you\u2019re using to read this blog can only function because of unidirectional biological wires called \u201cneurons\u201d; and disrupting how charge segregates in the mitochondrion is one weight loss trick that doctors definitely<\/a> don\u2019t want you to know about. The same is probably true for our Cables. They shunt electrons up<\/em> through the sediment to live, but what if another electrical pressure pushed those electrons down<\/em> the other way? What if a Cable, that can experience a thermoeletric effect, is arranged the wrong way<\/em> in the thermal gradient? I\u2019d imagine this would be no good. So given the existence of diodes, and one-way neuronal conduction, and the mechanisms in place to prevent mitochondrial \u201cdecoupling\u201d, I\u2019d posit that if Cables can experience a thermoelectric effect, I would also expect them have diode-like systems in place to deal with disruptive thermoelectric current. Further, Cables can move through bacterial gliding, so I\u2019d also expect them to try and optimally align themselves to thermal gradients.<\/p>\n\n\n\n

Our complete transition to renewable energy sources like solar is in part being stopped by a duck. The \u201cduck curve<\/a>\u201d results from the uneven rate at which solar energy is produced versus its consumption. During the day, when electrical lighting is least needed, we generate the most solar electrical energy, but we\u2019re not too good at storing that energy for later use in the evening. What I\u2019m driving at is that to efficiently use power from environmental sources, one needs to be able to rapidly adapt to changes in production efficiency. Sometimes it\u2019s really bright and if we don\u2019t act, we waste that overproduction, maybe even damage our power grid. Other times it\u2019s too dim and we better hope we made use of our previous brighter boon times. The same issues exist in microbial energetics. An equation has to be continually balanced between production and consumption. Fail to do so and at best you\u2019re being inefficient, but at worst you\u2019re accumulating toxic energetic byproducts or unnecessarily starving during future badtimes. So with thermal gradients being hard to predict, I\u2019d expect thermoelectric Cables to have machinery to react to electron gluts and dearths caused by fluctuating thermal gradients.<\/p>\n\n\n\n

All of this speculative navel gazing is to say, if Cable Bacteria are sensitive to thermoelectric effects, I\u2019d expect to observe these three biological features:<\/p>\n\n\n\n