Thermodynamic Warfare

Sometimes you just need an equation. Even if you’re writing a “popular” book where equations are discouraged. No, I’m not talking about E = mc² that shows up just to be associated with someone famous.

 

Karen Lloyd, the author of Intraterrestials is a superbly engaging writer. Her book is littered with well-chosen metaphors and analogies to explain how scientists study organisms hiding away deep in the subsurface of our planet. But I appreciate that the professor in her wants to teach her readers something useful and profound. She chose the Gibbs Free Energy equation:

 

I explain this equation every year to my G-Chem 2 students when we discuss thermodynamics. Lloyd does so with much more flair. In chapter 6 (“Breathing Rocks”) she opens with her life-harrowing yet exhilarating experience of sampling for microbes at a volcano caldera in Chile. After the scenario of physical heat and motion (get your samples quick so you don’t die!), she launches into the heat and motion associated with thermodynamics. She explains the Gibbs equation with colorful examples such as roller coasters and hand warmers. I could quote her for several paragraphs, but instead, I recommend you read her book in full. It’s a page-turner!

 

The crux is that subsurface organisms, unable to get their energy from the sun (like photosynthetic organisms) or eat food they can metabolize with oxygen (like most of us do) respire by breathing rocks. They eke out a low-energy lifestyle turning carbon dioxide (from carbonate rocks) into biomass with the help of nitrogen and sulfur compounds, also found in minerals. Such chemical reactions typically have a small negative delta-G, so you can’t get much energy from them, but they are still energetically “downhill” and thus favorable.

 

But things get even weirder when there’s competition for resources. In chapter 7 (“Life on the Edge”), Lloyd sets up the discussion with another vignette in the cold of Svalbard, Norway, where she is cutting sediment cores dug up for her research. While doing so, she ponders life in the cold Arctic with tremendously varying sunlight. And now I have to quote her: “But intraterrestials don’t care about sunlight or cold. They care about delta-G.” And unlike our familiar surface microbes that “secrete deadly antibotics, hoard nutrients, and grow ultrafast to get ahead” and beat out the competition, subsurface microbes have an additional weapon: “If one microbe’s delta-G is better than another one’s then the first microbe can asphyxiate the second.”

 

I was delighted that Lloyd chose sulfate-breathing microbes to illustrate her point since I’m studying the role of sulfur at the origin of life in competing autocatalytic cycles. She delves into the equation, now focusing on how delta-G can be modulated by Q, the reaction quotient. It’s counterproductive for them to grow big fast because that decreases sulfate diffusion in their cellular bodies. Releasing antibotics is also bad because it would kill symbiotic species in addition to its direct competitors. Molecular hydrogen is a required “food”; you can’t stop your competitors from getting it, but you can hoard enough so that you still have a borderline negative delta-G, while forcing the delta-G of your competitors to turn positive (“uphill”, energetically unfavorable) so their metabolism no longer yields energy and they die.

 

Lloyd writes: “Like a shipwrecked sailor dying of dehydration while surrounded by water, these microbes expire with their food right in front of them. Sulfate reducers win because they take the whole system to the bitter edge of their own thermodynamic capabilities, which pushes everyone else off the cliff.” Ugh. That’s war. But then as the amount of sulfate reduces, the sulfate reducers now face extinction. As they die off, their competitors (often methanogens) can access more hydrogen once again and a revolution takes place.

 

Life gets weirder still. Some microbes (methanogens!) can reverse their food and waste as delta-G switches, so they can keep eking energy. Others ferment; in Lloyd’s words: “takes one slice out of the pie and puts the rest back into the fridge for others to eat later. It’s very polite. Because of this restrained eating, fermentation ends up being one of the lowest-energy processes known to support life.” The low-energy living of intraterrestials suggests that they might live a long, long time without reproducing. It’s immortality of a sort, though not the one we might desire.

 

Reading Lloyd’s book rejuvenated my excitement about my research projects. It also reminded me that I want to be a better teacher and communicator. While this was a library book, I will be purchasing my own copy because it deserves re-reading, and I still need to delve into the scientific papers listed in the references!