The True Extreme Athletes - Extremophiles

Brad jumps back in to comment on the extreme lifestyles seen in some microbes.

Welcome back to Genomics Revolution.  This is Brad Goodner.  I freely admit that I am a sports nut.  I love watching humans compete against themselves and each other under stressful conditions.  I also grew up intrigued by extreme athleticism in non-human animals – the world’s fastest mammal or longest-distance migrant.  I don’t think I am alone in being intrigued by survival and even more so by high performance under conditions that seem way beyond the norm.  I bring this up today because when it comes down to it, microbes are the ultimate extreme athletes (1).

Many of the microbial genomes we have considered so far come from organisms that grow best under conditions that we humans consider very comfortable – plenty of oxygen as an electron acceptor for aerobic respiration, plenty of reduced organic compounds as food (think carbs and fats), temperatures between 20 and 45oC, a neutral pH around 7, and at most a touch of salt in the water.  For example, Agrobacterium tumefaciens strain C58, an organism I have worked on for over 25 years and spoke on in episode 7, would happily grow in a flask strapped to my hip containing a watery extract of the same food that I eat.  That said, many other microbes grow best under one or more conditions that we would consider extreme.  Let us consider four extreme conditions.

First and foremost, we humans are totally dependent on oxygen and we struggle to imagine life without it.  Antoine von Leeuwenhoek published a short report in 1680 on two extracts of ground pepper, one in an open tube and the other in a tube that was heat-sealed (2).  Leeuwenhoek was surprised that the sealed tube not only contained microbes, what he called “animacules”, but that the most abundant microbe in the sealed tube was not seen in the open tube.  He did not know how to explain the differences he saw, but his report certainly documents what we now know is anaerobic microbial life a century before the discovery of oxygen and its importance for aerobic life. Louis Pasteur, one of the founders of experimental microbiology, was the first to truly understand and write about anaerobic microbes in 1861 as he was studying a fermentation that produces butyric acid as a byproduct (3).  Such a fermentation leads to rancid butter.  Pasteur figured out that the microbes producting butyric acid grew in the absence of oxygen, but even more surprising that these same microbes could be killed by bubbling oxygen through the solution.  He wrote the first description of what we call obligate anaerobes.  The Bifidobacterium strains discussed in episode 17 do well in our colons because of the absence of oxygen at the end of our GI tracts.  The pathogen Clostridium perfringens in episode 24 and the archaeaon Methanococcus jannaschii in episode 28 are also obligate anaerobes.  Other anaerobes don’t use oxygen but are not killed by it.  They are called aerotolerant anaerobes.  Going one step further, some anaerobes grow fine without oxygen but will use oxygen for aerobic respiration when possible.  The pathogens E. coli O157:H7 from episode 8 and Vibrio cholerae strain El Tor 16961 from episode 9 act this way and are called facultative anaerobes.  Finally, some microbes use oxygen but only at very low concentrations.  These microaerophiles, as we call them, include the pathogen Campylobacter jejuni discussed in episode 16.  We humans cannot do any of these no-oxygen or very low-oxygen alternative growth strategies for more than a few minutes without severe consequences.

Second, I grew up in north central Texas where summer days reached 100 oF (37.8 oC) and more than occasionally 110oF (43.3oC).  Even in the shade, the heat would just wear you out.  We humans do best at slightly cooler temperatures and so do many microbes known as mesophiles whose optimum growth temperature is between 20 and 45oC.  Organisms whose optimum growth temperatures are between 45 and 80oC are called thermophiles including the archaeaon Thermoplasma acidophilum from episode 10.  Aeropyrum pernix strain K1 from episode 27 takes it up another notch as a hyperthermophile whose optimal growth temperature is greater than 80oC.  For reference, 80oC is twice as hot as your steaming hot tub on the back deck.  Yowsa! 

Third, if you are like me you have jumped when you have accidentally gotten some lemon juice in your eye or in a cut.  Imagine if you were bathed in lemon juice all the time.  That mouth-puckering citrus liquid has a pH between 3 and 2, meaning it is 10,000 to 100,000 times more acidic than water.  How can anything live in such an environment?  Ask Thermoplasma acidophilum who we already know is a thermophile, but it is also an acidophile that thrives at pH 2 and can still grow at pH as low as 0.5.  On the other end of the pH spectrum are alkaliphiles who prefer a very basic lifestyle at pH >9.

Fourth and finally, have you every soaked your aching feet in some water containing Epsom salt?  Great for a short while, but what living in a very salty solution all the time.  Our bodies spend a lot of energy and regulatory power to maintain internal cellular salt concentrations in a very narrow range.  So do most other organisms across all 3 domains of life.  Organisms in salt water have to work that much harder to osmotically regulate.  Seawater has an average salinity of 3.5% (35 grams of salts per liter), but there are microbes who love living in the Great Salt Lake or the Dead Sea at up to 27% and 33.7% salinity, respectively.  Halobacterium strain NRC-1 from episode 26 is one such extreme halophilic member of the Archaea that is part of a lineage that evolved a novel salt-dependent growth strategy that involves accumulating high internal levels of KCl instead of NaCl.

In the next several episodes, we will hear about many more extremophiles who live in amazing places right here on Earth.  Our guides will be more of my students from the 2019 Hiram College Genetics course.  Stay tuned.

For more information:
(1) Merino et al., 2019.  Living at the Extremes: Extremophiles and the Limits of Life in a Planetary Context.  Frontiers in Microbiology 10:780.

(2) Gest, 2004.  The discovery of microorganisms by Robert Hooke and Antoni van Leeuwenhoek, Fellows of the Royal Society,  The Royal Society 58:12.

(3) Pasteur, 1861. Animal infusoria living in the absence of free oxygen, and the fermentations they bring about [translated from French].  Competes Rendus de l’Academie des Sciences 52:344-7.