Life on Earth, Part 1: The Extremophiles

Life finds a way even under the harshest of climatic conditions.

Life on Earth and climate are interrelated in many ways. To have life requires the right conditions, and Earth has many: flowing water, energy availability (especially from the sun), nutrient cycling, protection from asteroid strikes (thanks to Jupiter’s massive gravity field), deflection of radiation (thanks to Earth’s magnetic field), and a dynamic and protective atmosphere (in part, thanks to the greenhouse effect). Evidence suggests liquid water existed as far back as 4.38 billion years ago, and about 300 million years later, we find the first evidence of life. These initial lifeforms were microscopic, and resilient. They probably had to withstand some impressively extreme conditions, as far as humans are concerned, such as superheated water near hydrothermal vents. Since then, life on Earth has both flourished at times, and faced mass extinction at others. Luckily, life has persisted, and we find ourselves living in the 21st Century.

I think a really cool way to think about the limits of life on Earth is to explore the range of climates in which life can survive. This could be super hot or super cold waters, exceedingly dry deserts, or even acid mine drainage. Have you heard about extremophiles before? These are lifeforms that can withstand extreme conditions. One such example is the tardigrade, a microscopic extremophile that is also referred to as a water bear or moss piglet. The tardigrade can endure temperature extremes, radiation exposure, and the vacuum of space. A google search for tardigrade brings up all sorts of interesting stories, memes, and GIFs. Here is one exciting aside, a story mentioned in Sierra Magazine (original source: Wired Magazine):

Apparently, when the Israeli lunar lander Beresheet crashed on April 11, 2019, it carried a box containing thousands of tardigrades. microscopic but incredibly tough creatures that may now be alive on the moon.

In other words, tardigrades may be the sole rulers of the Moon. However, additional searching yields this from Science:

Researchers have subjected tardigrades, microscopic creatures affectionately known as water bears, to impacts as fast as a flying bullet. And the animals survive them, too—but only up to a point. The test places new limits on their ability to survive impacts in space—and potentially seed life on other planets.

Right, so tardigrades probably cannot survive crash landings in space. RIP tardigrades on the Moon.

Ok, back to Earth. Extremophiles like the tardigrade can tell us something about the extreme limits of life on our own planet, and for that matter, the range of conditions that might support life on other planets.

The following factors and associated limits represent the range of climates in which extremophiles exist on Earth. For high temperatures, extremophiles can survive in places like submarine hydrothermal vents (230-250°F) - that’s hotter than boiling water, yikes! At low temperatures, extremophiles can survive in unfrozen hypersaline polar lakes (-0.4°F), in certain places on land, and apparently, our friend the tardigrade can lay dormant at 1 degree above absolute zero. Inconceivable! It’s worth pointing out that some microbes are good time travelers, because they can fall asleep (lie dormant) in ice or permafrost and wake up a million years later as if nothing has happened.

The lichen Xanthoria elegans can continue to photosynthesize at −11°F. [Wikipedia, CC BY-SA 3.0, Jason Hollinger]

Extremophiles have been found in wildly acidic environments (pH -0.06 to 1.0), like the geothermal pools in Yellowstone National Park. They also survive in basic environments at pH > 11, such as Lake Shala in the East African Rift Valley.

The bright colors of Grand Prismatic Spring, Yellowstone National Park, are produced by thermophiles, a type of extremophile. [Wikipedia, CC BY-SA 4.0, Carsten Steger]

For radiation, the tardigrade is the champion, withstanding radiation doses of 5,000 to 6,000 grays. Humans could withstand 5 grays, but not well.

Milnesium tardigradum, doing what it does, surviving. [Wikipedia, CC BY 4.0, Kiosya et al. 2021]

We also have salinity (saltiness). Extremophiles can survive in waters 40% saltier than the average ocean. For desiccation (to remove moisture), life survives the ultra-dry Atacama Desert (Chile) and McMurdo Dry Valleys (Antarctica). Some organisms can survive 99% loss of water for prolonged periods. Bacillus sphaericus spores survived 25 million years of complete desiccation in amber via a process known as anhydrobiosis. Finally, we have pressure, with extremophiles surviving 1,100 bar in the Mariana Trench, Pacific Ocean (35,000 feet deep). For comparison, the average atmospheric pressure on Earth at sea level is approximately 1.013 bar.

Microscopic image from the hypersaline Lake Tyrrell, in which the eukaryotic chlorophyte, Dunaliella salina, an extremophile, can be identified. [Wikipedia, CC BY-SA 4.0, Mike Dyall-Smith]

Well, there you go. We now have a pretty good idea about life on Earth. In the locations the extremophiles have found a way to survive, humans don’t stand a chance. In short, life is beyond awesome, it is everywhere on Earth, and conceivably, it is probably spread across the universe. Just for fun, let’s consider if there might be life on other planets or moons in our own Solar System.

First, let’s talk about Mars. Literally, Mars has always been the topic of speculation about otherworldly civilizations or potential interplanetary battles with Martians. Fantastic movies have been produced. Check out the following:

In the comedy film, Martians kidnap Santa Claus and two children and bring them back to Mars. The film regularly appears on lists of the worst films ever made. [Wikipedia, Public Domain]

At the beginning of the film, the sole survivor of a human mission to Mars is recovered by a second rocket ship, but a monster stows away on board for the trip back to Earth. Apparently, this film was the inspiration for the movie Alien. [Wikipedia, Public Domain]

Alright, so what does the science say about life on Mars? We know that there was likely water on Mars in the distant past. Certainly, from what we know about extremophiles, life on Mars seems possible. In fact, NASA’s Mars rover recently made a ‘fantastic’ find in the search for past life. Nature News details how the Perseverance Rover collected four rock samples from an ancient river delta in the Jezero Crater where organisms might have thrived. Of great significance, NASA will send other spacecraft to pick up the cores and return them to Earth for analysis by 2033. This means that within one decade, there is a reasonable chance that humans can prove the existence of life on another planet.

Another potential target for life in our solar system is on Jupiter’s moon Europa. Using information about extremophiles on Earth, scientists have identified the deep ocean of Europa, far from the life killing doses of radiation at the moon’s surface, as a potential location that can support life. If you want to nerd out, read the Marion et al. 2003 paper below.

Sources:
Marion et al. 2003. The Search for Life on Europa: Limiting Environmental Factors, Potential Habitats, and Earth Analogues. https://doi.org/10.1089/153110703322736105 [pdf]

One place we won’t be looking for life is the thick, toxic atmosphere of Venus, filled with carbon dioxide and sulfuric acid that trap heat for an average temperature of 864°F.

We can also look further afield - far outside our own Solar System - with the James Webb Space Telescope. It can “tell us about the atmospheres of extrasolar planets, and perhaps even find the building blocks of life elsewhere in the universe.” NASA has more info here.

Alright, that’s it for this installment of Paleoclimate. In the next newsletter post, I will discuss additional climatic constraints for complex lifeforms: hint, it’s all about stability!

Sincerely, 

TRJ, PaleoClimate Scientist