Some Like It Very, Very Hot

Organisms living in habitats that no sane person would call comfortable have fascinated biologists for decades. Extremophiles, as they are known, can withstand environmental conditions that would kill less hardy creatures both very quickly and in an extremely unpleasant manner. One subset of extremophilia that I find most fascinating is hyperthermophilia. As you can probably guess, hyperthermophila involves living and growing at rather high temperatures.

A hyperthermophile is defined as any organism that can function above 60ºC, but most of them have an optimal growth temperature of 80ºC or higher.¹ These creatures are truly amazing: To be able to function in such conditions, every part of their cellular structure needs to be adapted and tuned to ultra-stability. Many parts of the cell are highly dependent upon a constant, relatively low temperature in order to remain functional. Proteins, the workhorses of the cell, have an optimal temperature range within which their precise, three-dimensional shape is biologically active. Phospholipids will dissociate at high temperatures, rupturing the cell's plasma membrane. DNA, the essential blueprint at the heart of the cell, isn't safe either, and its familiar double helix shape will start to come apart once the temperature rises past a certain point, becoming useless.

To put these hyperthermophiles' biochemical achievements in perspective, proteins in our body have an optimal temperature of around 37ºC — which is, not coincidentally, our normal core temperature — but if exposed to the optimal temperatures of hyperthermophilic proteins, our precious proteins would instantly denature, jumbling into a quivering, biologically-useless tangle of amino acids. We didn't evolve to survive anything like that amount of heat.

So, you're probably all wondering:

• What are the known limits to hyperthermophilia?
• What are the most extreme members of this already-crazy group of creatures?
• How hot do we know they can go?

I now submit to you three champions of the hyperthermophilic realm.

First up, Pyrococcus furiosus. With a growth range of 70-103ºC and an optimal temperature of 100ºC, it has rather impressive thermal stats, but most interesting about this archaeal species are certain aspects of its biochemistry.² P. furiosus contains enzymes that incorporate tungsten into their structure, which is extremely rare, even in the weird and wacky world of microbes. This makes tungsten the heaviest element known to have a biological function, with second place going to iodine.³ But that's not all. It has a very simple respiratory system: The oxidation of sugars and amino acids reduces protons to hydrogen gas, which produces the energy to create a proton gradient and drive the synthesis of ATP, the basic unit of energy in the cell. And that's it!⁴ It's a much simpler system than the complicated electron transport chain we have in our mitochondria, leading researchers to wonder whether such a system was present in some of the earliest forms of life on the planet.

Next, Geogemma barossii, also known as "Strain 121." Like P. furiosus, it's a member of the domain Archaea (as many other extremophiles are), but when it was found and described in 2003, it smashed all known temperature records: It has an incredible growth range of 85-121ºC, with an optimal temperature of 115ºC!⁵ 121ºC is the sterilization temperature for laboratory autoclaves: G. barossii cells could be multiplying as you're trying to get rid of them! And even so, while its limit for growth is 121ºC, it can survive temperatures of 130ºC for up to two hours and remain viable! This is one hardy organism. Even the fact that it gains energy by reducing iron oxide (rust) to magnetite, a magnetic form of iron, is just an interesting side note in comparison to its temperature-conquering abilities.

But the current king of the hyperthermophiles is Methanopyrus kandleri. Again, it's an archaeal species, which reduces hydrogen and carbon dioxide to methane for energy, and was discovered in 1991 in the wall of a hydrothermal vent in the Gulf of California, 2450 m below the sea's surface. But it wasn't until 2008 that scientists realized, if cultivated at hydrostatic pressures similar to those in their native habitat, 40 megapascals in this case, their growth range was 90-122ºC, compared with 85-116ºC at 0.4 MPa (normal atmospheric pressure is approximately 0.1 MPa)!⁶ This one extra degree of temperature tolerance — 122ºC compared with G. barossii's 121ºC — along with experiments that showed that M. kandleri could remain viable at 130ºC for at least three hours — one hour longer than G. barossii — puts it on top as the most hyperthermophilic organism known.

The biosphere is a fascinating place, and we've only really scratched its surface in terms of the number of species we've discovered, described, and studied in any detail. Who knows how many more hyperthermophiles, and extremophiles, are living out there in volcanic springs, in rocks deep underground, or in the very depths of the ocean around hydrothermal vents? Hey, perhaps we'll someday find a microbe that lives in molten rock! You never know, nature always finds ways to surprise us.

Image Credit: NOAA (via Wikimedia)

References:

1. Stetter, K. (2006). Hyperthermophiles in the history of life. Philosophical Transactions of the Royal Society B 361, 1837-1843 DOI: 10.1098/rstb.2006.1907.

2. Fiala, G., & Stetter, K. (1986). Pyrococcus furiosus sp. nov. represents a novel genus of marine heterotrophic archaebacteria growing optimally at 100ºC. Archives of Microbiology 145, 56-61 DOI: 10.1007/BF00413027.

3. Mukund, S, & Adams, M. W. (1991). The novel tungsten-iron-sulfur protein of the hyperthermophilic archaebacterium, Pyrococcus furiosus, is an aldehyde ferredoxin oxidoreductase. Evidence for its participation in a unique glycolytic pathway. The Journal of Biological Chemistry 266, 14208-16 PMID: 1907273.

4. Sapra, R. et al. (2003). A simple energy-conserving system: Proton reduction coupled to proton translocation. Proceedings of the National Academy of Sciences 100, 7545-7550 DOI: 10.1073/pnas.1331436100.

5. Kashefi, K. (2003). Extending the upper-temperature limit for life. Science 301, 934-934 DOI: 10.1126/science.1086823.

6. Takai, K. et al. (2008). Cell proliferation at 122 C and isotopically heavy CH4 production by a hyperthermophilic methanogen under high-pressure cultivation. Proceedings of the National Academy of Sciences 105, 10949-10954 DOI: 10.1073/pnas.0712334105.