Dear Kenneth, Archaea is a correct  term, but Aquificae are Bacteria! not Archaea or Archaeobacteria (the outdated term). 

Elemental S is quite stable at anaerobic conditions at acidic pH and normal or high temperatures, however,  in the presence of sulfides/hydrogen sulfide it is able to form soluble polysulfides. This formation of polysulfides increases with increase of temperature or pH (and pH is much more important). At high (for microbes, let`s say 80-100C) temperatures and pH (e.g. 9) almost all elemental sulfur could be transfered to polysulfides, but at neutral pH and high T or high pH and normal temperatures there still will be high amounts of elemental S in equilibrium with significant amounts of polysulfides.

And the most important thing - dew to its solubility polysulfides are much more tasty for microbes than elemental sulfur.  In the same time concentration of oxygen, as I said, is quite low at high temperatures dew to low solubility making it difficult to utilize. This is a reason why the majority of extremely or hyperthermophilic microbes are sulfur (polysulfide) reducers and not oxygen reducers.

And the last remark: K. Stetter is one of the greatest microbiologists (being actually molecular biologist at the beginning), however, some  of his assumptions (at least this one) made in 80th-90th are not so correct to the current date. Reduced compounds of S, Fe not Oxygen are really important for thermophiles as well as for ancient life forms if we presume that the LUCA was a thermophile, what is quite obvious, I suppose. Moreover, as I know, 3500-4000 Ma ago, when life appeared, the planet was completely anaerobic.

Thanks, Kenneth, but I mean, at which temperatures and pressures can we expect to find life forms on this planet. The article I referred to is showing an abundance of life in oil reservoirs, at relatively high temperatures. I would expect that even when parts of the water is boiling, the rest of the water may host live organisms. However, the ultimate boundary will be met as water becomes supercritical, as it then has the property of dissolving all organic matter, including oil.

Yes, in some cases oxygen can be present, even in extremely hot and pressurized environments. So, what is the temperature limit for A. pyrophilus?

A good question, but I guess it has to be oxygen that has been split from binding in CO or H2O? But, if the Aquifex has a maxT of less than normal surface boiling temperature, I guess it does not occur very deep, i.e., in high pressure environments?

400C is too much :) In 1967 Brock written “Bacteria are able to grow… at any temperature at which there is liquid water…”, however later studies revealed that the limit is depended on stability of cells metabolites and compounds eg. lipids, aminoacids, DNA etc., thus should not be higher than ~140C. The current "leader" can grow at 122-123 (not optimally). So, every environment above 140 is sterile (spores can resist it for short period of time), but life is ready to colonize it when the temperature will be a bit lower.

Great, Ilya, this was the answer I was looking for. You see, the answer has two important components: the max temperature where life can be stable, and the precursor for life: you call it "..life is ready to colonize it when...." This means that the supercritical water phase, which occurs at temperatures around 400 C, depending on what elements are dissolved, may represent a very powerful precursor for life. When this water phase cools down to below the critical point, then it condenses to pure water. Thereafter all the other ingredients that were transported with the supercritical water will condense or precipitate out and perhaps be colonized by primitive life forms.

exactly! this is how thermal springs and especially deep-sea vents (eg. black smokers) "work" and support abundant life in close proximity to these vents...

Hi Martin, this info could be useful: 

Jones et al., 2007. Crude-oil biodegradation via methanogenesis in

subsurface petroleum reservoirs. Nature Letters

Dear Kenneth,  first of all, Aquificae is not an archaeal but bacterial phylum. Second, they aren`t a deepest lineage among bacteria. To the current knowledge, Thermotogae is a deepest phylum with cultivated representatives and all of them are strict anaerobes. Moreover, not every Aquificae are microaerophiles, the deepest lineage among them, Desulfurobacteriacea family, represented exclusively by strictly anaerobic bacteria. Thus it is quite logic, that the first Aquificae were anaerobes, which developed (or acquired by HGT, I don`t remeber)  the enzymatic machinery for oxygen respiration much later. Among the other things, oxygen has very low solubility at high temperatures, that`s why the majority of extremelly thermophilic and hyperthermophilic prokaryotes are strict anaerobes.

Dear Kenneth, Archaea is a correct  term, but Aquificae are Bacteria! not Archaea or Archaeobacteria (the outdated term). 

Elemental S is quite stable at anaerobic conditions at acidic pH and normal or high temperatures, however,  in the presence of sulfides/hydrogen sulfide it is able to form soluble polysulfides. This formation of polysulfides increases with increase of temperature or pH (and pH is much more important). At high (for microbes, let`s say 80-100C) temperatures and pH (e.g. 9) almost all elemental sulfur could be transfered to polysulfides, but at neutral pH and high T or high pH and normal temperatures there still will be high amounts of elemental S in equilibrium with significant amounts of polysulfides.

And the most important thing - dew to its solubility polysulfides are much more tasty for microbes than elemental sulfur.  In the same time concentration of oxygen, as I said, is quite low at high temperatures dew to low solubility making it difficult to utilize. This is a reason why the majority of extremely or hyperthermophilic microbes are sulfur (polysulfide) reducers and not oxygen reducers.

And the last remark: K. Stetter is one of the greatest microbiologists (being actually molecular biologist at the beginning), however, some  of his assumptions (at least this one) made in 80th-90th are not so correct to the current date. Reduced compounds of S, Fe not Oxygen are really important for thermophiles as well as for ancient life forms if we presume that the LUCA was a thermophile, what is quite obvious, I suppose. Moreover, as I know, 3500-4000 Ma ago, when life appeared, the planet was completely anaerobic.

Thanks a lot for these interesting facts and theories.

Dear Kenneth, I`m a bit confused why are you avoided other arguments pointed out exclusively the final phrase, which was not written "with such authority" but using "as I know" extenuation. Possibly this is dew to my english, sorry for that. Concerning this phrase, I can not say that I`m a big specialist in this field, but I thought that it is well-known that the early Earth (I`m speaking about 3000-4000 Ma) was reduced. Eg, I was interested in copper enzymes of one of our microbes, and found that copper is regarded as modern metal among bioelements since it was not bioavailable before 3.000 Ma dew to reduced conditions on Earth. (eg. review Copper-A “Modern” Bioelement. Wolfgang Kaim and Jochen Rall). Other elements (Fe, for example), as I remember, also denoted the anaerobic conditions on early Earth. Russell in his paper "The emergence of life from iron monosulphide bubbles at a submarine hydrothermal redox and pH front" also stated: "The proposition that anaerobic life preceded aerobic life, with hydrogen and sulphide available from the beginning and oxygen as a late-comer, is supported by both the geochemical and the biochemical evidence (Goldschmidt 1952; Wald 1964; Schopf 1983; de Duve 1997). Mulkidjanian, Galperin, Koonin and coauthors in their Zn-world theory argue with Russel on the place of origin of life and biochemistry of LUCA, however they also agree with an ancient anaerobic world: in the paper "On the origin of life in the Zinc world. 2. Validation of the hypothesis on the photosynthesizing zinc sulfide edifices as cradles of life on

Earth" they have stated: "The reduced state of the cytoplasm indicates that the first cells have evolved – and the principal biochemical pathways have been established – before the atmosphere became oxygenated (which was due to the activity of cyanobacteria at 2 – 2.5 Ga [127])". And there are many other papers, supporting that. Possibly there were several local spots of high oxygen (from where?), but LUCA and its first descendants did not used it. Moreover, after cyanobacteria began to produce oxygen and its concentrations became significant, microbes began to develop defense mechanism since oxygen was a kind of a poison for them (and still for anaerobes). Some of these mechanisms (cytochrome oxydases) later evolved and obtained another function - oxygen respiration. Sure, in the field of geology you should know much more than me, however you can see that many biologists support anaerobic origin of life using geological, chemical and  phylogenetic data. Nevertheless, I would be interested in any other theory. Is this "There is a substantial body of evidence that it was not" being a kind of side theories on the subject or it is a newest and currently popular theory? I would gladly read your book or paper, but which one?

To come back to the original question: Life at termperatures up to the critical point of water would be fascinating. But isn't the experimentally proven limit much lower? Have there been any reports on actively growing organisms above 130 C ?

No. 122C - maximum to the current knowledge.

Methanopyrus kandleri is actively (not optimally) growing at 122C at high pressure. The title is: "Cell proliferation at 122°C and isotopically heavy CH4 production by a hyperthermophilic methanogen under high-pressure cultivation". Pyrolobus fumarii is growing optimally at 106C - the highest temperature for optimal growth to the moment. The title is: "Pyrolobus fumarii, gen. and sp. nov., represents a novel group of archaea, extending the upper temperature limit for life to 113°C". you can google the papers...

The boundary for life on Earth is almost nonexistent.  Growth has been observed above normal autoclaving temperatures to -20C or below (if you include active metabolism).  If you had decades to consistently look I'm sure you would see growth/doubling well below -20C too. Life also exists in fresh water up to saturated brine and in the Atacama Desert, which receives little to no rain (sometimes < 1-3 mm a year).  The polar regions can also be considered a desert and organisms grow happily there too.  Organisms can happy survive kiloGrays of ionizing radiation and high levels of UV radiation.  Life can be found over the entire range of pH from less than 1 (even negative pH) up to 12.  To your point about the subsurface, organisms are routinely isolated from the South African Gold Mines at depths of 3-4 km.  They are also isolated from several km above the Earth's surface in the stratosphere (these were isolated about 22-27 km above the surface!).

Simply put, the bin that contains all the hypotheses of scientists who set boundaries for Life is chock full.  Therefore to put limits on life is a bit arrogant.  Just because we (humans) don't understand or can't conceive of a chemistry that would support life in X environment doesn't mean there isn't one and Life hasn't already found a way.

Kenneth,

I would say yes.  The 'limits' to Life are usually defined by our technology.  Growing an organism at 122+ C is not the easiest thing in the world, nor is waiting for a year to see growth on a plate in a cold room that contains a portion of a Greenland ice core sample.  That coupled with the fact that we (humans) rarely think about other possible forms of life (e.g. non-carbon based, not reliant on liquid water) makes detecting them probably impossible (if they even exist).

About the liquid water part of the equation, xerophiles and halophiles grow quite well in an environment with low water activity/availability.  Regions of the Atacama Desert go for up to 4 years (or more) without seeing any water. Microorganisms in the polar regions also do quite well in what is ostensibly a desert (e.g. the Dry Valleys near McMurdo Sound) .  They also grow well in places where there is little liquid water like the ice and snow.  It has actually been shown that in these cases microorganisms generally occupy the briny channels between ice crystals (so water yes, but very saline so low water availability).  On the Mars front, I think there is now an abundance of evidence that at the very least points to the fact that there was water on the surface of Mars at some point in its history.  Water has also been found by the Spirit rover (2007) in the subsurface of Mars.  Plus there is about 210 ppm of water vapor in the Martian atmosphere, so again, more than life requires on Earth.

About the carbon part, like the water part,  I'm not sure what you mean by your statement that Mars has no carbon.  The major component (96%) of Mars' atmosphere is carbon dioxide so more than enough carbon there to support life.

Thanks James, for your enlightning and daring comments to the limits of life.

I totally agree with you, that it is our technological limitations that hinder us from defining any clear limits to life. This is why the absolute limit has to be set at the fourth phase of water (not ice, not liquid, and not vapor), the supercritical phase of water. This is where water turns into a non-polar liquid, which dissolves all organic matter, i.e, there is no place for organisms... But, there may even here be a small niche where it is possible to hide a small organic molecule. If the water turning into the supercritical phase consists of a solute of salt, the salt has to precipitate into minute salt particles, at these high pressures and temperatures (300 bars, and 410 degrees C, for seawater), as shown by Tester et al., 1993. Inside these salt particles, there may be a possibility of hiding components of life, but - I doubt so.  Perhaps there is a way, with molecular simulation or something similar, it will be possible to prove that it is not possible.

- I don't know why this question should reflect any arrogance...?

Thanks, Kenneth,

are there absolutely H2O-free environments on Earth, other than in the dry Laboratory? Even if there was a place temporarily "dry", like in a desert or salt-dried (salt is hygroscopic, and mops up H2O), or even freeze dried, location on Earth, there could be particles containing organic molecules, that would be 'dormant' as long as no water was added, however, that become active immediately water is added?  

It is possible for life to exist in a form other than that obtaining on our planet EARTH!!! Is also plausible for a life to thrive on energy itself without recourse to Water and Oxygen; however SCIENCE till date has not come across such an incidence. An interesting corollary , though, could be found in a book named “ALIEN INTERVIEW” by Lawrence R Spencer that discloses astonishing information in this respect; but it is reader’s choice to believe or disbelieve in the facts stated in this book….

Thanks, Kenneth, for reminding me of this ancient question again. This spring I have made som interesting new observations on cyanobacteria. I have concluded that primitive life on Earth is vulnerable for exposure to extreme cold, e.g., direct exposure to air temperatures of -8 to -12 C, for several days. However, if the organisms are protected by a thin layer of snow, say 2 cm snow cover, they will probably survive.

I base this conclusion on the following observations: Over the two last years, here in SW Norway, we have had rather mild winters, except for this last one (dec. 2017 - March 2018), when we have had periods of several days with temperatures below -10 C (e.g., down to - 12 C), without snow on the ground, e.g., hoar frost.

In our garden we have three terrasses with impregnated wooden surface, they are up to 20 sq m. After the two mild and wet winters, they had all attracted cyanobacteria and green algal growth, perhaps a mm or so thick. My wife decided that we need to clean them this year, a rather hard and tiring job (normally done with a high-pressure hoze and water).

However, during late March, I discovered that the organisms were actually dead, and had loosened from the wooden substratum. By chance, I tried washing one of the wooden surfaces with a wet rag (with warm water and a little soap). The rag became black/purple after just one washing. Normally this does not happen, even after hard rubbing. This discovery resulted in us washing the terrases with scrubbers and warm water with a little chlorine. They became really clean as never before!

My conclusion is that the cyanobacteria (black and purple growth) and the green algae were 'dead' - at least, they had lost their grip (temporarily?) on the wood after the hard winter this year.

The limit for life is therefore set at perhaps minus 12 C (prolonged exposure, several days), including lack of water. When it comes to the upper limit, it has to be the phase transition curve (boiling curve / supercritical ), which must be the upper limit, also this perhaps for several hours or perhaps days.

Yes, I agree, Kenneth,

you need protection against certain radiation, including UV. This is managed by the atmosphere, and perhaps especially by O3 (Ozone).