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Selfsim
2013-Aug-22, 07:38 AM
New study published in PLOS ONE: A Low Temperature Limit for Life on Earth (http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0066207)


There is no generally accepted value for the lower temperature limit for life on Earth. We present empirical evidence that free-living microbial cells cooling in the presence of external ice will undergo freeze-induced desiccation and a glass transition (vitrification) at a temperature between −10C and −26C. In contrast to intracellular freezing, vitrification does not result in death and cells may survive very low temperatures once vitrified.

The high internal viscosity following vitrification means that diffusion of oxygen and metabolites is slowed to such an extent that cellular metabolism ceases. The temperature range for intracellular vitrification makes this a process of fundamental ecological significance for free-living microbes. It is only where extracellular ice is not present that cells can continue to metabolise below these temperatures, and water droplets in clouds provide an important example of such a habitat.

In multicellular organisms the cells are isolated from ice in the environment, and the major factor dictating how they respond to low temperature is the physical state of the extracellular fluid. Where this fluid freezes, then the cells will dehydrate and vitrify in a manner analogous to free-living microbes.

Where the extracellular fluid undercools then cells can continue to metabolise, albeit slowly, to temperatures below the vitrification temperature of free-living microbes. Evidence suggests that these cells do also eventually vitrify, but at lower temperatures that may be below −50C.

Since cells must return to a fluid state to resume metabolism and complete their life cycle, and ice is almost universally present in environments at sub-zero temperatures, we propose that the vitrification temperature represents a general lower thermal limit to life on Earth, though its precise value differs between unicellular (typically above −20C) and multicellular organisms (typically below −20C). Few multicellular organisms can, however, complete their life cycle at temperatures below ~−2C.

Journo article here... (http://phys.org/news/2013-08-lowest-temperature-life.html)

'When you have a single-celled organism and cool it until ice forms in the external medium, in every case we looked at the cells dehydrated and then vitrified between -10C and -25 C. There were no exceptions,' explains Clarke.

Selfsim
2013-Aug-22, 08:18 AM
Probably of greater significance, is their conclusion:

However the ability of multicellular organisms to regulate their internal body fluids means that some terrestrial species living in permanently or seasonally cold environments can allow these fluids to undercool, and hence their cells to metabolise, down to temperatures well below the lower thermal limit for growth in free-living microbes. Endotherms (mammals and birds) can regulate the temperature of their interior such that at least one species, the Emperor Penguin, Aptenodytes forsteri, can complete its live cycle at temperatures below −40C in the depths of the Antarctic winter.

Undercooling is not an option for unicellular organisms, where ice is almost invariably present in the environment once the temperature drops below 0C. Nevertheless there are reports in the literature of apparent microbial metabolism at temperatures well below −20C (discussed above). We therefore suggest that any proposal for metabolising cells under these conditions must meet a minimal set of criteria to distinguish true metabolism from geochemical processes that mimic metabolism. A possible set of criteria might include some or all of:


- Utilisation of an electron donor;
- Reduction of an electron acceptor;
- Maintenance of membrane potential and protonmotive force;
- Synthesis of ATP;
- Transport of metabolites across the cell membrane;
- Membrane viability;
- Ribosomal viability.

This is a tough set of criteria, but the theoretical arguments elaborated above and our experimental data indicate clearly that the burden of proof lies with the demonstration of microbial metabolism below −20C. Unless these criteria can be met, we must assume that what is being measured is concentrated solution chemistry and not integrated metabolism.

Our results also imply that for any cells with an intracellular milieu similar to that of cells on Earth the window of opportunity for metabolism to proceed in the present climate on Mars is very limited indeed. It would, however, seem perfectly possible, given sufficient free energy, for cells to exist and metabolise in the water hypothesised to exist below the surface ice layer on Europa. Furthermore, our results suggest that microbial cells could feasibly be transferred across interplanetary space were they to be transported in water with sufficient solutes to induce dehydration of the cells through freeze concentration, and thereby promote vitrification. Once vitrified, cells could withstand the very low temperatures (<10 K) encountered in interplanetary space.I hope this conclusion is also noticed by the designers of remote probe life detection instruments, (if and when such attempts are made again).

Colin Robinson
2013-Aug-22, 10:45 PM
Probably of greater significance, is their conclusion:
I hope this conclusion is also noticed by the designers of remote probe life detection instruments, (if and when such attempts are made again).

The set of criteria proposed by Andrew Clarke include synthesis of the coenzyme ATP (Adenosine triphosphate). What would you call a structure that behaves like a cell in the way it processes energy and materials, but which uses a molecule other than ATP as an energy carrier?

Why should designers of remote probe life detection instruments proceed from a set of criteria so restrictive that anything with a different energy-carrying molecule wouldn't qualify?

Hlafordlaes
2013-Aug-22, 11:02 PM
I'd venture to guess it's because ATP is what we know to look for. Unless there is a strongly supported alternate (i.e., passes at the very minimum advanced computer models) we could hunt for, seeking any and every sign of energy would be a wild goose chase among myriad non-organic, natural chemical processes, wouldn't it?

Selfsim
2013-Aug-22, 11:22 PM
The set of criteria proposed by Andrew Clarke include synthesis of the coenzyme ATP (Adenosine triphosphate). What would you call a structure that behaves like a cell in the way it processes energy and materials, but which uses a molecule other than ATP as an energy carrier? Well, firstly Clarke would say that the burden of proof for the idea that the structure behaves like a cell, would be on the discoverers.

Secondly, if you're asking me, (in particular), then given your very specific set of criteria, I might be inclined to call such a structure 'alien' to our form of life. (With an abundance of good accompanying reasons for doing so).

Thirdly, well, Hlafordlaes covered my third point .. :)


Why should designers of remote probe life detection instruments proceed from a set of criteria so restrictive that anything with a different energy-carrying molecule wouldn't qualify?"Extraordinary claims call for extraordinary evidence", no? :)

Cheers

Colin Robinson
2013-Aug-22, 11:44 PM
I'd venture to guess it's because ATP is what we know to look for.

So we'll look for ATP on Mars or Europa, and if we don't find it, then no ATP means no metabolism is occurring (according to Andrew Clarke's criteria for metabolism), so we will conclude that there is no life there?


Unless there is a strongly supported alternate (i.e., passes at the very minimum advanced computer models) we could hunt for, seeking any and every sign of energy would be a wild goose chase among myriad non-organic, natural chemical processes, wouldn't it?

"Wild goose chase" is an interesting metaphor... Wild geese can be hunted and caught, but it helps if you know the terrain you're hunting in. A problem with the Viking mission was that so little was known then about the Martian terrain.

Colin Robinson
2013-Aug-22, 11:56 PM
"Extraordinary claims call for extraordinary evidence", no? :)

The question here is whether adenosine triphosphate is so extraordinary that no other molecule could perform a comparable function?

Selfsim
2013-Aug-23, 12:16 AM
Why should designers of remote probe life detection instruments proceed from a set of criteria so restrictive that anything with a different energy-carrying molecule wouldn't qualify?"Extraordinary claims call for extraordinary evidence", no? :)-defaultThe question here is whether adenosine triphosphate is so extraordinary that no other molecule could perform a comparable function?(Re - underlined text): Well, we already know that's not the case. TTP, CTP, UTP, etc, all perform similar functions, but in more specific processes in biology. So, I personally, wouldn't be particularly concerned about such a question. ATP is the most basic unit of energy storage in all species thus far studied. If its not present, then we can say "something's afoot".

The name-of-the-game in remote testing, is to return results which are capable of eliminating as many 'other possibilities', as is possible. A 'tough set of criteria' is exactly what facilitates this.

Colin Robinson
2013-Aug-23, 01:30 AM
(Re - underlined text):Well, we already know that's not the case. TTP, CTP, UTP, etc, all perform similar functions, but in more specific processes in biology. So, I personally, wouldn't be particularly concerned about such a question. ATP is the most basic unit of energy storage in all species thus far studied.

Yes. But what we don't know, is whether they all use ATP because they have a common ancestor that used ATP? Or whether it is because ATP is uniquely suited to that role?


If its not present, then we can say "something's afoot".

I'm not sure what you mean by "something's afoot". Would you like to explain?


The name-of-the-game in remote testing, is to return results which are capable of eliminating as many 'other possibilities', as is possible. A 'tough set of criteria' is exactly what facilitates this.

What if we design a probe to test whether Europa (for instance) has a biochemistry involving ATP?

That would be a gamble, in the sense that a yes answer would give us specific information of great scientific value, but a no answer would tell us very little.

We still wouldn't know specifically what Europa does have — what sort of chemistries, what sort of complex systems, do any of them fit a functional definition of life? (By a functional definition, I mean a definition based on what living things do, rather than on their composition.)

Selfsim
2013-Aug-23, 06:41 AM
... I'm not sure what you mean by "something's afoot". Would you like to explain?If ATP synthesis and these other criteria were not present in a sample at low temperatures, and yet 'metabolism' is somehow claimed, (by some other means), then someone's going to have to do some pretty fancy footwork to rule out the possibility that some other equally interesting geochemical process is mimicking 'metabolism' in the results.


What if we design a probe to test whether Europa (for instance) has a biochemistry involving ATP?

That would be a gamble, in the sense that a yes answer would give us specific information of great scientific value, but a no answer would tell us very little.

We still wouldn't know specifically what Europa does have what sort of chemistries, what sort of complex systems, do any of them fit a functional definition of life? (By a functional definition, I mean a definition based on what living things do, rather than on their composition.)I think you have to look at the whole set of criteria Clarke is suggesting, which would provide the basis for distinguishing bio-metabolism from geochemistry.

Colin Robinson
2013-Aug-23, 07:34 AM
If ATP synthesis and these other criteria were not present in a sample at low temperatures, and yet 'metabolism' is somehow claimed, (by some other means), then someone's going to have to do some pretty fancy footwork to rule out the possibility that some other equally interesting geochemical process is mimicking 'metabolism' in the results.

As you said in an earlier message: Extraordinarily claims require extraordinary evidence.

I agree. I'm just not sure which of the following propositions is more extraordinary:

* The proposition that life elsewhere is chemically the same as Earth life?
* Or the proposition that life elsewhere is chemically somewhat different?

Paul Wally
2013-Aug-23, 07:54 AM
"Extraordinary claims call for extraordinary evidence", no? :)



There we go again ... :rolleyes:

galacsi
2013-Aug-23, 08:21 AM
So we'll look for ATP on Mars or Europa, and if we don't find it, then no ATP means no metabolism is occurring (according to Andrew Clarke's criteria for metabolism), so we will conclude that there is no life there?



"Wild goose chase" is an interesting metaphor... Wild geese can be hunted and caught, but it helps if you know the terrain you're hunting in. A problem with the Viking mission was that so little was known then about the Martian terrain.

It reminds me of that story about crazy looking for his keys , at night , under a street lamp. . .
But what are you doing , say to him to the policeman ? I'm watching you and you were turning in circles for an hour!
I am looking for my keys!
You're sure you've lost them in this place?
No, not a t all , but here there is some light. . .

Hlafordlaes
2013-Aug-23, 01:01 PM
.... I'm just not sure which of the following propositions is more extraordinary:

* The proposition that life elsewhere is chemically the same as Earth life?
* Or the proposition that life elsewhere is chemically somewhat different?

I don't think those are the right options. It's more like:
* Search now locally (our system) for life that we are currently capable of identifying and detecting.
* Search for alternate forms of life if and when we are able to properly define that search.

If not for cost, perhaps the best option would be a permanent base around Jupiter, sending prolonged expeditions down to Europa and examining everything we find, be they identified as organic or non-organic processes or artifacts, and see what comes up.

At any rate, my take is that our entire Solar System participated in generating the conditions and chemical precursors for life, and any local life in the system is very likely to be similar to that on Earth. Once the rocket scientists here get off their butts ;) and gimme my warp drive, then we could do a long mission to another system and examine it with an mind toward finding new life processes.

Colin Robinson
2013-Aug-23, 10:50 PM
If not for cost, perhaps the best option would be a permanent base around Jupiter, sending prolonged expeditions down to Europa and examining everything we find, be they identified as organic or non-organic processes or artifacts, and see what comes up.

I agree that sort of option would be better than just testing for particular molecules supposedly characteristic of life (e.g. ATP, nucleic acids, proteins).

When looking for microbial life operating at different temperatures here on Earth, it may indeed be a reasonable working assumption that any which do exist would be made of proteins, keep their genetic information in nucleic acids, and use ATP as an energy carrier.

But is such a working assumption still reasonable when looking for life on other worlds — life which might be genetically unrelated to any form of Earth life?


At any rate, my take is that our entire Solar System participated in generating the conditions and chemical precursors for life, and any local life in the system is very likely to be similar to that on Earth.

How similar is similar?

I would agree that life elsewhere in the Solar System is very likely to be based on organic compounds, i.e. molecules consisting of chains of carbon atoms with other atoms hanging off them. After all, carbon is a common element throughout the Solar System, it is chemically very versatile, and we already know of several places in the Solar System where it has formed into long chain compounds.

For these reasons, it is important to learn more about which places have organic compounds, what organics they have, and how the organics are behaving.

However, the chemical versatility of carbon also means that the number of possible compounds become greater and greater the longer the chains we are considering. That is why I think it is not reasonable (even as a working assumption) to suppose that life unrelated to Earth life would use the same compounds (e.g. ATP).

Selfsim
2013-Aug-23, 11:33 PM
I agree that sort of option would be better than just testing for particular molecules supposedly characteristic of life (e.g. ATP, nucleic acids, proteins).

When looking for microbial life operating at different temperatures here on Earth, it may indeed be a reasonable working assumption that any which do exist would be made of proteins, keep their genetic information in nucleic acids, and use ATP as an energy carrier.

But is such a working assumption still reasonable when looking for life on other worlds life which might be genetically unrelated to any form of Earth life?
I think its more than just an assumption, Colin. Ie: there are no empirically evidenced exceptions and there's a myriad of evidence that ATP synthesis is the primary molecule used for energy derivation in metabolism. If our model of life references our own, for developing tests for carbon-based life, (and it necessarily does), then it gives us a firm footing for explaining whatever else we might find, one way or the other. This is how testing uncertainty is eliminated. If we couldn't eliminate uncertainties, then testing becomes unfeasible. (Admittedly, this is taking it to the extreme position, but the general point is valid).

This doesn't necessarily mean that an ATP test would exclude the capability of detecting other similar molecules which we know can also be used in similar ways to ATP. ATP is detected using photometry (I think), which is a very simple test .. one I think which would be a 'push-over' for say Curiosity/SAM(?) There is only one carbon molecule difference between ATP and say, UTP (I think the QMS/GCMS system might be able to resolve these molecules, although its worthwhile checking this, to confirm .. I'm in a bit of a rush at the moment ..)


How similar is similar? Don't know till we actually find something ..


I would agree that life elsewhere in the Solar System is very likely to be based on organic compounds, i.e. molecules consisting of chains of carbon atoms with other atoms hanging off them. After all, carbon is a common element throughout the Solar System, it is chemically very versatile, and we already know of several places in the Solar System where it has formed into long chain compounds.

For these reasons, it is important to learn more about which places have organic compounds, what organics they have, and how the organics are behaving.

However, the chemical versatility of carbon also means that the number of possible compounds become greater and greater the longer the chains we are considering. That is why I think it is not reasonable (even as a working assumption) to suppose that life unrelated to Earth life would use the same compounds (e.g. ATP)... See above comment ..

Colin Robinson
2013-Aug-24, 12:55 AM
I think its more than just an assumption, Colin. Ie: there are no empirically evidenced exceptions and there's a myriad of evidence that ATP synthesis is the primary molecule used for energy derivation in metabolism.

Yes, but all that evidence comes from living things which are genetically related to one another.

Living things on Mars/Europa might be expected to metabolize the same way IF

* they are in fact related genetically to Earth life (e.g. via meteor transfer), or
* if abiogenesis is strongly deterministic process: i.e. if it must result in biochemistry using ATP if it results in biochemistry at all.

But we don't know whether either of these propositions is true.


If our model of life references our own, for developing tests for carbon-based life, (and it necessarily does),

Is testing for carbon-based life necessarily based on the idea that those catalytic molecules used by all (related) Earth life will also be used by all (possibly unrelated) extraterrestrial life? I don't think so.

Testing can be based on reasonable conjectures (based on known principles of chemistry, thermodynamics, and systems theory) about which features common to all Earth life are more likely to be common to all life anywhere, and which features common to all Earth life are simply there because the life-forms are genetically related.

After all, why do we think that life-forms on Earth are in fact related? Isn't it because, in terms of certain basics, they are much more similar than unrelated life-forms would reasonably be expected to be?


This doesn't necessarily mean that an ATP test would exclude the capability of detecting other similar molecules which we know can also be used in similar ways to ATP.

I'd suggest that the thing to do is to find whether various planets/moons in fact have organic molecules, and then get a detailed inventory of what organic molecules they have. If there happens to be ATP there, I'd agree that would be a very interesting find.

Selfsim
2013-Aug-24, 02:53 AM
Colin;
If we both came to the view (in post #11 (http://cosmoquest.org/forum/showthread.php?145833-Lowest-temperature-for-metabolism&p=2152591#post2152591)) that:

If ATP synthesis and these other criteria were not present in a sample at low temperatures, and yet 'metabolism' is somehow claimed, (by some other means), then someone's going to have to do some pretty fancy footwork to rule out the possibility that some other equally interesting geochemical process is mimicking 'metabolism' in the results. .. which was the original point in the study anyway. The ATP matter, in the sense you are raising it, really becomes a matter of opinions about "if/then/elses ", no?

I checked, (http://cosmoquest.org/forum/showthread.php?143405-Mars-Soil-Resembles-Veggie-Garden-Dirt-Lander-Finds&p=2129149#post2129149) and Curiosity's SAM can inventory organics of up to 20 carbon atoms. Sounds like it might be up to the task of distinguishing ATP from UTP, TTP etc, anyway.

Colin Robinson
2013-Aug-24, 03:52 AM
Colin;
If we both came to the view (in post #11 (http://cosmoquest.org/forum/showthread.php?145833-Lowest-temperature-for-metabolism&p=2152591#post2152591))

??? This is what I said in post 11


As you said in an earlier message: Extraordinarily claims require extraordinary evidence.

I agree. I'm just not sure which of the following propositions is more extraordinary:

* The proposition that life elsewhere is chemically the same as Earth life?
* Or the proposition that life elsewhere is chemically somewhat different?

I don't quite see why you think we both came to a single view there?


The ATP matter, in the sense you are raising it, really becomes a matter of opinions about "if/then/elses …", no?

In post 2, you quoted Andrew Clarke's criteria for distinguishing between metabolism and geochemistry, one of which is synthesis of ATP, and you went on to say



I hope this conclusion is also noticed by the designers of remote probe life detection instruments, (if and when such attempts are made again).

I've tried to explain why I think it would be a mistake for a life detection space mission to be based on the assumption that the chemistry of life there will be the same as the chemistry of life here, in terms of particular co-enzyme such as ATP.

Selfsim
2013-Aug-24, 07:46 AM
Man this conversation is so messed up .. (I think), by so many undeclared assumptions ... and folk jumping to premature conclusions(?)

I'll try and explain where I'm coming from (at least) ..

Ok, so a probe goes to some cold planet (<<-20C). It executes tests on a sample of interest using Clarke's criteria*.

Next, there are many permutations possible .. lets say one is:
No ATP detected: .. that would rule out the presence of Earth-like life. (This is what I meant by: "Extraordinary claims call for extraordinary evidence". Ie: the strict criteria of 'ATP' has provided the 'extraordinary evidence', by ruling out Earth-like life, upfront) ...

OK so then, two possibilities might then be:
i) no metabolism => conclusion = geochemistry only or;
ii) metabolism => conclusion = 'something's afoot' (meaning something other than ATP might be being used to generate an apparent metabolism, or geochemistry is causing it);
iii) So then, test using a SAM-like QMS/GCMS test, looking for long chain carbon-based molecules => conclusion, (say), might be molecule length of 9, so => conclusion = maybe 'its' using UTP instead of ATP;
iv) If (iii) above isn't conclusive, "fancy footwork" etc, is then required to rule out that geochemistry might still be causing the apparent metabolism.

Hope that makes sense?

Footnote:
*Note the quote in Post #2 says:
A possible set of criteria might include some or all of: ...Clarke is not fixated solely ATP detection .. this inference was injected into the conversation by Colin.