In the second part of the article there is an explanation which for me is the most plausible, and which would not be applicable to Martian soil.
Even if they killed all living beings in the soil, after their death the enzymes that are the catalysts for metabolism would just become dispersed in the soil and they continue to catalyze reactions like those of the Krebs cycle.
After many years of storage the molecules of the enzymes will be degraded, i.e. they will break into fragments. That again does not mean much, because the catalytic action of the enzymes is typically caused by very small parts of the enzymes, which can remain intact even after fragmentation.
In general, the biggest part of an enzyme is just a scaffold that attaches the enzyme in precise places of a cell, usually on some intracellular membranes, so that a great number of enzymes can be assembled like a production line in a factory, to coordinate the metabolic reactions for maximum efficiency.
After death and enzyme fragmentation, even after many years, the catalytic fragments of the enzymes can still catalyze reactions like those of the Krebs cycle.
It is also possible that some of the observed chemical reactions are catalyzed by minerals present in the soil and not by remnants of the enzymes from the dead cells, but for now no evidence has been gathered about this.
Moreover, there are enzyme residues which are difficult to distinguish from abiotic minerals. Some of the enzymes involved here contain a catalytic part formed by a cluster of iron and sulfur atoms, which are attached to a protein molecule. That iron-sulfur cluster is pretty much identical with a very small fragment of an iron sulfide mineral.
We've found amino acids almost everywhere we look, including astroids [1].
It seems that the building blocks of life pretty naturally and readily form. Which is a pretty strong indicator that life is likely fairly common outside earth.
The amino acids that can be found everywhere include ten of the simpler amino-acids that are used in proteins (glycine, the 2 acids, the 3 branched, alanine, proline and the 2 alcohols).
The other 11 amino-acids from proteins have never been found where life does not exist. They are more complex and they seem to have been developed by living beings long after the appearance of life and the appearance of the genetic code (they seem to have substituted later the simpler amino-acids in certain locations of the map of the original genetic code, which encoded fewer amino-acids).
Moreover, while the simple amino-acids, including the ten that are used in proteins, can be found pretty much everywhere, wherever they were not produced by living beings they have been found in racemic mixtures, i.e. in equal amounts of left-handed and right-handed isomers, while in proteins only the left-handed isomers are used, so the living beings normally produce almost only left-handed isomers. Very small quantities of right-handed isomers are produced by some living beings, for other purposes than making proteins.
So it is relatively easy to distinguish amino-acids that have been produced by living beings from amino-acids that have been produced in abiotic conditions (i.e. the amino-acids produced in abiotic conditions are recognized by the absence of complex amino-acids and by the presence of great quantities of right-handed isomers).
I don't see the left handed aspect as necessary for life. To me this just suggests our common ancestor for life on earth made use of this chirality. Another group of organisms somewhere else could have evolved from an ancestor that makes use of right handed chirality. Or one that is hand-blind.
It seems that the choice between left-handed and right-handed amino-acids was random.
However, it is unlikely that other kinds of life forms could use both kinds indiscriminately, because mixing them creates difficulties in the assembling of polymers. So it is likely that amino-acids produced by some extra-terrestrial life form would also be predominantly of only one orientation, but it could happen to be the right-handed variant.
Moreover, extra-terrestrial life forms could use very different complex amino-acids, because there are much more of those than the 11 that have been added to the simple amino-acids in the terrestrial proteins.
That makes sense that the chirality can affect downstream polymer assembly or even folding in the higher order structures.
Likely we are all left handed on earth because our left handed ancestor outcompeted the right handed organisms in the primordial soup. Or the right handed organisms just didn't evolve in the first place here on earth and there was nothing to outcompete. There might still be some higher order advantages to shifting chirality one way or another. Certain molecules, such as methamphetamine, have differing bioactivity based on chirality. Maybe this can be regulated in some way such as to control the rate of some other downstream process. In an abstracted sense, chemists here on earth are already this organism as they refine reactions to produce desired chirality and reduce expenditure on undesired chirality.
ET could be using different amino acids, or more or fewer. I would hazard to guess there is immense selection to reduce the amino acid set to its most necessary components. This pressure has gotten to the point here on earth where even these necessary components might not all be produced endogenously by the organism who needs them, but consumed from the environment saving energy spent on synthesis. But this requires your neighbor to be producing these AAs, such that you consume them, and you having sufficient feedback mechanisms to not immediately consume all of your neighbor's species and put your own insufficient lineage to extinction.
Life can even use something other than amino acids. They are really inconvenient when you think about it. Fixed nitrogen is extremely rare, and there are no nitrogen-containing minerals other than some exotic exceptions.
Amino acids are useful because they can be easily joined together and split apart (via the C-N bond). But there are other types of "molecular glues" that are viable, like sulfur or phosphorus.
The living beings use much more amino acids than those that compose proteins.
The relatively low number of amino acids that are used in proteins appears to be caused by the difficulty of modifying the genetic code by adding not yet encoded amino acids to the set of encoded amino acids.
Variations of the genetic code are known at various living beings, but nonetheless they are very rare, because a change in the genetic code requires a lot of other coordinated changes. A new kind of transfer RNA must be encoded in the genome (the only likely origin of such a new tRNA is a mutation in one of the existing) and that RNA molecule must be able to bind preferentially to the codons that are repurposed to encode a new amino acid, and also to molecules of that amino acid, which requires a lot of favorable change is the molecular structure of that RNA.
It seems that in the earliest form of genetic code, there were only 4 distinct symbols, i.e. of the 3 nucleobases of a codon only the central one was meaningful and the 2 peripheral nucleobases did not encode information.
The 4 original symbols selected between 4 major kinds of amino acids: the special amino acid glycine, an acid amino acid, a hydrophobic amino acid and an amino acid with intermediate behavior, like alanine or proline.
These variations would have been enough to build proteins with specific conformations.
The fact that a codon had 3 nucleobases, presumably to ensure the binding to transfer RNA molecules, even if only one of them encoded information, appears to have been a great luck, because this allowed later the expansion of the genetic code, because 3 bases give 64 combinations allowing the encoding of up to 64 symbols.
Most of the possible codons have remained ambiguous until today, but the number of encoded amino acids has increased slowly in time, up to 21, the most recent additions to the encoded set being those of the sulfur-containing amino acids, aromatic amino acids and selenium-containing amino acids.
As you say, there are disadvantages in using many kinds of amino acids, but there are also advantages, by allowing the creation of proteins with properties that are not achievable with a smaller set of amino acids.
The balance between advantages and disadvantages appears to have slowed down continuously the rate of adding new amino acids to the set encoded in the genetic code, so that the majority of the living beings of today have not added any new amino acid since several billion years ago.
Most of the expansions of the genetic code happened before the last common ancestor of all living beings of today, so that today there are very few living beings with more recent modifications in the genetic code.
Wouldn't that require a constant filter, to ensure that L-processes never use R-amino compounds? Seems almost like a Maxwell's Demon would be required in every cell...
Tangentially related, this is a bit trying to sell upcoming book, but the discussion of origins of life was interesting to me. https://www.newscientist.com/article/2526959-how-a-radical-n...
YMMV but It’s free via my local library and Libby if you are stopped by the subscription nag.
Even if they killed all living beings in the soil, after their death the enzymes that are the catalysts for metabolism would just become dispersed in the soil and they continue to catalyze reactions like those of the Krebs cycle.
After many years of storage the molecules of the enzymes will be degraded, i.e. they will break into fragments. That again does not mean much, because the catalytic action of the enzymes is typically caused by very small parts of the enzymes, which can remain intact even after fragmentation.
In general, the biggest part of an enzyme is just a scaffold that attaches the enzyme in precise places of a cell, usually on some intracellular membranes, so that a great number of enzymes can be assembled like a production line in a factory, to coordinate the metabolic reactions for maximum efficiency.
After death and enzyme fragmentation, even after many years, the catalytic fragments of the enzymes can still catalyze reactions like those of the Krebs cycle.
It is also possible that some of the observed chemical reactions are catalyzed by minerals present in the soil and not by remnants of the enzymes from the dead cells, but for now no evidence has been gathered about this.
Moreover, there are enzyme residues which are difficult to distinguish from abiotic minerals. Some of the enzymes involved here contain a catalytic part formed by a cluster of iron and sulfur atoms, which are attached to a protein molecule. That iron-sulfur cluster is pretty much identical with a very small fragment of an iron sulfide mineral.