
How Ancient Microbes Gave Us Iron - Amorymeltzer
http://www.newyorker.com/tech/elements/how-ancient-microbes-gave-us-iron
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padobson
I really enjoyed the abstractions in this piece, particularly this:

 _Iron from the mantle, released at black smokers, has a predictable ratio of
iron-56 (full fat) to iron-54 (lite), but in the Hamersley rocks the ratio is
skewed; the iron is, on average, lower-fat than expected._

It made for enjoyable reading what was essentially a piece describing why most
of the mined iron deposits we use to make steel have a certain isotope -
single celled organisms de-oxygenating rust molecules and expelling iron-54 as
a waste product.

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protonfish
Jumping to the conclusion that there were ancient terrestrial microorganisms
that metabolized iron as an energy source to explain odd isotopic ratios seems
like a stretch to me. In the same way that it is unnecessary to pad out a
science article with questionable metaphors.

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nitrogen
What is your alternative explanation for the claimed isotope imbalance?

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protonfish
They mention in the article non-biological process that can affect isotope
ratios. This seems much more plausible. Suggesting that there were large
enough amounts of terrestrial chemotrophs to change isotopic ratios is not
supported by anything else we know about this time period, or life on Earth in
general. The burden of proof is still in their camp to demonstrate:

1\. Any evidence to suggest biological origin 2\. Reasons why biological
process are more likely to account for the observed iostope ratio that non-
biological.

Until then, this is only an interesting, though unlikely, explanation.

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dalke
Could you explain what you mean by 'or life on Earth in general'? Kinetic
fractionation is a well-known process. Quoting
[https://en.wikipedia.org/wiki/Kinetic_fractionation](https://en.wikipedia.org/wiki/Kinetic_fractionation)
:

> Biological processes are generally unidirectional and are very good examples
> of "kinetic" isotope reactions. All organisms preferentially use lighter
> isotopic species, because "energy costs" are lower, resulting in a
> significant fractionation between the substrate (heavier) and the
> biologically mediated product (lighter). As an example, photosynthesis
> preferentially takes up the light isotope of carbon 12C during assimilation
> of an atmospheric CO2 molecule. This kinetic isotope fractionation explains
> why plant material (and thus fossil fuels, which are derived from plants) is
> typically depleted in 13C by 25 per mil (2.5 per cent) relative to most
> inorganic carbon on Earth.

That sounds like life on Earth in general can form large geological
concentrations of isotopically shifted material, and it's well accepted.

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protonfish
Also, you can't make the same assumptions with chemotrophs as you can with
phototrophs. Photosynthetic organisms use sunlight to power their chemical
reactions so they can thrive anywhere there is sunshine, water and CO2.
Chemotrophs can only thrive where there is a local concentration of their
"food" (iron, in this case) compared to the average concentration.

Let me explain it this way. Say you have an environment rich in elemental
oxygen and iron. What's to stop the oxidation from occurring naturally before
an organism can facilitate it and harness the energy? Nothing. This is why you
only see chemotrophs around local high concentrations (like deep sea vents.)
This means it is inherently impossible for chemotrophs to ever become
widespread.

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dalke
Do you have any evidence that chemotrophs do not have an isotopic preference?
That would contradict the "all life" from the Wikipedia quote, and contradict
what little I know about the universality of the mechanism. A quick search
finds:

Detmers, J., Brüchert, V. Habicht, K.S. and Kuever, J. (2001) Diversity of
Sulfur Isotope Fractionations by Sulfate-Reducing Prokaryotes, Applied and
Environmental Microbiology, 67, 888-894.
[http://www.ncbi.nlm.nih.gov/pmc/articles/PMC92663/](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC92663/)

> All of the 32 sulfate-reducing bacteria discriminated against 34S during
> sulfate reduction. Desulfonema magnum showed the largest fractionation (ɛ =
> 42.0‰), and Desulfovibrio halophilus showed the smallest (ɛ = 2.0‰)

It looks like the scientists working in this field have not made the
assumption but have actually tested it.

> This is why you only see chemotrophs around local high concentrations

I lack the biochemistry knowledge. My limited understanding is that life was
restricted to the ocean. If the Fe and O react on land (with limited oxygen,
that produces iron(II), yes?), then it will be dissolved in fresh water. But
when it hits the ocean, it's going to interact with the salts and other
substances which a billion years of rain bought into the sea.

In that case, the ocean edge will have the local high chemical concentrations.
Sure, over time that will equilibrate. But any life at that boundary will be
able to take advantage of the chemical gradient in the way you mentioned. We
see this now, where continental runoff is a source of nutrients for ocean
life, similar to how volcanic eruptions can add nutrients to soil through the
weathering process of volcanic rocks.

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Turing_Machine
Similar processes are going on even now. "Bog iron" was very widely used in
the days before long-range transportation.

[https://en.wikipedia.org/wiki/Bog_iron](https://en.wikipedia.org/wiki/Bog_iron)

