Theoretical origin of life; aged subterranean extremophiles (Introduction)

by David Turell , Saturday, May 15, 2021, 19:25 (1069 days ago) @ David Turell

A new study:

https://aeon.co/essays/deep-beneath-the-earths-surface-life-is-weird-and-wonderful?utm_...

The deep Earth supports an entire biosphere, largely cut off from the surface world, and is still only beginning to be explored and understood.

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"We know now that the deep terrestrial subsurface is home to one quintillion simple (prokaryotic) cells. That is two to 20 times as many cells as live in all the open ocean. By some estimates, the deep biosphere could contain up to one third of Earth’s entire biomass.

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"Cold is not a problem down there, however. Quite the opposite: rainwater that percolates kilometres deep into the crust along fractures and faults between rocks can reach temperatures of 60°C (140°F) or higher. The further down you go from the surface, the closer you are to the mantle. Heat rising from the inner Earth is what warms the fissure water. Additionally, the water is under high pressure, contains very little or no oxygen, and is bombarded by radiation from natural radioactive elements in the rocks.

"Within this hellish environment, though, are crucial ingredients for nurturing life. Underground water reacts with minerals in the continental crust, and the longer the water has been trapped down there, the more time there has been for the results of those reactions to accumulate along the flow path. The slow reactions between water and rock dissolve minerals into the water, and break up some of the water molecules, producing molecular hydrogen. This hydrogen is an important fuel for microorganisms in the deep subsurface.

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"Old-water ecosystems are dominated by hydrogen-utilising microorganisms such as sulphate-reducing bacteria and methane-producing archaea. Those methane-producing archaea, or methanogens, are microbes that visually resemble bacteria but are so structurally and genetically distinct that they belong to a completely separate domain of life. Sulfate-reducing bacteria and methanogens are among the life forms that appeared earlier in the evolutionary history. In contrast, young-water ecosystems are dominated by metabolically diverse and versatile bacteria of the phylum proteobacteria.

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"Subsurface microorganisms are estimated to be extraordinarily long-lived. In our studies, they show a turnover time as slow as 1,000 years, meaning that they divide only once every few thousand years. To put it in perspective, the common gut bacterium E.coli divides once every 20 minutes. One of the long-standing questions is, how do the deep microbes achieve such a slow-motion lifestyle?

"It is not easy to make a living in the subsurface because the biochemical reactions to harness energy from minerals and geological gases – a set of processes known as chemotrophy – are not as efficient as photosynthesis, the process that green plants use to capture energy from photons of sunlight on the surface. Some subsurface microorganisms can form stress-resistant spores and remain inactive in order to withstand extreme subsurface conditions; otherwise, microorganisms have to invest at least a certain amount of energy, which varies from one taxa (evolutionary population) to another, to maintain the integrity and functionality of the cells.

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"The molecular data, together with isotope geochemistry and thermodynamic modelling, presented a unified story that the most successful group down there is the betaproteobacteria, a class of proteobacteria that obtain energy through a coupling of nitrate reduction and sulphur oxidation in order to fix carbon dioxide for cellular growth. The demand for nitrate among deep microbes was unexpected; it had gone unnoticed prior to our study because the measured nitrate concentrations in the subsurface water samples were tiny. More interesting, we deduce that deep microbial groups have established strong, paired metabolic partnerships, or syntrophic relationships, which helps the organisms overcome the challenges of extracting the limited energy that originated from rocks.

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"humans need 21 per cent oxygen in our atmosphere to be able to breathe. Nematodes can make do indefinitely with only 0.5 per cent oxygen, and many species can survive extended periods with less or no oxygen at all. (my bold)

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"Finally, we recognise that we have probably explored only a tiny fraction of the deep biosphere, and might not yet have encountered its most significant inhabitants. It stands to reason that, if cosmopolitan species from the surface can survive in the deep subsurface, isolated from their surface brethren, then over a long period of time some organisms might have adapted to even more extreme conditions deeper in the subsurface. It could be that the real treasure trove of new and weird life forms still awaits discovery far beneath our feet."

Comment: Note my bold. Oxygen is only needed by complex life forms. Living forms can be anywhere.