[LINK] "How do creatures survive without sunlight?"

[rfmcdonald]

Over at the Globe and Mail, April Holladay examines this question. This has implications for extraterrestrial life inhabiting the sub-surface water oceans of outer-system moons like Europa and Enceladus, since such life would have to exist independently of the energy output of good ol' Sol.

Such organisms use inorganic chemicals (usually hydrogen and hydrogen sulfide obtained from rocks and sea water) for energy instead of organic matter. They utilize carbon dioxide as their carbon source.

Geothermal, rather than solar, energy catalyzes chemical reactions that create life-sustaining inorganic molecules. Organisms consume the inorganic chemicals and convert them to life's fuel ¯ usually sugar. Water is the only absolutely essential ingredient deep organisms need in addition to the inorganic chemicals they mine from their surroundings.

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It not only does not need the Sun but also does not need any other life. Most bacteria sponge off other species for some needs -- for example, bacteria around sea vents rely on planktonon the sea surface to produce oxygen from photosynthesis. Then sea-vent bacteria merely take oxygen from deep seawater put there by the surface-dwelling plankton.

But Dax audaxviator is unique in that it survives alone in deep rock. Dax must extract all its needs from its sterile surrounds and then, by itself, manufacture organic molecules out of water, inorganic carbon and nitrogen (from ammonia) it gets from surrounding rocks and fluid.

Decaying uranium indirectly fuels Dax' energy needs. As uranium decays into lighter elements, it releases energy. The freed energy catalyzes chemical reactions that produce hydrogen and sulfate -- Dax chow. Dax releases the liberated energy in a series of careful steps to power cell work . For instance, it combines hydrogen and sulfate to produce lower-energy hydrogen sulfide, which Dax exports into the environment -- Dax poop.

By the way, if Dax were to release its energy in a single step, the energy stored in molecular bonds would escape in the form of heat, bursting Dax into flames. So Dax proceeds with its potentially hazardous task gingerly, in many small controlled reactions.

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Modern relatives of D. audaxviator living in hydrothermal vents (geysers on the seafloor) get their primary energy from chemical bonds. Heat from molten rock far below the seafloor raises trickle-down seawater temperatures to well above 350 degrees C (660 F). The hot seawater reacts with ocean-crust rocks causing the hot water to pick up hydrogen sulfide, which then up-wells with the vent water.

“Vent bacteria (Dax' cousins) break the chemical bonds of the up-gushing hydrogen sulfide and use the bond energy to combine oxygen (or nitrate) with carbon dioxide (which comes from seawater) into stable, biologically useable compounds, such as glucose”, says marine bioscientist Barbara J. Campbell of the University of Delaware.Dax' cousins make sugars within their one-cell bodies and use energy from the sugars to power cell life.

Other vent organisms that can't synthesize their own food gobble these compounds. Sometimes vent organisms also eat the vent bacteria or their waste products. Dax' cousins thus contribute nutrients to the vent community and form the base of the food web.

These thermophiles never see the Sun or encounter a breath of oxygen, but survive 2.4 kilometers (1.5 mi) below the sea surface.