Between a rock and a hard place
 How do animals or insects that live deep under water or
far in the Earth get vitamins and energy from the sun? Do not all living things
need the sun to live? Jean-Pierre, Windsor, Canada
Where does the energy to sustain life around sea vents come from? I
have heard that sulfur plays an interesting role.
Harry, Newark, Delaware, USA
Tubeworms
(lacking a mouth, a digestive tract and anus) live off sugars, fatty acids and
amino acids made by symbiotic bacteria dwelling inside the tubeworms. The
blood-red worms can grow up to 6 feet tall near hydrothermal vents on the
seafloor. Photo courtesy of
Woods Hole Oceanographic Institution, copyright, used with permission.
Your questions lead us along lifes most basic path survival of the
species the primary function of all organisms. When a creature successfully
manages to reproduce, it has done its job. The task, though, can be difficult,
especially along the fringes of the web of life in the deep dark.
Almost all life on Earth is part of a web that gets its energy
ultimately from the Sun. Each life form in the web usually exchanges nutrients
with other life forms. But, if the environment can supply basic needs, strange
singular forms can exist independently of all other life and even of the Sun.
All cells must have three things to survive:
- Energy to run the
activities of their cells (such as, combining nutrients to make sugar and
thereby release energy).
- Liquid water to dissolve chemicals and allow them to mix together
and react. "Liquid" because liquid water is the right
temperature for essential chemical reactions to occur in cells.
- Chemical building blocks.
- Carbon for its ability to form long
chain-like molecules (like sugars and proteins). Every living thing on Earth is made from a set
of molecules built around carbon atoms.
- Hydrogen and oxygen to bond with
carbon and also make water. By the way, of the approximately twenty-four atoms required, 95 % of the
human body is made up of just four atoms (carbon, hydrogen, oxygen and
nitrogen)
- Nitrogen likewise to bond with carbon
and also to bond with hydrogen and oxygen and form stable, large molecules.
- Other elements: sulfur, phosphorus,
sodium, potassium, magnesium, calcium, manganese, iron, cobalt, copper and
zinc.
Carbon is the key. "The concentration of carbon in living matter (18%)
is almost 100 times greater than its concentration in the Earth (0.19%). So
living things extract carbon from their nonliving environment," says biologist
John W. Kimball author of Biology. Given
energy, though, organisms can do the extraction work.
How cells manage to survive without the Sun
You ask specifically about those animals that live in the deep dark of rock or sea. These creatures, over millions of years,
evolved to use energy supplied from our planet rather than our sun.
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.
Let's investigate two species to see how they get energy ― without direct
sunlight ― one living in deep rock (deeper than we have found any other
creature) and a distant cousin living in the deep sea.
Deep rock life
The
D. audaxviator bacterium lives 2.8 km (about 2 mi) below the ground, in a South
African gold mine.
Micrograph courtesy of
Greg Wanger and
Gordon Southam, University of Western Ontario, used with permission.
The microscopic
(4-micron) creature lies within a cage of surrounding hard,
dense volcanic rock (basalt) in a dark, sulfur-stinking pool of scalding-hot
salty ancient water. Millions of tons of rock press in all directions upon its tiny body 2.8
kilometers below ground and raise the temperature of its
home to 60 degrees Celsius (140 F). No whiff of air, no glimmer of
sunlight ever penetrates this far beneath Earth's surface. It's species name is D. audaxviator ― bold traveler.
I call the organism "Dax."
This type of heat-loving bacterium (called a
thermophile) has lived between 3 to 25 million years ― totally
cut off from surface life, imprisoned in 3-billion year old basaltic rock.
You ask how it manages. The D. audaxviator species is doubly unusual.
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 plankton on 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 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.
Revving up its one-cell factory, Dax makes cellular building blocks:
amino acids for proteins, genetically-coded chains of DNA and RNA for
reproduction and lipid fats for cell membranes.
However, Dax' food supply is too meager for
much more than survival in
this perhaps harshest of Earth's living spaces. Furthermore, oxygen kills
the organism, which implies it may have been separated from Earth's surface for
millions of years.
"What's remarkable is that Dax, on its own, carries out the cellular
functions that entire communities of bacteria are required to do in other
environments, says geoscientist
Tullis C. Onstott
of Princeton University.
Indeed, "The fact that the community contains only one species stands one of
the basic tenets of microbial ecology on its head," says astrobiologist
Carl
Pilcher of NASA.
In 2006, geoscientist Tullis Onstott, Lisa Pratt
of Indiana University and their team from nine collaborating institutions
discovered Dax and his fellow species bacteria living totally alone ―
isolated from all other bacteria cultures ― deep within South
Africa's deepest goldmine. The water that sustains the bacteria was
undiluted by surface water and between three and 25 million years old. D. audaxviator
is a distant ancestor of many thermophiles.
In 2008,
Dylan Chivian of the Lawrence Berkeley National Laboratory, California
analyzed and sequenced Dax' genome. The genetic
structure revealed by Chivian's analysis informs us of Dax's potential
abilities.
Deep sea life
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.
Life's start (and waning years)
We may never know how life started, but I wonder if life retreated to a safe
refuge in the deep sea or below Earth's crust when huge
meteorites crashed into our planet. Certainly the environment was
less hellish below. How about Mars? Maybe her surface is lifeless
now, but her deep rocks, like ours, may harbor abundant life.
"With the discovery of an organism capable of living in complete isolation
from the photosphere, we can begin in earnest to search for subsurface life on
planets like Mars," says geoscientist
Lisa Pratt
of Indiana University.
In 1992, astronomer
Thomas Gold of Cornell University speculated that the mass of all
subterranean microbes equals the mass of all organisms on or above Earth's
surface.
Further Reading:
Deep-sea tubeworms get versatile 'inside' help, Oceanus, the online magazine
of research from Woods Hole Oceanographic Institution, 12 January 2007
Environmental Genomics Reveals a Single-Species Ecosystem Deep Within Earth,
Science 10 October 2008: Vol. 322. no. 5899, pp. 275 - 278 DOI:
10.1126/science.1155495
Two miles underground, strange bacteria are found thriving, Mars Today.com,
October 2006
These bacteria use radiated water as food, Indiana University Bloomington
press release, October 19, 2006
The ingredients of life, BBC, 24 September 2009
The way we work, by David Macaulay, Houghton Mifflin Company Boston 2008
The deep hot biosphere by Thomas Gold, Proceedings of the National Academy
of Science,
PNAS July 1, 1992
vol. 89 no. 13
Dive and discover: Bacteria at Hydrothermal Vents, Woods Hole Oceanographic
Institution, 2005
Microbe Survives in Ocean's Deepest Realm, Thanks to Genetic Adaptations,
National Science Foundation Press Release February 5, 2009
Cellular respiration, 5 March 2008, John W. Kimball, Kimball's Biology Page.
Carbohydrates, 9 December 2004, John W. Kimball, Kimball's Biology Page.
ATP,
Biology online
(Answered 12 October 2009)
Comment
|