News: Shallow Origins
Robert Karl Stonjek
December 22nd, 2009 in Space & Earth / Earth Sciences
In finding answers to the mystery of the origin of life, scientists may not
have to dig too deep. New research is shedding light on shallower waters as
a possible location for where life on Earth began.
Hydrothermal vents have been a focus of origin of life studies ever since
the first one was discovered in 1977. These were mainly deep vents that
averaged 2,100 meters down on the ocean floor. The hot gasses emanating from
the center of the Earth through these vents could reach temperatures greater
than 300 degrees Celsius.
These high temperatures caused some scientists to reject the possibility
that life originated at deep sea hydrothermal vents, since organic molecules
are unstable at such high temperatures.
In a paper published in the November issue of the journal Astrobiology,
scientists point to shallow hydrothermal vents, at depths of 200 meters or
less, as a possible location where the first signs of life emerged.
�Shallow water hydrothermal vents have been dismissed,� says lead author
Marcelo Guzman, origins of life postdoctoral fellow at the School of
Engineering and Applied Sciences and Department of Earth and Planetary
Sciences at Harvard University. �There are plenty of shallow hydrothermal
vents but they have been studied less.�
The shallow depth makes it possible for the Sun�s energy to reach the vents.
Depths of 200 meters or less consist of the �photic zone,� regions of the
ocean through which sunlight can penetrate, providing the required energy
for chemical reactions. Sunlight is completely filtered out at depths beyond
200 meters. Temperature is also a factor.
�Shallow hydrothermal water systems are more temperate,� says Guzman.
In shallow hydrothermal vents, temperatures range from 10 to 96 degrees
Celsius, much milder than those of deep hydrothermal vents.
Shallow hydrothermal vents aren�t that common today, but they were probably
more prevalent about four billion years ago when the Earth�s mantle had just
cooled enough to form. There may have been less water on Earth at that time
as well, since many scientists believe a majority of Earth�s water was
delivered after formation, by asteroids and comets.
The earliest examples of ancient life are stromatolites - pillars of rock
created by microbial mat colonies. Stromatolites are rare today, but usually
form in shallow water. It would be a case of straight-forward evolution if
the ancient stromatolites formed in the same environment where life itself
was born.
Starting the Cycle
Scientists who study the origin of life tend to fall into one of two camps -
geneticists or metabolists. Proponents of the "metabolism first" view
believe that complex chemical reactions provided the environment from which
a genetic system developed. The supporters of the "genetics first" theory
argue that replicating polymers came first and made way for metabolism
through evolution.
Though Guzman promotes the "metabolism first" perspective, he also believes
there were "several mechanisms happening simultaneously, and the first
original cell had both genetics and metabolism."
Assuming that is true, there are three requirements for a primitive
metabolism: energy, a mineral catalyst and a perpetuating chemical cycle.
The energy for the shallow hydrothermal environment would have been provided
by sunlight and the temperate heat of the vents. The mineral catalysts would
have been part of the rocks that made up the vent structures. The
perpetuating chemical reaction that Guzman and his colleague, Scot Martin,
imagine to have existed on early Earth is the reverse Krebs cycle (also
called the reductive tricarboxylic acid (rTCA) cycle or the reductive citric
acid cycle), which uses carbon dioxide and water to make carbon compounds.
Even though the Krebs cycle is one of the most basic cycles life can use to
�fix� carbon, Guzman says, it is still complex because specific enzymes work
during each step. Guzman does not believe that enzymes existed before life o
riginated, and therefore the Krebs cycle most likely evolved from something
even more primitive.
�We're talking about prebiotic metabolism,� Guzman says. �Maybe metabolites
in the environment allowed the first cells to have the chemistry they needed
to run. Maybe in the first cell, a mineral catalyzed certain reactions.�
Guzman and Martin tried to chemically replicate the Krebs cycle - that is,
without enzymes playing a role. They experimented with the semi-conducting
mineral zinc sulfide as the catalyst. But iron, cadmium and manganese can
also be used, says Guzman. Using a �colloidal suspension� (a chemical
mixture in which a solid is suspended in a liquid) of zinc sulfide and
sodium sulfide and exposing it to UV light, the research team was able to
reproduce about 70 percent of the cycle.
�The inevitability of certain compounds appearing again and again kind of
links to what people have been thinking about - the core metabolism as being
the essential starting point,� says George Cody, senior research scientist
at the Carnegie Institution of Washington.
Cody says he views this research study very favorably, and even though the
finding is �not a quantum leap� in this area of research, �it�s an
interesting bit of chemistry,� he says.
�It�s a set of experiments that other people aren�t doing,� Cody says. �It
highlights how much work needs to be done. There are many different
environments where one can simulate experiments, but there's a lot more to
the story than just that. One has to link reliable chemistry with
geochemical reality.�
In future research, Guzman hopes to be able to accurately simulate the
chemical conditions of a hydrothermal vent in the lab.
Source: Astrobio.net
http://www.physorg.com/news180726917.html
Posted by
Robert Karl Stonjek