1. The Emergence of Metabolism at Hydrothermal Vents
Jessica Nuñez
Minority Student Programs Intern, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak
Grove Drive, Pasadena CA 91109
Citrus College. Address, Glendora, CA
Background
Hydrothermal vents on the early earth may have been the
hatchery for the emergence of life. The Hadean ocean was
highly acidulous and concentrated with carbon dioxide and
iron cations. Water seeped in to the ocean’s crust causing
serpentinization reactions which resulted in the convection
of alkaline hydrothermal fluid as well as the production of
hydrogen and hydrosulfide anions (Russell and Hall 2006).
Mineral precipitates formed chimney-like structures at the
interface of the acidic ocean and alkaline hydrothermal
fluid. The two solutions were far from equilibrium in
regards to charge and composition. The Hadean ocean
contained ferrous/ferric ions, nitrite, nitrate, and
phosphate while the reducing hydrothermal fluid contained
hydrogen, methane, amino acids and formate (Russell and
Hall 2006). This disequilibrium resulted in the formation
of steep gradients across the hydrothermal vent chimney.
The mineral precipitates at the solutions’ interface would
act as inorganic membranes transected by significant pH,
thermal, and electric potential gradients. The energetic
capacity of these gradients is thought to provide the
energy necessary to propel the endergonic processes, which
are imperative first steps towards the emergence of life,
through the coupling of endergonic reactions with exergonic
reactions (Branscomb and Russell 2012).
Nature’s Engines
Green Rust
Green Rust is a mineral of particular interest for the
emergence of life in hydrothermal systems because of its
redox active and catalytic properties (Russell et al.
2014). In the iron rich Hadean ocean, green rust would have
been the likely analog to the magnesium hydroxide
precipitate that is found in modern hydrothermal vent
chimneys (Kelley et al. 2005). Green Rust is of interest
primarily due to its ability to concentrate an array of
substances within its structure including phosphates and
sulfates (Nawalde 2002, Arrhenius 2003) as well as
biomolecules such as amino acids, proteins and RNA (Oh et
al. 2009; Nawalde 2002).
Small compartments form within Green Rust’s double layered hydroxide
structure, restricting diffusion and “forcing” reactions (Russell et
al. 2014) Image Credit: Applied Mineralogy. Double Layered Hydroxide.
22 July 2014. Web.
Pyrophosphate
It has been suggested that pyrophosphate acted as a
predecessor to ATP at the emergence of life (Baltscheffsky
1999), and the stability of pyrophosphate in simulated
mineral precipitated hydrothermal chimneys has recently
been demonstrated in the laboratory (Barge et al. 2014).
The simultaneous presence of polyphosphates and amino acids
within Green Rust is now hypothesized to be able to
facilitate synergistic activity between the two substances,
in a process by which the interaction of amino acids and
polyphosphates synthesizes peptides, which in turn act as
catalysts in the formation of polyphosphates (Milner-White
and Russell 2010).
Methods
The experimental procedure was to form Iron (II/III) hydroxide chimney structures by slowly
injecting an alkaline hydrothermal fluid simulant into an acidic ocean simulant. The
hydrothermal simulant was injected into the ocean simulant over a number of hours and pictures
were taken throughout. Post-injection, the chimneys were removed from the fluid and their
structures were analyzed visually and with JPL’s environmental scanning electron microscope
(ESEM). The objective of this research was to analyze how chimney structure and organic
concentration are affected by experimental variables including the solution concentrations,
the presence of amino acids, and the presence of pyrophosphate.
Iron (II/III) hydroxide control experiments
The Iron (II/III) hydroxide chimneys would form various branches which
grew in unpredictable directions. They were reddish brown in structure.
Although they branches of these chimneys appeared frail they would often
remain upright and intact after the surrounding fluid was removed.
FeOH chimney following a 3 hour
injection.
ESEM image of the solids from an FeOH chimney experiment.
Iron (II/III) hydroxide + phosphate/pyrophosphate
Fe(II/III)-hydroxide chimney containing
phosphate.
Fe(II/III) chimney containing
pyrophosphate.
Both the phosphate and pyrophosphate containing chimneys were larger than
the control chimneys containing only Fe(II/III)-hydroxide. The addition of
phosphate to the acidic ocean simulant caused an orange coating with a fuzzy
appearance to accumulate on the outer surface of the chimney. The addition
of pyrophosphate caused the accumulation of a translucent green film
extending outward from the outer chimney surface. In both sets of chimneys
the accrued substance would fall off from the main structure when the
chimney was removed from the surrounding fluid. The chimney structures
appeared identical to that of the pure Fe(II/III)-hydroxide chimneys with
the exception of being larger.
Iron(II/III) hydroxide + pyrophosphate + alanine
Iron(II/III)-hydroxide chimney
containing pyrophosphate + alanine.
Some Fe(II/III)-hydroxide chimneys were
grown with amino acids (alanine)
included in the injected hydrothermal
simulant. The alanine-containing
chimneys formed thick, straight upward
structures rather than thin branches.
When alanine was included in
pyrophosphate-containing Fe(II/III)-
hydroxide chimneys, the addition of
alanine caused a much greater amount of
the translucent green substance to
accumulate on the chimney structure
relative to experiments containing only
Fe-hydroxide and pyrophosphate. Though
the addition of alanine produced chimney
structures that were larger and appeared
sturdier, they often collapsed
completely the surrounding fluid was
removed.
Environmental Scanning Electron Microscopy
ESEM image of solid from an Iron (II/III) hydroxide
+ pyrophosphate chimney.
ESEM image of solid from an Iron (II/III) hydroxide
+ pyrophosphate + alanine chimney.
Under the environmental scanning electron microscope the pyrophosphate chimneys
appeared less rigid than the iron (II/III)-hydroxide control chimneys. The
pyrophosphate + alanine iron(II/III)-hydroxide chimneys appeared the least rigid in
structure out of all the chimney experiments which were performed.
Conclusions
The chimney experiments performed here are in some ways analogous to the processes
which would have occurred on the Hadean earth at hydrothermal vents. The mineral
precipitated chimneys - inorganic membranes - would have transected thermal, pH, and
electrical potential gradients while providing energy for the emergence of life. The
double layered hydroxide structure of these Fe(II/III)-containing chimneys would have
provided an ideal environment for the earliest geochemical “engines” to drive reactions
towards the emergence of metabolism. This also has implications for the possible
emergence of life on other worlds; especially those which may have hosted hydrothermal
activity in the past (e.g. Mars) and those that currently have liquid water oceans in
contact with a rocky seafloor (e.g. Europa).
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