In the strange, dark world of the ocean floor, underwater fissures, called
hydrothermal vents, host complex communities of life. These vents belch
scorching hot fluids into extremely cold seawater, creating the chemical
forces necessary for the small organisms that inhabit this extreme
environment to live.
In a newly published study, biogeoscientists Jeffrey Dick and Everett Shock
have determined that specific hydrothermal seafloor environments provide a
unique habitat where certain organisms can thrive. In so doing, they have
opened up new possibilities for life in the dark at the bottom of oceans on
Earth, as well as throughout the solar system. Their results have been
published in the Journal of Geophysical Research: Biogeosciences.
On land, when organisms get energy out of the food they eat, they do so
through a process called cellular respiration, where there is an intake of
oxygen and the release of carbon dioxide. Biologically speaking, the
molecules in our food are unstable in the presence of oxygen, and it is that
instability that is harnessed by our cells to grow and reproduce, a process
called biosynthesis.
But for organisms living on the seafloor, the conditions for life are
dramatically different.
"On land, in the oxygen-rich atmosphere of Earth, it is familiar to many
people that making the molecules of life requires energy," said co-author
Shock of Arizona State University's School of Earth and Space Exploration
and the School of Molecular Sciences. "In stunning contrast, around
hydrothermal vents on the seafloor, hot fluids mix with extremely cold
seawater to produce conditions where making the molecules of life releases
energy."
In deep-sea microbial ecosystems, organisms thrive near vents where
hydrothermal fluid mixes with ambient seawater. Previous research led by
Shock found that the biosynthesis of basic cellular building blocks, like
amino acids and sugars, is particularly favorable in areas where the vents
are composed of ultramafic rock (igneous and meta-igneous rocks with very
low silica content), because these rocks produce the most hydrogen.
Besides basic building blocks like amino acids and sugars, cells need to
form larger molecules, or polymers, also known as biomacromolecules.
Proteins are the most abundant of these molecules in cells, and the
polymerization reaction (where small molecules combine to produce a larger
biomolecule) itself requires energy in almost all conceivable environments.
"In other words, where there is life, there is water, but water needs to be
driven out of the system for polymerization to become favorable," said lead
author Dick, who was a postdoctoral scholar at ASU when this research began
and who is currently a geochemistry researcher in the School of Geosciences
and Info-Physics at Central South University in Changsha, China. "So, there
are two opposing energy flows: release of energy by biosynthesis of basic
building blocks, and the energy required for polymerization."
What Dick and Shock wanted to know is what happens when you add them up: Do
you get proteins whose overall synthesis is actually favorable in the mixing
zone?
They approached this problem by using a unique combination of theory and
data.
From the theoretical side, they used a thermodynamic model for the proteins,
called "group additivity," which accounts for the specific amino acids in
protein sequences as well as the polymerization energies. For the data, they
used all the protein sequences in an entire genome of a well-studied vent
organism called Methanocaldococcus jannaschii.
By running the calculations, they were able to show that the overall
synthesis of almost all the proteins in the genome releases energy in the
mixing zone of an ultramafic-hosted vent at the temperature where this
organism grows the fastest, at around 185 degrees Fahrenheit (85 Celsius).
By contrast, in a different vent system that produces less hydrogen (a
basalt-hosted system), the synthesis of proteins is not favorable.
"This finding provides a new perspective on not only biochemistry but also
ecology because it suggests that certain groups of organisms are inherently
more favored in specific hydrothermal environments," Dick said. "Microbial
ecology studies have found that methanogens, of which Methanocaldococcus
jannaschii is one representative, are more abundant in ultramafic-hosted
vent systems than in basalt-hosted systems. The favorable energetics of
protein synthesis in ultramafic-hosted systems are consistent with that
distribution."
For next steps, Dick and Shock are looking at ways to use these energetic
calculations across the tree of life, which they hope will provide a firmer
link between geochemistry and genome evolution.
"As we explore, we're reminded time and again that we should never equate
where we live as what is habitable to life," Shock said.
Reference:
Jeffrey M. Dick et al, The Release of Energy During Protein Synthesis at
Ultramafic‐Hosted Submarine Hydrothermal Ecosystems, Journal of Geophysical
Research: Biogeosciences (2021).
DOI: 10.1029/2021JG006436
Tags:
Space & Astrophysics
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