Marine Ecosystems Research Department - jamstec japan agency ...

Japan Marine Science and Technology Center
Frontier Research System for Extremophiles
tallinity shows higher dissolution temperature, while
surface area or crystalline form does not affect the dissolution
temperature significantly.
(b) Colloidal dispersions in supercritical water
Water from hydrothermal vents often contains a
high amount of inorganic particles that are colloidal in
size. This motivated us to study colloidal dispersions
in supercritical water (SCW). From the viewpoint of
colloid science, SCW is a unique media in the following
aspects.
(i) Solvent properties can be manipulated widely
and continuously by temperature and pressure
(ii) Intense density fluctuation is present in the
vicinity of the critical point
In SCW, interparticle interactions such as van der
Waals or electrostatic interactions can be controlled
through the solvent properties by changing temperature
and pressure because of (i). In the close vicinity
of the critical point, where the effect of (ii) is significant,
the behavior of colloidal particles is expected to
be different from that in normal medium. The solvent
molecules may move cooperatively in the density fluctuation,
and such cooperativity would alter the motion
of the dispersed particles from Brownian to non-
Brownian. The interparticle interactions would also be
affected by the fluctuation, leading to change of colloidal
stability.
We have studied colloidal dispersions in supercritical
water in order to understand the effect of (i). On
the other hand, study of (ii) has been hampered by the
anisotropy of the density fluctuation, induced by gravity
on Earth. Possible solution of the problem is to perform
the experiments under microgravity. Our
research proposal entitled "Behavior of Colloidal
Particles in Critical Density Fluctuation" has been
accepted as a part of "Ground-based Research
Announcement for Space Utilization" promoted by
Japan Space Forum.
Preliminary experiments were performed on a dispersion
of monodisperse polystyrene latex (m in
diameter) at ambient condition. As predicted by theory,
displacement of the lattices measured in second
interval followed normal distribution around the origin.
Diffusion coefficient, calculated from the dispersion
of the distribution, was x - m /s, which is in
good agreement with the value calculated from
Einstein-Stokes equation.
(c) Thermal stability of hyperthermophiles under subcritical
aqueous conditions
Adaptive abilities of hyperthermophiles to high
temperature environments are mainly characterized
by their growth temperatures, which are around
˚C. Another measure of thermal adaptation is thermal
death temperature (TDT), at which a microorganism
dies due to thermal degradation of cell-components
such as proteins or lipid membranes.
Hyperthermophiles have been isolated from deep-sea
hydrothermal vents at temperatures over ˚C. The
result indicates that the TDT of the hyperthermophiles
is much higher than the growth temperature. However,
TDT of hyperthermophiles have not been studied well.
The purpose of this work is to obtain systematic
experimental data of TDT for hyperthermophiles.
TDT would give important information for estimating
the distributions of hyperthermophiles in the
hydrothermal systems, and help to understand the
thermal adaptation mechanisms.
Preliminary batch-wise experiments using E. coli
W revealed that its D value (the time at which
viable cell counts reduced to % of the initial count)
is min at ˚C but less than sec at ˚C. In order to
measure the D value at even shorter time scale, we
have developed a new flow-type apparatus. The apparatus
is designed to operate at temperatures and pressures
up to ˚C and MPa. According to the present
design, microorganisms are subjected to high
temperature for . seconds. Experimental results
employing this apparatus for E. coli W are in
good agreement with the previous data obtained by the
batch-wise methods. Considering the strong temperature
dependence of the D value, this assures that the
new flow-type apparatus operates as designed, and can
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