YSM Issue 93.2

WHAT’S IN THE
EARTH’S MANTLE?
HOW WEAKER MATERIAL IN OLDER PLATES
MAY INCREASE DEFORMATION
BY DRUHV PATEL
ILLUSTRATION COURTESY OF ANMEI LITTLE
Tectonic plates and subducting slabs, portions of
tectonic plates that have slid under other plates, hold
the secrets to the movement of land masses. Previous
evidence showed that older slabs, which are colder and
supposedly stronger, deform in the mantle more than warm,
and presumably weaker, plates. This phenomenon puzzled
researchers and scientists. Professor Jennifer Girard of Yale
University’s Department of Earth and Planetary Sciences,
along with a team of researchers including Anwar Mohiuddin
and Shun-ichiro Karato, aimed to uncover the basis for this
unexpected deformation. They used a high-pressure and
high-temperature press to simulate the conditions in the
Earth’s mantle, allowing them to study the deformation and
subduction of slabs on a smaller scale.
The team found that when a subducting slab made
mostly of large olivine mineral plunges into the mantle, the
increased pressure causes olivine to transform into finegrained
ringwoodite. “The study clearly shows that newly
formed fine-grained ringwoodite is significantly weaker
than the coarse-grained olivine,” Girard said. While newly
formed ringwoodite in cold slabs grows slowly, higher
temperatures in warmer slabs cause grains of ringwoodite
to grow much faster; this causes the young slabs to become
much stronger as the grain grows. In fact, the team believes
that this inhibited growth rate may be the reason that cold
slabs deform while warmer slabs do not. These findings
will help researchers further explore and understand the
unexpected behavior of tectonic plates. ■
Mohiuddin, A., Karato, S. & Girard, J. (2020). Slab
weakening during the olivine to ringwoodite transition
in the mantle. Nature Geoscience, 13, 170–4. https://doi.
org/10.1038/s41561-019-0523-3
www.yalescientific.org
ANALYZING
AUTOINDUCER-3
UNDERSTANDING
CRITICAL BACTERIA
BY JERRY RUVALCABA
ILLUSTRATION COURTESY OF SOPHIA ZHAO
Escherichia coli is a dominant bacterial member of the human
intestinal tract and a major model organism in biology. Some
members of E. coli contribute to a healthy gut ecosystem, whereas
others are pathogens causing over a million infections worldwide
that often develop antibiotic resistance. Despite this, mechanisms for
regulation of its population-level phenotypes, which are associated
with pathogenesis, have remained elusive. Researchers in Professor
Jason Crawford’s group in the Departments of Chemistry and
Microbial Pathogenesis at Yale University, however, have illuminated a
key pathway underlying this phenomenon at the molecular level. They
have discovered the structure and pathway of autoinducer-3 (AI-3), an
uncharacterized signal responsible for regulating virulence.
This signal is secreted from the bacteria during growth, accumulating
as cells divide and allowing the bacteria to assess their numbers.
Researchers isolated the metabolite by applying cellular stress. Then,
the structure was determined using one- and two-dimensional
Nuclear Magnetic Resonance spectroscopy. Additionally, the effects of
AI-3 were tested on both bacteria and human tissue. Upon introducing
the metabolite to a strain of E. coli that causes intestinal lesions and
kidney failure, the bacteria became more virulent. When introduced
to human tissue, an inflammatory effect was observed, indicating
human cells can detect and combat these signals.
The elucidation of the AI-3 structure and pathway is a crucial step
forward in microbial pathogenesis. “It can be used to determine
the collection of genes regulated by the AI-3 molecule in other
pathogenic bacteria,” Crawford said. These findings pave the way to
combatting virulence in a variety of pathogens. ■
Kim, C.S., Gatsios, A., Cuesta, S., Chong Lam, Y., Wei, Z.,
Chen, H., Russell, R.M., Shine, E.E., Wang, R., Wyche, T.P.,
Piizzi, G., Flavell, R.A., Palm, N.W., Sperandio, V., & Crawford,
J.M. (2020). Characterization of Autoinducer-3 Structure and
Biosynthesis in E. coli. ACS Central Science, 6(2), 2020, 197–206.
https://doi.org/10.1021/acscentsci.9b01076
September 2020 Yale Scientific Magazine 7