CREDITS (FROM TOP): SUNET AL.; FABIAN PLOCK / ALAMY STOCK PHOTO840 22 MAY 2020 • VOL 368 ISSUE 6493 sciencemag.org SCIENCE
metrics exceeded those of
conventional silicon transistors
with similar dimensions. —PDS
Science, this issue p. 850CELL BIOLOGY
Supramolecular attack
particles
Cytotoxic T cells (CTLs) are at
the front lines against cancer
and chronic infection. T cells
kill by secreting caspase-
activating granzymes and the
pore-forming protein perforin
from dense core granules.BIOFILMS
Bacteria maintain
motile reserves
During biofilm formation, bac-
terial cells switch from a motile
to a generally sessile, matrix-
producing state. However,
biofilms formed by Bacillus
subtilis can spread to overtake
and kill neighboring colonies of
competitor species. Steinberg
et al. found that a motile
subpopulation of cells within
B. subtilis biofilms was required
for the biofilms to spread over
foreign objects. This process
required the matrix protein
TasA, which stimulated a sub-
set of cells within the biofilm to
revert from a matrix-producing
to a motile state, thus ensuring
that the colony could spread.
—AMV
Sci. Signal. 13 , eaaw8905 (2020).DEVICE TECHNOLOGY
Aligning dense carbon
nanotube arrays
Although semiconducting
carbon nanotubes (CNTs)
are promising candidates toreplace silicon in transistors
at extremely small dimen-
sions, their purity, density, and
alignment must be improved.
Liu et al. combined a multiple
dispersion sorting process,
which improves purity, and
a dimension-limited self-
alignment process to produce
well-aligned CNT arrays on a
10-centimeter silicon wafer.
The density is sufficiently
high (100 to 200 CNTs per
micrometer) that large-scale
integrated circuits could be
fabricated. With ionic liquid
gating, the performanceTROPICAL FORESTSThermal sensitivity of tropical trees
A
key uncertainty in climate change models is the thermal sensitivity of tropical
forests and how this value might influence carbon fluxes. Sullivan et al. mea-
sured carbon stocks and fluxes in permanent forest plots distributed globally.
This synthesis of plot networks across climatic and biogeographic gradients
shows that forest thermal sensitivity is dominated by high daytime temperatures.
This extreme condition depresses growth rates and shortens the time that carbon
resides in the ecosystem by killing trees under hot, dry conditions. The effect of
temperature is worse above 32°C, and a greater magnitude of climate change thus
risks greater loss of tropical forest carbon stocks. Nevertheless, forest carbon stocks
are likely to remain higher under moderate climate change if they are protected
from direct impacts such as clearance, logging, or fires. —AMS
Science, this issue p. 869Throughout the tropics,
carbon stocks in forests, such
as this one in Liberia, will be
reduced in response to higher
daytime temperatures.RESEARCH
IN SCIENCE JOURNALS
Edited by Michael FunkDNA frame for
nanotube devices
Sun et al., p. 874Published by AAAS