APPLIED PHYSICS

A leak of information

"Pavlo Zubko and Jean-Marc Triscone"

As capacitors, the ubiquitous components of electronic circuitry, get smaller, keeping them insulating is a challenge. But that’s not necessarily bad news — some conductivity might be just the thing for data storage.

Figure 1 | An approach to data storage. IBM’s Millipede project involves a technique, depicted here in an artist’s impression, for reading and writing data that consists of operating, in parallel, an array of atomic-force-microscopy cantilevers with sharp tips. Just as punch cards were used in the first computers, the tips are used to punch nanometre-scale indentations, representing the ‘ones’ and ‘zeros’ of binary information, into a silicon chip coated with a thin plastic film. Garcia and colleagues’ demonstration1 of the operation of a single tip to read and write ferroelectric domains could begeneralized to use a similar array of cantilevers.

Pavlo Zubko and Jean-Marc Triscone are in the

Department of Condensed Matter Physics,

University of Geneva, 24 Quai Ernest-Ansermet,

CH-1211 Geneva 4, Switzerland.

e-mails: pavlo.zubko@unige.ch;

jean-marc.triscone@unige.ch

IMMUNOLOGY

A metabolic switch to memory

"Martin Prlic and Michael J. Bevan"

Two therapeutic drugs have been found to enhance memory in immune cells

called T cells, apparently by altering cellular metabolism. Are changes in

T-cell metabolism the key to generating long-lived immune memory?

T lymphocytes respond to an acute infection

with a massive burst of proliferation, generating

effector T cells that counteract the pathogen.

When the infection is cleared, most of these

effector T cells die (the contraction phase of

the immune response), but a minority lives

on and changes into resting memory T cells

that rapidly respond to future encounters with

the same pathogen1. In this issue, Pearce et al.2

(page 103) and Araki et al.3 (page 108) report

that two drugs, one used to control diabetes

and the other to prevent organ-transplant

rejection, markedly enhance memory T-cell

development. Through their actions on major

metabolic pathways in the cell, these drugs

seem to promote the switch from growth to

quiescent survival.

While investigating the role of a protein

called TRAF6, which is a negative regulator

of T-cell signalling, Pearce et al.2 noted that,

although T cells in which TRAF6 was knocked

out mounted a normal effector response to a

pathogen, they left behind few if any memory

cells. The authors performed a microarray

analysis comparing the genes expressed by

normal and TRAF6-deficient T cells at the

time they change from effector to memory

cells. In a eureka moment, they realized that

TRAF6-knockout T cells display defects in

the expression of genes involved in several

metabolic pathways, including the fatty-acid

oxidation pathway, implying that a metabolic

switch in T cells might be affecting memorycell

generation.

Pearce et al. followed up on this clue, and

showed that the inability of TRAF6-deficient

T cells to spawn long-lived memory T cells

could be reversed by treatment with either the

antidiabetes drug metformin or the immunosuppressant

rapamycin. Both drugs are known

to affect cellular metabolism, and treatment

with either drug not only restored the memory

T-cell response in TRAF6-deficient cells,

but also greatly enhanced memory T-cell

formation in normal cells, resulting in a

superior recall response to a second infection.

In an independent study, Araki et al.3

examined the effect of treating mice with

rapamycin during the various phases of a

T-cell response to viral infection. Giving

rapamycin during the first 8 days after infection

(the proliferative phase) markedly

increased the number of memory T cells

5 weeks later. This was due to an enhanced

commitment of effector T cells to become

memory precursor cells. When the authors

administered rapamycin during the contraction

phase of the T-cell response (days 8–35

after infection), the number of memory T cells

did not increase, but there was a speeding up of

the conversion of effector T cells to long-lived

memory T cells with superior recall ability.

Rapamycin inhibits mTOR (‘mammalian

target of rapamycin’), a protein-kinase enzyme

found in at least two multiprotein complexes

— mTORC1, which is rapamycin sensitive,

and mTORC2, which is largely resistant to

inhibition by rapamycin4. To pinpoint the

cellular target of rapamycin in their studies,

Araki et al.3 used RNA-interference knockdown

techniques to demonstrate that the

mTORC1 complex, acting intrinsically in

T cells, regulates memory-cell differentiation.

So both rapamycin and metformin seem

to enhance T-cell memory formation. But do

both drugs affect the same pathway(s), are the

pathways interconnected, or do two different

mechanisms lead to a similar outcome?

Metformin activates AMPK, an enzyme that

can inhibit mTOR activity in several ways,

including directly targeting raptor, a component

of rapamycin-sensitive mTORC1 (ref. 5).

Both AMPK and mTOR sense and control the

energy status of a cell (ATP:AMP ratio) and

regulate key aspects of cell growth and, as part

of this, glucose metabolism.

In a quiescent cell, most energy (in the form

of ATP) is generated in the mitochondria

through oxidative phosphorylation, including

the oxidation of fatty acids and amino

acids — catabolic metabolism. On activation,

T cells massively increase their glucose uptake

and shift to producing ATP by glycolysis

(anabolic metabolism) instead of catabolically

(Fig. 1). mTOR is activated by signalling

molecules, growth factors and antigen-induced

T-cell-receptor signalling, and its activity

enables a cell to increase glycolysis and ATP

accumulation, which opposes AMPK activation6.

Although some of the processes involved

in the switch from catabolic to anabolic metabolism

are fairly well understood, the reversal

from an anabolic to a catabolic state is not as

well characterized. One could speculate that

rapamycin and metformin facilitate the switch

from a glucose-dependent anabolic state (effector

T cell) to a catabolic state of metabolism

(memory T cell) by blocking mTORC1 activity

(Fig. 1). But how a change in the metabolic

signature of a T cell could enhance memory

T-cell numbers and function is unknown.

How can rapamycin, a drug known for its

immunosuppressive effects, enhance the function

and formation of T-cell memory? The

answer may lie in dosage and, more importantly,

timing. Whereas treatment with a low dose of

rapamycin during the first 8 days after T-cell

activation enhanced the numbers and function

of memory T cells, a higher dose, closer to therapeutic

levels, hampered the T-cell response,

as would be expected of an immunosuppressant3.

Interestingly, both papers2,3 clearly show

that the higher dose of rapamycin enhanced

memory T-cell function and recall ability if

administered after day 8. At this point, the

vigorous cell proliferation that is characteristic

of the effector stage has ceased and cells begin

to enter the more quiescent memory state.

Recent data4 suggest that mTOR can form different

complexes (aside from mTORC1 and

mTORC2) depending on the phase of the cell

cycle, and little is known about their inter action

with rapa mycin. In addition, as the metabolic

signature of a cell changes along with its activation

state, rapamycin might differentially

affect a cell depending on its cell cycle and

metabolic state.

A long-standing paradigm in immunology

proposes that, after the peak of the proliferative

response, the programmed cell death of

effector T cells is caused by a lack of growth

and survival factors — conditions that could

also affect cell metabolism. However, recent

experiments7 indicate that, in a physiological

setting, effector T-cell viability and conversion

to memory T cells are not regulated by competition

for growth and survival factors. Thus, it

is more likely that the metabolic switch is either

programmed early after T-cell activation or

occurs as a secondary effect after a quiescent

stage has been entered.

Both Pearce et al.2 and Araki et al.3 establish

a crucial role for mTOR-mediated metabolic

changes in enhancing T-cell memory. Does

changing the metabolism of T cells through

manipulation of mTOR hold promise for

improving future vaccination strategies?

mTOR is involved in regulating a plethora of

functions in many cell types, and rapamycin

administration is associated with many side

effects. Thus, a more targeted approach will be

required to harness their memory-enhancing

ability. Identifying the downstream signalling

pathways that lead to enhanced T-cell memory

on inhibition of mTOR complexes will be a first

step in that direction.

Martin Prlic and Michael J. Bevan are in the

Department of Immunology, University of

Washington, Seattle, Washington 98195-7370,

USA.

e-mails: mprlic@u.washington.edu;

mbevan@ u.washington.edu

BIOGEOCHEMISTRY

Climatic plant power

"Yves Goddéris and Yannick Donnadieu"

Levels of atmospheric carbon dioxide constrain vegetation types and thus

also non-biological uptake during rock weathering. That’s the reasoning used

to explain why CO2 levels did not fall below a certain point in the Miocene.

The world is currently at risk of overheating

in response to all the carbon dioxide being

pumped into the atmosphere from the use of

fossil fuels: the current atmospheric concentration

of CO2 is about 385 parts per million

(p.p.m.), compared with a ‘pre-industrial’

level of around 280 p.p.m. But overheating is

an atypical menace in the recent history of the

Earth. Over most of the past 24 million years,

it was the possibility of cooling that posed the

main threat to life. Cooling, however, did not

reach the levels of severity that might have

been expected, and on page 85 of this issue

Pagani et al.1 put forward a thought-provoking

case as to why that was so.

Since the end of the Eocene, around 40

million years ago, Earth’s climate has been

naturally getting colder. In temporal terms,

the cooling has sometimes occurred in discrete

steps, sometimes as a long-term trend2.

Over the same interval, levels of atmospheric

CO2 have fallen from around 1,400 p.p.m. at

the end of the Eocene to possibly as low as

200 p.p.m. during the Miocene3 — the geological

period between around 24 million and 5 million

years ago.

This long-term history of atmospheric CO2

is the result of the interplay between several

pro cesses. The degassing of the Earth through

magmatic activity (for instance volcanic

eruptions) is the main source of CO2, and

the dissolution of continental rocks captures

atmospheric CO2, which is eventually stored

as marine carbonate sediments4. The efficiency

of the dissolution process — chemical

weathering — is heavily dependent on climate,

but also depends on vegetation and physical

erosion. The last two parameters boost CO2

uptake by rock weathering. In particular, land

plants promote rock dissolution through the

mechanical action of roots, and by acidifying

the water in contact with rocks5. Acidification

occurs through the release of organic acids and

the large-scale accumulation of CO2 in soil

through root respiration. Removing plants,

particularly trees, may strongly decrease the

dissolution rate of rocks and the ability of this

process to consume atmospheric CO2.

In their paper, Pagani and colleagues1 consider

the potential role of the rise of many mountainous

regions (orogens) over the past 40 million

years, especially in the warm and humid

low-latitude areas. In these mountain ranges,

physical erosion would break down rocks and

expose them to intense chemical weathering.

The uptake of atmospheric CO2 would consequently

increase, as indicated by the levels of

CO2 measured, which could have declined to

the lowest levels since multicellular life evolved

on Earth some 500 million years ago.

But what could stop this CO2 uptake pump?

Degassing through magmatic activity was

probably declining at the same time (at best it

remained constant), and tectonic activity accelerated

mountain uplift over the past 24 million

years. According to this line of evidence, CO2

levels should have plunged to below 200 p.p.m.,

with ice ultimately covering large surfaces of

the Earth as a consequence. But that was not

the case. Earth even experienced a warm spell

between 18 million and 14 million years ago2.

Pagani et al.1 propose an exciting hypothesis

to explain why, 24 million years ago, CO2

might have levelled off at about 200 p.p.m., and

then stuck there. They suggest that, when CO2

levels became too low, forests became starved

and were progressively replaced by grasslands,

particularly in the low-latitude orogens. Grasslands

exert a much less vigorous effect on rocks

than do trees. In consequence, runs the thinking,

CO2 consumption due to weathering

declined because of changing terrestrial ecosystems,

which in turn stabilized atmospheric

CO2 at around 200 p.p.m.

Overall, the authors’ model provides an

elegant twist on several ideas about the Earth

system that emphasize the role of vegetation

in dynamically regulating and fixing the lower

limit of atmospheric CO2. But it also raises contentious

issues.

First, in the model1, forest starvation is triggered

by the low level of atmospheric CO2,

and by elevated temperature. But do proxy

estimates of conditions at that time confirm

this paradoxical combination? Proxy measurements

of CO2 levels include marine carbonate

boron isotopes6, carbon-isotope values

of alkenones produced by oceanic algae3 and

the density of stomata — a measure of gas

exchange — in fossil leaves7. Unlike the geochemical

proxy records3,6, the more recent

estimates based on the stomatal index7 depict

a highly variable CO2 trend over the Miocene

(in good agreement with climatic fluctuations),

rather than a CO2 level stuck at 200 p.p.m.

Furthermore, the estimates show that CO2

concentrations are above the forest-starvation

level most of the time, oscillating between 300

and 500 p.p.m.

Second, in their model Pagani et al. assume

that rock weathering generated by mountain

uplift would have continuously consumed

atmospheric CO2 until it reached the foreststarvation

level. But there is evidence that the

extra consumption of CO2 due to the Himalayan

uplift, the most important orogeny of

the recent past, occurred mainly through the

burial of organic matter in the Bengal fan,

and not through rock weathering8,9. In addition,

the tectonic history of the past 24 million

years is still subject to debate, and the timing

of the uplift of the main mountain ranges, such

as the Himalaya and Andes, is far from fully

constrained10.

Finally, the link between weathering and

continental vegetation is well recognized. But

it is complex. Apart from acidifying water and

mechanical effects, land plants also control the

hydrology of soils. In humid tropical environments,

about 70% of the rainfall is absorbed by

land plants and then evaporates through their

leaves. This effect should inhibit weathering

reactions by limiting the amount of water available

for rock dissolution. Also, in equatorial

uplifted areas, intense erosion occurs through

landslides triggered by heavy rainfall11. These

landslides bring fresh rock material in contact

with water by removing the soil mantle, promoting

weathering and CO2 consumption. The

role of vegetation cover in these systems might

not be as significant as Pagani et al. suggest.

The authors themselves acknowledge some

of these limitations, and all in all have put

forward a bold and provocative hypothesis.

But accounting for all of the processes and

constraints involved is probably beyond the

capabilities of the first-order global models

that Pagani et al. used, and more-complex and

process-based modelling12,13 will be required to

test their conclusions. Whatever the outcome,

that should prove to be a fruitful exercise for

carbon-cycle modellers intent on understanding

the processes that drove climate and CO2

oscillations during the Miocene.

Yves Goddéris is at the LMTG-Observatoire

Midi-Pyrénées, CNRS, Université de Toulouse III,

Toulouse F-31400, France. Yannick Donnadieu

is at LSCE, CNRS-CEA, Gif-sur-Yvette F-91191,

France.

e-mails: godderis@lmtg.obs-mip.fr;

yannick.donnadieu@lsce.ipsl.fr

Evolution’s influence on art nouveau

Characterized by sinuous shapes and subtle colours, the glass vases, bowls and other objects made by the nineteenth-century French artistdesigner Emile Gallé and his factory are still highly regarded. But his interest in botany and evolution is less well known. An exhibition this summer in Moselle, northeastern France, explores how Gallé’s work was influenced by nature and by Japanese art and design. The show is co-curated by scientist François Le Tacon of the French National Institute for Agronomic Research in Nancy, France. Gallé was one of the founders of art nouveau, an influential art and design movement. He learnt glass-making skills as an apprentice, but after taking over his father’s factory in Nancy in 1875, he created designs that were made by its artisan employees. He exploited the sensual properties of glass, using acid etching to form opaque vessels with layered colours and shaped surfaces. His organic designs did not simply borrow from nature — they expressed contemporary thought and politics. Gallé was aware of Darwin’s work by 1877 and owned a copy of Ernst Haeckel’s Art Forms in Nature (1899–1904). Plant, insect and animal motifs were derived from his careful observations, but also held deeper meanings. “Beetles symbolized industriousness; the thistle symbolized Nancy, Lorraine and separation from Germany; the rose symbolized France and the lover,” note Jennifer Hawkins Opie in Art Nouveau 1890–1914 (V&A Publications, 2000). Gallé was also inspired by aquatic flora and fauna, as shown in nine works incorporating themes such as undulating strands of seaweed and deep sea colours. Japanese artwork displayed in Europe at the end of the nineteenth century captivated many western artists and designers. Gallé himself owned woodblock prints by Hokusai and others, and he befriended the Japanese botanist Hokkaï Takashina after meeting him at a horticultural exhibition in Nancy in 1886. They were both interested in chrysanthemums, a potent symbol in Japan, which Gallé used as a decorative motif. The internally crackled and coloured glass used by Gallé and his artisans, although made in France, was inspired by Japanese watercolours and lacquerware, as well as carved rock crystal and jade from China.

From 1886 until his death in 1904, Gallé investigated evolutionary mechanisms in botany, an interest covered in the exhibition. Displayed, for example, are plates illustrating the orchid Aceras hircina, from a paper on polymorphisms in orchids local to Lorraine that Gallé presented at an international botanical congress in Paris in 1900. They underscore how Gallé’s biological exactitude and interest in symbolism generated his incomparable designs.

Colin Martin is a writer based in London.

HOT TOPIC

CLIMATE’S SMOKY SPECTRE

With their focus on greenhouse gases, atmospheric scientists have largely

overlooked lowly soot particles. But black carbon is now a hot topic among

researchers and politicians.

“Sea ice may be

melting anyway, but

black carbon can

cause more melting

and earlier melting.”

— Andreas Stohl

Some Indian cooking

methods are contributing

to atmospheric soot levels.

Stopping the soot

Last month the eight-nation

Arctic Council appointed a

task force to look at ways to

reduce black carbon and other

key pollutants responsible

for rapid Arctic warming. It

was a sign that the scientists

pushing the link between

black carbon and climate

(see main article) are getting

their message through to

governments. The question

faced by the council, and by

policy-makers across the

world, is what to do.

Black carbon, a primary

component of soot, is a

ubiquitous product of

incomplete combustion,

formed by natural forest fires,

motor vehicles, coal plants

and myriad other sources.

Soot contains both black

carbon and light-coloured

particles that cool the planet;

smoke produced by sources

such as cooking stoves and

diesel engines tends to be rich

in darker particles. Reducing

black-carbon emissions isn’t a

technical problem — modern

stoves and filters can do most

of the work — so much as

an issue of governance and

resources.

“Black carbon is perhaps

the biggest, fastest bite we

can take out of the climate

problem,” says Durwood

Zaelke, who heads the

Institute for Governance and

Sustainable Development

in Washington DC and

has helped spearhead the

movement internationally. “It

needs, however, to be followed

with aggressive regulatory

action.”

Global soot emissions have

been rising steadily since

the mid-1800s, although in

recent decades the source of

emissions has shifted from

industrialized to developing

nations. Pinning down actual

emissions is difficult, but Tami

Bond, a researcher at the

University of Illinois, Urbana–

Champaign, estimates that

diesel combustion and

residential fuel use (from coal,

wood and agricultural debris)

each produce roughly onequarter

of the total; another

40% comes from wildfires and

controlled agricultural burning;

various industrial sources

make up the remainder.

Industrialized nations could

clean up fossil fuels further

and reduce agricultural

emissions at home. But

much of the focus will be on

developing countries. The

hope there is that current

concerns over climate change

will energize existing efforts

to clean up diesel emissions

and replace inefficient cooking

stoves.

The precedent is there.

China delivered roughly 150

million stoves to rural areas

in the 1980s and early 1990s

in an effort to reduce fuel use,

says Kirk Smith, a rural energy

expert at the University

of California, Berkeley.

Smith is working with local

communities to encourage

the use of locally produced,

clean-burning biomass stoves,

which reduce emissions of

carbon dioxide, methane and

other dangerous compounds.

The impetus for the work has

been to improve public health

and reduce greenhousegas

emissions, but the new

attention on black carbon

doesn’t hurt, says Smith.

“It’s sort of the pollutant of

the month and you need to

take advantage of what’s on

people’s minds.”

Hybrid

Reprocessing plants, such as

the Olkiluoto Nuclear Power

Plant in Finland, only reduce

the volume of fission waste

by a factor of two or three.

The hybrid returns

Slotting a fusion reactor into the heart of a nuclear fission plant could accelerate the development

of waste-free nuclear energy. So why are all the designs still on paper, asks Ed Gerstner.

“The most ambitious and

comprehensive piece of climate change

legislation anywhere in the world.”

Scotland’s climate-change minister, Stewart

Stevenson, as his country passed legislation on

24 June to reduce greenhouse-gas emissions by

42% by 2020, and by 80% by 2050.

“A bold and necessary step.”

US President Barack Obama hails the passage of

the Waxman–Markey energy and climate-change

bill through the House of Representatives on

26 June — by the narrow margin of 219 to 212 votes

(see Nature 459, 493; 2009).

“We must move the debate from a

stand-off over hypothetical figures to

active negotiation on real mitigation

actions and real contributions.”

UK prime minister Gordon Brown suggests on

26 June that an international fund of US$100 billion

a year will be needed by 2020 to help developing

countries to mitigate and adapt to climate change

African research

African institutions gain

support networks

African research, by many measures

the least competitive in the world, got a

financial pick-me-up this week.

On 2 July, the UK Wellcome Trust

launched a £30-million (US$50-million)

plan to support more than 50 institutions

across the continent, organized in

themed networks that will study water

and sanitation, infectious diseases and

population health.

“There are other collaborations and

networks but the lack of research capacity

in Africa is a huge problem and it’s going

to take more than one initiative to achieve

this,” says Jimmy Whitworth, the Wellcome

Trust’s head of international activities.

The funding will be used to revamp

laboratories, to train laboratory personnel

and to support competitive grants aimed at

encouraging African scientists to remain

working in their home countries.

International Whaling Commission (IWC)

Impasse at talks leaves

whales high and dry

The 2009 meeting of the International

Whaling Commission (IWC) in Madeira,

Portugal, has ended in deadlock.

No consensus could be reached on Japan’s

proposal that it be allowed to resume

commercial whaling in its own coastal

waters — banned since 1986 — in exchange

for reducing its quota of whales killed for

‘scientific research’ in Antarctic waters.

Scott Baker, a researcher at Oregon State

University’s Marine Mammal Institute in

Newport, told the meeting that the number

of coastal whales killed as ‘by-catch’ in

fishing nets, and sold on Japanese markets,

is under-reported. His team found that

by-catch numbers approach 150 minke

whales a year, roughly equivalent to those

killed in Japan’s North Pacific offshore

whaling programme. A 2007 study

(C. S. Baker et al. Mol. Ecol. 16, 2617–2626;

2007) found similar coastal by-catch

depletion in South Korea.

The IWC also postponed a decision

on Denmark’s request for Greenland’s

indigenous Inuits to hunt 10 humpback

whales a year.

PICTURE ALERT

Heritage alert for Central American reef

Belize’s barrier-reef system (pictured) — the largest of its kind in the Northern Hemisphere — has been put on a danger list by the World Heritage Committee to encourage international support for its preservation. The committee, part of the United Nations Educational, Scientific and Cultural Organization (UNESCO), included the reef because it is threatened by development and the cutting back of mangroves. It has been on the World Heritage List of naturally important sites since 1996. Also placed on the danger list is the Los Katios National Park in Colombia, which is at risk from logging. UNESCO added a further two natural sites to its World Heritage List: the Wadden Sea wetlands belonging to Germany and the Netherlands, and the Dolomites mountain range in Italy.

SNAPSHOT

Vanishing meadows

Seagrass meadows around the world are in dire shape, according to the first comprehensive global assessment of these economically and biologically essential areas. More than a quarter of all seagrass meadows have disappeared in the past 130 years, says a new synthesis of quantitative data from 215 sites The rate of decline has grown from less than 1% per year before 1940, to 7% per year since 1990. As well as supporting wildlife such as dugong (Dugong dugon, right) and green turtles (Chelonia mydas), seagrass meadows also serve as a vital nursery for fish, supporting populations for coral reefs and commercial fisheries.

Thank You Daniel Cressey

GENOMICS

Murky associations

(2009)

Genome-wide association studies have

been hailed for their ability to find genetic

variations that may contribute to disease risk.

But assigning meaning to these variations is

more difficult. Peter Holmans of the MRC

Centre for Neuropsychiatric Genetics and

Genomics in Cardiff, UK, and his colleagues

have developed an approach that they call

ALIGATOR.

They look for gene ontology categories

— agreed-upon terms used to define the

function and activity of gene products — that

regularly pop up in the candidates exposed

by genome-wide association, reasoning

that many associations in the same category

signal functional relevance. For Crohn’s

disease, which is immunological in origin,

overrepresented ontological categories

included immune functions, as expected.

For bipolar disorder, they included hormone

activity and RNA splicing, processes with as

yet unknown roles in the condition.

ASTRONOMY

Little neighbours

How does one find dwarves in a crowd of

giants? Evgenya Shkolnik at the Carnegie

Institution of Washington in Washington

DC and her colleagues searched X-ray

data gathered by the now-defunct German

satellite ROSAT for nearby M-class dwarf

stars less than 300 million years old.

Previous surveys placed greater emphasis

on higher mass, higher luminosity Sunlike

stars. The team identified 185 likely

candidates before ruling out older interlopers

by spectroscopy. The 144 remaining, some

as small as 10% of the mass of the Sun, have

a better chance than easier-to-spot, highermass

stars of revealing the early formation of

rocky Earth-like planets.

If any of the dwarves do host planets,

their proximity to Earth (most are within 25

parsecs) will make them relatively easy to

study in detail.

CHEMISTRY

Fire boxed

Science 324, 1697–1699 (2009)

White phosphorus combusts violently in air,

making it a dangerous agent, and one that has

become controversial for its military uses.

of Cambridge, UK, and his co-workers have

constructed a molecular cage to defuse

this version of phosphorus. The cage selfassembles

in water from organic groups and

iron ions. A solution of the cage can suck up

solid white phosphorus, trapping its small,

tetrahedral molecules within the cages’ larger

ones (pictured, below).

The cage doesn’t prevent oxygen reaching

the white phosphorus, but does stop it

reacting because there isn’t enough room

within the cage for the normal products of

the reaction to form.

Benzene can displace the incendiary

molecule, releasing it from its confinement.

BIOLOGY

Shell shocker

Big eggs risk being crushed by the big birds

that sit on them. Eggshells must be sturdy, but

not so sturdy that they entomb chicks. These

conflicting demands placed on eggshells

set the upper size limit for birds, according

to Geoffrey Birchard of George Mason

University in Fairfax, Virginia, and Charles

Deeming of the University of Lincoln, UK.

They are also, the authors suggest, the reason

that females outweigh males in the largest of

species.

The researchers analysed shell thickness,

body mass and incubation behaviour for 968

species of bird. Having relatively small males

do the incubating allows eggs to be bigger

yet thinner-shelled. The largest specimens

of extinct giants such as the 400-kilogram

elephant bird might all be female, they

speculate.

EVOLUTIONARY DEVELOPMENT

The birth of a thymus

In a broad-ranging study, Thomas Boehm of

the Max Planck Institute of Immunobiology

in Freiburg, Germany, and his colleagues

attempt to reconstruct the evolutionary

history of the thymus, the immune-system

organ where T cells accumulate and mature.

The authors looked at the expansion and

diversification of genes implicated in T-cell

and thymus development in 13 species

spanning the chordate phylogenetic tree.

They paid special attention to the jawed

vertebrates, which have a thymus, and jawless

fishes such as lampreys, which do not. The

authors conclude that the latter have many

but not all of the genes required to develop a

thymus. The study begins to show how the

duplication and cooption of genetic pathways

leads to the development of a complex organ.

ECOLOGY

Putting height on the map

Average plant height peaks in the tropics

and declines towards the poles, says an

international team after analysing a data set

of almost 6,000 species. Plants living near

to the equator are 29 times taller on average

than those found between 60–75 °N (in

Iceland, say) and 31 times taller than those

at 45–60 °S (such as on the South American

archipelago of Tierra del Fuego).

Plant height drops 2.4-fold at the edge of the

tropics, suggesting that temperate and tropical

species pursue different ecological strategies,

according to Angela Moles of the University of

New South Wales in Sydney, Australia, and her

colleagues. Cold or dry places support plants

with a range of heights, but there are few short

species in warm, wet environments. Rainfall

in the wettest month of the year is the best

predictor of plant height, the researchers say.

PHYSIOLOGY

Ground control

The limits on acceleration are commonly

described in terms of horsepower, be it from

internal combustion or accompanied by real

hoofbeats.

But motorbike and drag-racing aficionados

know that too much acceleration can lift front

wheels off the ground, sacrificing control.

Sarah Williams and her colleagues from the

Royal Veterinary College in Hatfield, UK, have

found that quadrupeds face the same problem.

Data from racing greyhounds (pictured,

below) and polo ponies showed that

acceleration limits at high speeds are

predicted by models of muscle power. At

lower speeds, however, animals do not

reach the acceleration theoretically possible

from muscle power. Instead they seem to

be limited by the rate that would cause the

quadruped equivalent of a ‘wheelie’.

NEUROSCIENCE

Early bird learns the tune

Scientists have assumed that vocal learning

in songbirds starts with subsongs — the bird

equivalent of infant babbling. But Wan-chun

Liu from the Rockefeller University, New

York, and his colleagues, have determined

that vocal learning begins much earlier —

when juveniles first beg for food.

When begging, juvenile male chipping

sparrows (Spizella passerina) exhibit neural

activity in part of the forebrain associated with

learned song, the team found. And begging

patterns appear in the sparrows’ first subsongs.

Auditory feedback is crucial to vocal

learning, and distinguishes it from innate

calls. Deafening affected the structures of

males’ begging calls, but had no such effect

on females; female chipping sparrows do not

sing and their begging calls are innate.

CANCER BIOLOGY

Double agent

A protein associated with some cancers acts

in the energy-producing organelles of the cell

called mitochondria.

The protein, STAT3, regulates gene

expression. It is activated by the addition

of a phosphate group, which sends the

protein into the nucleus. Transformation

of healthy cells into cancerous ones by the

small protein Ras is inhibited by an absence

of normal STAT3. But David Levy of New

York University School of Medicine and his

colleagues found that STAT3 mutants that

cannot accept the phosphate or cannot bind to

DNA still allow Ras to transform cells.

Surprisingly, the researchers found

that STAT3 is active in mitochondrial

metabolism, and during transformation

mediates metabolic changes necessary for

cancers to grow.

Losing Louisiana

Losing Louisiana

Nature Geosci. 2, 488–491 (2009)

The Mississippi Delta is sinking, and to rebuild it

some have proposed diverting the Mississippi River

to carry sediment to coastal areas. But a new study

shows that the delta’s rivers don’t have enough

sediment to counter predicted sea-level rise.

Mike Blum and Harry Roberts, both then

working at Louisiana State University in Baton

Rouge, found that the rivers currently transport

less sediment than the amount that was needed

for delta formation. Sinking land and accelerating

sea-level rise will submerge 10,500–13,500 square

kilometres by 2100, they predict.

Even if sediment trapped in dams was freed, the

duo found, the rivers still couldn’t supply the 18–24

billion tonnes needed to keep the delta above water

until the end of the century.

Carbon Dioxide

Time for early action

Carbon dioxide is not the only warming agent

worth tackling now in the bid to cool the planet.

The US House of Representatives must be commended for passing

a comprehensive climate bill last week that would finally set the

United States on a path to lower its greenhouse-gas emissions.

The pending legislation is far from perfect and will face a tough test

in the Senate. But it is a necessary first step for the country that has so

far added the most carbon dioxide to the atmosphere.

However, it will take several decades, if not longer, for the United

States and other nations to significantly reduce carbon dioxide emissions.

This fact has spurred many scientists to intensify research

into techniques that might provide a more immediate way to turn

down the planetary thermostat. Some solutions seek to fine-tune

Earth’s climate through large-scale geoengineering projects, such as

pumping sulphates into the atmosphere to reflect more sunlight back

into space. But another approach — an ‘early action’ climate agenda

increasingly being pushed by environmentalists and some scientists

— might prove safer and much easier to sell to governments and

populations around the world.

A good place to start is with black carbon, the sooty, dark component

of smoke that emanates from diesel engines, inefficient cooking stoves,

forest fires and the like. Black carbon is a danger to human health, having

been implicated in a variety of cardiovascular and respiratory diseases,

and may also be a major player when it comes to global warming,

particularly in regions that spend at least part of the year covered with

ice and snow (see page 29). But unlike carbon dioxide, which stays in

the atmosphere for centuries, black-carbon particles remain in the air

for just a matter of weeks. So, in principle, efforts to eliminate emissions

could quickly reduce the warming power of this pollutant.

Unfortunately, it is not that simple. First, not all black carbon is

anthropogenic in origin. Second, black carbon is accompanied to

varying degrees by its lighter-coloured cousin, organic carbon, which

cools the planet along with most other reflective aerosols. Third,

despite more than a decade of research, the chain of reactions by

which black carbon warms the atmosphere and melts snow remains

surprisingly hazy. All of these factors make black carbon’s effect on

climate difficult to quantify.

However, none of the caveats is a reason for nations not to try their

utmost to control it. Even if the climate benefits turn out to be less

than hoped, cleaner air would save hundreds of thousands of lives

a year. Governments should already be working to clean up diesel

emissions and to improve cooking stoves in southeast Asia, where

the health problems are most acute. But it may be that the threat of

global warming is more effective than health advocacy.

Accompanying black carbon on the early-action agenda are methane

and hydrofluorocarbons (HFCs). The latter are used predominantly

as refrigerants, and can be more than a thousand times more powerful

than CO2 as greenhouse gases (see Nature 459, 1040–1041; 2009).

The international ‘Methane to Markets’ programme provides

money and expertise to help countries to capture methane from

sources such as landfills, farms and coal mines. Once up and running,

these projects produce energy at a profit by making use of a

clean-burning compound that would otherwise have wreaked havoc

in the atmosphere for years. The programme has been a resounding

success, and other nations should find the money and will to

replicate it.

The case for HFCs is a bit more complicated, but no less strong.

HFCs were developed to replace ozone-depleting chlorofluorocarbons

that were phased out in response to the 1989 Montreal

Protocol. Because they don’t affect atmospheric ozone levels, they

were pulled under the United Nations’ Kyoto climate treaty. But

there are now calls to take the chemicals out of the climate treaty

and put them into the Montreal Protocol. This is the right thing to

do. Montreal regulators have already proved their ability to implement

worldwide curbs on emissions, and there is little doubt that they

could handle this problem faster and more cost-effectively than could

a cumbersome treaty aimed at targeting CO2.

Notably, each of these solutions could be pursued immediately,

precisely because they make sense on multiple levels; global warming

might be a driving factor, but it is not the only one. There is no

need to wait for international negotiators to strike a deal on a climate

treaty that would lay the groundwork for a global carbon market. And

they do not pose the ethical or legal challenges that geoengineering

schemes so often face.

Some fear that even talking about such subjects could distract from

the main problem, which is CO2. The opposite is true. Providing workable

solutions in other areas will build momentum and simultaneously

ease the burden that remains. What is there to lose?