Arctic in bloom: algae under ice

Scientists sponsored by Nasa, the US space agency, have made an ecological discovery on Earth, which they say is as unexpected as finding a rainforest in the middle of a desert.

Beneath a metre of Arctic sea ice – and 100km from the nearest unfrozen waters – they found higher concentrations of microscopic phytoplankton, the foundation of the marine food chain, than in any other ocean on the planet. The intense sub-glacial “algal bloom”, which marine biologists would previously have said was impossible, may be a sign of global warming, as Arctic sea ice thins sufficiently for summer sunshine to filter through to the water beneath.

The discovery, published online by the journal Science, was made during the Icescape expedition, in which Nasa sent an icebreaker through the Chukchi ice shelf north-west of Alaska.

“Part of Nasa’s mission is pioneering scientific discovery, and this is like finding the Amazon rainforest in the middle of the Mojave desert,” says Paula Bontempi, head of the agency’s ocean biology programme. “We embarked on Icescape to validate our satellite ocean-observing data in an area of the Earth that is very difficult to get to. We wound up making a discovery that hopefully will help researchers and resource managers better understand the Arctic.”

Marine biologists had believed phytoplankton grew only in open water. Now it seems that the ice can be thin enough to allow sunlight to catalyse algal blooms without it melting completely.

Indeed the Icescape researchers found that the ice shelf melts in a way that concentrates the sunlight and enhances algal growth. Extensive pools of meltwater form on top of the ice, reducing its reflectivity and acting like magnifying lenses.

Under these advantageous conditions, “growth rates under the ice are higher than I thought was possible for Arctic phytoplankton,” says Kevin Arrigo of Stanford University, head of the Icescape scientific team. “Algal cells that would normally take three days to divide were doubling more than once a day.”

Although this is the first observation of a bloom under the ice, the conditions that allow it exist over a rapidly expanding area of the Arctic. The discovery has implications for the broader ecosystem because vast amounts of phytoplankton are consumed by small ocean animals that in turn feed larger creatures including fish, whales and birds. Which species benefit and which lose out remains to be seen.

There may also be consequences for the oceans’ energy balance, and for the global carbon cycle, if sub-glacial phytoplankton absorb significant amounts of carbon dioxide.

Size matters: how cells know when to grow

How does a cell sense its size and know when to stop growing? A British-Israeli research collaboration is currently answering that important biological question.

The scientists have discovered that “molecular motors”, proteins that transport molecules within the cell, play a key role in sensing and maintaining size. These motors not only carry biochemical cargo but also represent a signalling system that operates like radar or sonar – in slow motion.

Michael Fainzilber and colleagues at the Weizmann Institute, working with Giampietro Schiavo of Cancer Research UK and Elizabeth Fisher of University College London, reached their conclusions – published in the journal Cell Reports – by combining experimental biology, computer simulation and signalling theory.

Although all cells need to sense their size, the problem is most acute in the longest ones: nerve cells. A human neuron may extend for as much as a metre from its central “cell body” to the tip of its tentacle-like “axon”.

One type of molecular motor transports cargo outward from the cell body to the periphery, while another type carries molecules back to the centre. Both travel along a nano-scale track called a microtubule. The neuron calculates its length from the movement of these motors travelling along the microtubule. The processing is similar to that in radar and sonar systems, which calculate the distance to an object from the time taken for pulses of radio or sound waves to bounce off and return to a receiver. But while radar and sonar give a near-instantaneous measurement, the cellular system is slower. The molecular motors travel a few centimetres a day (unlike the speedy transmission of electrical signals along nerves), but this is sufficient for the biological purpose of measuring cell length.

Fainzilber hopes that manipulating the size sensing system will be a new route to stimulating the growth of damaged neurons.

New evidence for Devon’s deep freeze

Glaciers extended further south in England during the last Ice Age than climate historians had previously realised – and Dartmoor had an ice cap up to 100 metres thick – according to an extensive study of geological features in Devon.

Researchers from Durham and Exeter universities and Stockton Riverside College concluded that Dartmoor’s distinctive features could only have been shaped by a thick ice cap.

The evidence was obtained from detailed observations on the ground and aerial photography. Glacial features include elongated rounded mounds (known as drumlins) and hummocky landforms that suggest the moraines of ancient glaciers.

The distribution of Dartmoor’s characteristic granite outcrops or tors is also compelling, the researchers say. On the highest summits in the middle of Dartmoor, the ice cap either destroyed and carried away tors or prevented their formation over thousands of years. However, the distinctive outer tors survived because the ice layer there was not substantial enough to destroy them.

Although the geology does not reveal the age of Dartmoor’s most recent glaciation, the researchers believe it was during the so-called Younger Dryas cold period, which ended 11,500 years ago.

“A landscape that has been regarded as a classic example of cold, non-glacial processes was in fact covered by a glacial ice cap,” says David Evans, geography professor at Durham. The evidence for this is “so subtle that researchers had missed it for almost 100 years.”

How humans nearly became extinct

About 100,000 years ago, the steadily growing population of our human ancestors in Africa experienced a sudden decline, pushing humanity to the brink of extinction with fewer than 10,000 people in existence, writes Katherine Rowland. Then, over the millennia that followed, the predecessors of modern-day humans bounced back and began to range around the globe.

Long at a loss to explain this mysterious evolutionary shift, scientists now propose that infectious disease may have played a central role in both decimating numbers and determining survival.

A new genetic analysis of molecular fossils, led by researchers at the University of California San Diego, suggests that a “massive pathogenic menace” swept through the populations of early man. The findings, published in the Proceedings of the National Academy of Sciences, indicate that bacterial infections particularly lethal to infants targeted two genes related to the immune system, called Siglec-13 and Siglec-17.

While the genes remain present in chimpanzees, man’s closest evolutionary cousins, Siglec-17 is inactive and Siglec-13 has been deleted altogether from the genome in all living humans. The researchers suggest that this genetic mutation among the tiny, emergent population of anatomically modern humans likely aided the survival of the species.

Though many factors shaped human origins, the researchers say: “We think infectious agents are one of them.”

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