An international team of scientists, led by physicist Nathaniel M. Gabor at the University of California, Riverside, has now constructed a model that reproduces a general feature of photosynthetic light harvesting, observed across many photosynthetic organisms.

Light harvesting is the collection of solar energy by protein-bound chlorophyll molecules. In photosynthesis — the process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water — light energy harvesting begins with sunlight absorption.

The researchers’ model borrows ideas from the science of complex networks, a field of study that explores efficient operation in cellphone networks, brains, and the power grid. The model describes a simple network that is able to input light of two different colors, yet output a steady rate of solar power. This unusual choice of only two inputs has remarkable consequences.

“Our model shows that by absorbing only very specific colors of light, photosynthetic organisms may automatically protect themselves against sudden changes — or ‘noise’ — in solar energy, resulting in remarkably efficient power conversion,” said Gabor, an associate professor of physics and astronomy, who led the study appearing today in the journal Science. “Green plants appear green and purple bacteria appear purple because only specific regions of the spectrum from which they absorb are suited for protection against rapidly changing solar energy.”

Gabor first began thinking about photosynthesis research more than a decade ago, when he was a doctoral student at Cornell University. He wondered why plants rejected green light, the most intense solar light. Over the years, he worked with physicists and biologists worldwide to learn more about statistical methods and the quantum biology of photosynthesis.

Richard Cogdell, a botanist at the University of Glasgow in the United Kingdom and a coauthor on the research paper, encouraged Gabor to extend the model to include a wider range of photosynthetic organisms that grow in environments where the incident solar spectrum is very different.

“Excitingly, we were then able to show that the model worked in other photosynthetic organisms besides green plants, and that the model identified a general and fundamental property of photosynthetic light harvesting,” he said. “Our study shows how, by choosing where you absorb solar energy in relation to the incident solar spectrum, you can minimize the noise on the output — information that can be used to enhance the performance of solar cells.”

Coauthor Rienk van Grondelle, an influential experimental physicist at Vrije Universiteit Amsterdam in the Netherlands who works on the primary physical processes of photosynthesis, said the team found the absorption spectra of certain photosynthetic systems select certain spectral excitation regions that cancel the noise and maximize the energy stored.

“This very simple design principle could also be applied in the design of human-made solar cells,” said van Grondelle, who has vast experience with photosynthetic light harvesting.

Gabor explained that plants and other photosynthetic organisms have a wide variety of tactics to prevent damage due to overexposure to the sun, ranging from molecular mechanisms of energy release to physical movement of the leaf to track the sun. Plants have even developed effective protection against UV light, just as in sunscreen.

“In the complex process of photosynthesis, it is clear that protecting the organism from overexposure is the driving factor in successful energy production, and this is the inspiration we used to develop our model,” he said. “Our model incorporates relatively simple physics, yet it is consistent with a vast set of observations in biology. This is remarkably rare. If our model holds up to continued experiments, we may find even more agreement between theory and observations, giving rich insight into the inner workings of nature.”

To construct the model, Gabor and his colleagues applied straightforward physics of networks to the complex details of biology, and were able to make clear, quantitative, and generic statements about highly diverse photosynthetic organisms.

“Our model is the first hypothesis-driven explanation for why plants are green, and we give a roadmap to test the model through more detailed experiments,” Gabor said.

Photosynthesis may be thought of as a kitchen sink, Gabor added, where a faucet flows water in and a drain allows the water to flow out. If the flow into the sink is much bigger than the outward flow, the sink overflows and the water spills all over the floor.

“In photosynthesis, if the flow of solar power into the light harvesting network is significantly larger than the flow out, the photosynthetic network must adapt to reduce the sudden over-flow of energy,” he said. “When the network fails to manage these fluctuations, the organism attempts to expel the extra energy. In doing so, the organism undergoes oxidative stress, which damages cells.”

The researchers were surprised by how general and simple their model is.

“Nature will always surprise you,” Gabor said. “Something that seems so complicated and complex might operate based on a few basic rules. We applied the model to organisms in different photosynthetic niches and continue to reproduce accurate absorption spectra. In biology, there are exceptions to every rule, so much so that finding a rule is usually very difficult. Surprisingly, we seem to have found one of the rules of photosynthetic life.”

Gabor noted that over the last several decades, photosynthesis research has focused mainly on the structure and function of the microscopic components of the photosynthetic process.

“Biologists know well that biological systems are not generally finely tuned given the fact that organisms have little control over their external conditions,” he said. “This contradiction has so far been unaddressed because no model exists that connects microscopic processes with macroscopic properties. Our work represents the first quantitative physical model that tackles this contradiction.”

Next, supported by several recent grants, the researchers will design a novel microscopy technique to test their ideas and advance the technology of photo-biology experiments using quantum optics tools.

“There’s a lot out there to understand about nature, and it only looks more beautiful as we unravel its mysteries,” Gabor said.

A team of scientists from the Max Planck Institute for Solar System Research, the University of Göttingen and New York University Abu Dhabi has now succeeded in drawing the most comprehensive picture of the plasma flows in nort-south-direction to date. The researchers have found a remarkably simple flow geometry: the plasma describes a single turnover in each solar hemisphere, which lasts for about 22 years. In addition, the flow in the direction of the equator at the bottom of the convection zone causes spots to form closer and closer to the equator during the solar cycle.

The number of sunspots on the visible solar surface varies; sometimes there are more, sometimes fewer. The distance between two sunspot maxima is about eleven years, after 22 years the sunspots are again magnetically polarized in the same way. During the maximum not only large sunspots appear, but also active regions. In addition, impressive arcs of hot plasma reach far into the solar atmosphere, particles and radiation are hurled into space in violent eruptions. At the activity minimum, however, the sun calms down noticeably.

“Over the course of a solar cycle, the meridional flow acts as a conveyor belt that drags the magnetic field along and sets the period of the solar cycle,” says Prof. Dr. Laurent Gizon, MPS Director and first author of the new study. “Seeing the geometry and the amplitude of motions in the solar interior is essential to understanding the Sun’s magnetic field,” he adds. To this end, Gizon and his team used helioseismology to map the plasma flow below the Sun’s surface.

Helioseismology is to solar physics what seismology is to geophysics. Helioseismologists use sound waves to probe the Sun’s interior, in much the same way geophysicists use earthquakes to probe the interior of the Earth. Solar sound waves have periods near five minutes and are continuously excited by near surface convection. The motions associated with solar sound waves can be measured at the Sun’s surface by telescopes on spacecrafts or on the ground.

In this study, Gizon and his team used observations of sound waves at the surface that propagate in the north-south direction through the solar interior. These waves are perturbed by the meridional flow: they travel faster along the flow than against the flow. These very small travel-time perturbations (less than 1 second) were measured very carefully and were interpreted to infer the meridional flow using mathematical modeling and computers.

Because it is small, the meridional flow is extremely difficult to see in the solar interior. “The meridional flow is much slower than other components of motion, such as the Sun’s differential rotation,” Gizon explains. The meridional flow throughout the convection zone is no more than its maximum surface value of 50 kilometers per hour. “To reduce the noise level in the helioseismic measurements, it is necessary to average the measurements over very long periods of time,” says Dr. Zhi-Chao Liang of MPS.

The team of scientists analyzed, for the first time, two independent very long time series of data. One was provided by SOHO, the oldest solar observatory in space which is operated by ESA and NASA. The data taken by SOHO’s Michelson Doppler Imager (MDI) covers the time from 1996 until 2011. A second independent data set was provided by the Global Oscillation Network Group (GONG), which combines six ground-based solar telescopes in the USA, Australia, India, Spain, and Chile to offer nearly continuous observations of the Sun since 1995.

“The international solar physics community is to be commended for delivering multiple datasets covering the last two solar cycles,” says Dr. John Leibacher, a former director of the GONG project. “This makes it possible to average over long periods of time and to compare answers, which is absolutely essential to validate inferences,” he adds.

Gizon and his team find the flow is equatorward at the base of the convection zone, with a speed of only 15 kilometers per hour (running speed). The flow at the solar surface is poleward and reaches up to 50 kilometers per hour. The overall picture is that the plasma goes around in one gigantic loop in each hemisphere. Remarkably, the time taken for the plasma to complete the loop is approximately 22 years — and this provides the physical explanation for the Sun’s eleven-year cycle.

Furthermore, sunspots emerge closer to the equator as the solar cycle progresses, as is seen in the butterfly diagram. “All in all, our study supports the basic idea that the equatorward drift of the locations where sunspots emerge is due to the underlying meridional flows,” says Dr. Robert Cameron of MPS. “It remains to be understood why the solar meridional flow looks like it does, and what role the meridional flow plays in controlling magnetic activity on other stars” adds Laurent Gizon.

Dogs play an important role in human life all over the world — whether as a family member or as a working animal. But where the dog comes from and how old various groups of dogs are is still a bit of a mystery.

Now, light has been shed on the origin of the sledge dog. In a new study published in SCIENCE, researchers from the Faculty of Health and Medical Sciences, University of Copenhagen, show that the sledge dog is both older and has adapted to the Arctic much earlier than thought. The research was conducted in collaboration with the University of Greenland and the Institute of Evolutionary Biology, Barcelona.

“We have extracted DNA from a 9,500-year-old dog from the Siberian island of Zhokhov, which the dog is named after. Based on that DNA we have sequenced the oldest complete dog genome to date, and the results show an extremely early diversification of dogs into types of sledge dogs,” says one of the two first authors of the study, PhD student Mikkel Sinding, the Globe Institute.

Until now, it has been the common belief that the 9,500-year-old Siberian dog, Zhokhov, was a kind of ancient dog — one of the earliest domesticated dogs and a version of the common origin of all dogs. But according to the new study, modern sledge dogs such as the Siberian Husky, the Alaskan Malamute and the Greenland sledge dog share the major part of their genome with Zhokhov.

“This means that modern sledge dogs and Zhokhov had the same common origin in Siberia more than 9,500 years ago. Until now, we have thought that sledge dogs were only 2-3,000 years old,” says the other first author, Associate Professor Shyam Gopalakrishnan, Globe Institute.

The Original Sledge Dog

To learn more about the origins of the sledge dog, researchers have further sequenced genomes of a 33,000-year-old Siberian wolf and ten modern Greenlandic sledge dogs. They have compared these genomes to genomes of dogs and wolves from around the world.

“We can see that the modern sledge dogs have most of their genomes in common with Zhokhov. So, they are more closely related to this ancient dog than to other dogs and wolves. But not just that — we can see traces of crossbreeding with wolves such as the 33,000-year-old Siberian wolf — but not with modern wolves. It further emphasises that the origin of the modern sledge dog goes back much further than we had thought,” says Mikkel Sinding.

The modern sledge dogs have more genetic overlap with other modern dog breeds than Zhokhov has, but the studies do not show us where or when this occurred. Nevertheless, among modern sledge dogs, the Greenland sledge dogs stands out and has the least overlap with other dogs, meaning that the Greenland sledge dog is probably the most original sledge dog in the world.

Common Features with Inuit and Polar Bears

In addition to advancing the common understanding of the origin of sledge dogs, the new study also teaches the researchers more about the differences between sledge dogs and other dogs. Sledge dogs do not have the same genetic adaptations to a sugar and starch rich diet that other dogs have. On the other hand, they have adaptations to high-fat diets, with mechanisms that are similar to those described for polar bears and Arctic people.

“This emphasises that sledge dogs and Arctic people have worked and adapted together for more than 9,500 years. We can also see that they have adaptations that are probably linked to improved oxygen uptake, which makes sense in relation to sledding and give the sledding tradition ancient roots,” says Shyam Gopalakrishnan.