Scientists from around the world have reconstructed changes in Earth’s ancient ocean chemistry during a broad sweep of geological time, from about 2.5 to 0.5 billion years ago. They have discovered that a deficiency of oxygen and the heavy metal molybdenum in the ancient deep ocean may have delayed the evolution of animal life on Earth for nearly 2 billion years.

The researchers arrived at their result by tracking molybdenum in black shales, which are a kind of sedimentary rock rich in organic matter and usually found in the deep ocean. Molybdenum is a key micronutrient for life and serves as a proxy for oceanic and atmospheric oxygen amounts.

Following the initial rise of oxygen in the Earth’s atmosphere 2.4 billion years ago, oxygen was transferred to the surface ocean to support oxygen-demanding microorganims. Yet the diversity of these single-celled life forms remained low, and their multicellular ancestors, the animals, did not appear until about 600 million years ago, explained Timothy Lyons, a professor of biogeochemistry in the Department of Earth Sciences at the University of California, Riverside, and one of the study’s authors.

Suspecting that deficiencies in oxygen and molybdenum might explain this evolutionary lag, Lyons and his colleagues measured abundances of molybdenum in ancient marine sediments over time to estimate how much of the metal had been dissolved in the seawater in which the sediments formed.

The researchers found significant, firsthand evidence for a molybdenum-depleted ocean relative to the high levels measured in modern, oxygen-rich seawater.

Molybdenum is of particular interest because it is used by some bacteria to convert the element nitrogen from a gas in the atmosphere to a form useful for living things — a process known as “nitrogen fixation.” Bacteria cannot fix nitrogen efficiently when they are deprived of molybdenum. And if bacteria can’t fix nitrogen fast enough then eukaryotes — a kind of organism that includes plants, pachyderms and people — are in trouble because eukaryotes cannot fix nitrogen themselves at all.

“These molybdenum depletions may have retarded the development of complex life such as animals for almost two billion years of Earth history,” Lyons said. “The amount of molybdenum in the ocean probably played a major role in the development of early life. As in the case of iron today, molybdenum can be thought of as a life-affirming micronutrient that regulates the biological cycling of nitrogen in the ocean.

“At the same time, molybdenum’s low abundance in the early ocean tracks the global extent of oxygen-poor seawater and implies that the amount of oxygen in the atmosphere was still low.

“Knowing the amount of oxygen in the early ocean is important for many reasons, including a refined understanding of how and when appreciable oxygen first began to accumulate in the atmosphere,” Lyons said. “These steps in oxygenation are what gave rise ultimately to the first animals almost 600 million years ago — just the last tenth or so of Earth history.”

Earth’s oxygenation

For animal life to commence, survive and eventually expand on Earth, a threshold amount of oxygen — estimated to be on the order of 1 to 10 percent of present atmospheric levels of oxygen — was needed.

Past research has shown that Earth’s oxygenation occurred in two major steps:

• The first step, around 2.4 billion years ago, took place as the ocean transitioned to a state where only the surface ocean was oxygenated by photosynthesizing bacteria, while the deep ocean was relatively oxygen-free.
• The second step, around 600 million years ago, marked the occasion when the entire ocean became fully oxygenated through a process not yet fully understood.

“We wanted to know what the state of the ocean was between the two steps,” said Clinton Scott, a graduate student working in Lyons’s lab and the first author of the research paper. “By tracking molybdenum in shales rich in organic matter, we found the deep ocean remained oxygen- and molybdenum-deficient after the first step. This condition may have had a negative impact on the evolution of early eukaryotes, our single-celled ancestors. The molybdenum record also tells us that the deep ocean was already fully oxygenated by around 550 million years ago.”

According to Scott, the timing of the oxygenation steps suggests that significant events in Earth history are related. Scientists have long speculated that the evolution of the first animals was linked somehow to the so-called Snowball Earth hypothesis, which posits that the Earth was covered from pole to pole in a thick sheet of ice for millions of years at a time. “The second oxygenation step took place not long after the last Snowball Earth episode ended around 600 million years ago,” Scott said. “So one question is: Did this global glaciation play a role in the increasing abundance of oxygen which, in turn, enabled the evolution of animals?”

The study results appear in the March 27 issue of Nature.

Scott and Lyons were joined in the research by A. Bekker of the Carnegie Institution of Washington, DC; Y. Shen of the Université du Québec à Montréal, Canada; S.W. Poulton of Newcastle University, Newcastle upon Tyne, United Kingdom; X. Chu of the Chinese Academy of Sciences, Beijing, China; and A.D. Anbar of Arizona State University, Tempe, Ariz.

The research was supported by grants from the U.S. National Science Foundation Division of Earth Sciences and the NASA Astrobiology Institute.

More about molybdenum as a proxy for ocean chemistry

Molybdenum, a metal abundant in the ocean today but less so at times in the past, is an excellent tracer of ancient chemistry for two reasons. First, the primary source of molybdenum to the ocean is oxidative weathering of continental crust, requiring oxygen in the atmosphere. Second, molybdenum is removed primarily in marine sediments where oxygen is absent and sulfide is abundant. Thus the enrichment of molybdenum in ancient organic-rich shales requires oxygen in the atmosphere but high sulfur and very low or no oxygen in the deep ocean. This combination is relatively rare today but may have been common when oxygen was less abundant in the earlier atmosphere.

When oxygen is available in the atmosphere, the amount of dissolved molybdenum in seawater is determined by the extent of hydrogen-sulfide-containing sediments and bottom waters (the colder, more isolated, lowermost layer of ocean water). Where sulfidic environments are widespread, the pool of molybdenum remaining in seawater is small, growing as the sulfidic environments shrink. The amount of molybdenum in the seawater is reflected in the magnitude of molybdenum enrichment in shales deposited in the deep ocean.

The UCR-led team of researchers estimated the size of the oceanic reservoir, and thus the extent of sulfidic bottom waters and sediments, based on the concentration of molybdenum in ancient black shales. They did so by dissolving the samples in a cocktail of acids and analyzing the dissolved rock for concentration using a mass spectrometer. The amount of this metal in the shales tracks the oxygen state of the early ocean and atmosphere and also points to the varying abundance of this essential ingredient of life. Molybdenum limitations may have delayed the development of eukaryotes, including the first animals, our earliest multicellular cousins.

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Mantis shrimp can see the world in a way that had never been observed in any animal before, researchers report in the March 20th Current Biology, a Cell Press publication. The discovery–which marks the fourth type of visual system–suggests that the ability to perceive circular polarized light may lend mantis shrimp a secret mode of communication.

“Mantis shrimp ventured into a new dimension of vision,” said Justin Marshall of the University of Queensland in Australia. Also known as stomatopods, mantis shrimp are large and particularly violent marine crustaceans that aren’t actually a kind of shrimp but look something like one.

Marshall describes circular polarized light as a spiraling beam that spins either to the left or the right. Scientists had shown before that some animals, such as scarab beetles, reflect that kind of light, but they hadn’t shown that any animal could actually see it–until now, that is.

“It’s complicated physics,” Marshall said, “but that makes it all the more amazing that some animals would use it for something.” Using it required the stomatopods to evolve a kind of filter in their eyes oriented at a precise 45 degree angle to photoreceptors underneath that pick up on linearly polarized light. The filter turns the circularly polarized light into its linear form. Many animals make use of linearly polarized light, Marshall said. To people, however, it is only glare, hence the need for polarized sun glasses.

In the new study, the researchers describe the anatomical basis for stomatopods’ remarkable vision in detail and show that these structures are stimulated when circular polarized light shines into them. They also offer behavioral proof of the stomatopods’ ability by training them to associate either left-handed or right-handed circular polarized light (L-CPL or R-CPL) with a food reward.

During tests, when no food was present, the researchers presented the animals with two feeding tubes, one reflecting L-CPL and the other R-CPL. The stomatopods chose the tube reflecting the CPL handedness to which they had originally been trained at levels significantly above chance, the researchers found.

Although it’s not yet clear exactly what the mantis shrimps’ newfound visual ability is good for in nature, Marshall said it’s likely all about sex.

Stomatopods are known to use highly specialized color and linear polarization signals for complex social interactions, he noted. And by using circular-polarization imaging, his team has identified three species of stomatopods (within the genus Odontodactus) where CPL is reflected from the cuticles of males but not females. Those sex-specific reflective areas are on parts of the body that stomatopods frequently use for behavioral displays.

“The precise role that these signals, visible to a CPL visual system, play in stomatopod sexual signaling is not yet known, but we speculate that these CPL reflections could act as a secret communication channel,” the researchers concluded. “Linear polarization signals, used by marine invertebrates, are visible to animals like cephalopods that prey on stomatopods and are therefore open to exploitation. Also, other genera of stomatopods that we have examined have variable CPL sensitivity, and may be unable to view the sexual displays of Odontodactylus species, making this a private channel of communication, unavailable to both predators and potential stomatopod competitors.

“Whatever the use of CPL signals and CPL vision to stomatopods, comparing design features of their CPL reflectors and sensors to those of man-made systems will be interesting,” they added. “Humans use CPL filters and imaging in everyday photography, medical photography, and object-detection systems in turbid environments. The reefs and waters that many stomatopods inhabit are often turbid, and it is perhaps no surprise that, perhaps as long as 400 million years ago (when stomatopod crustaceans first appeared), nature got there first.”

The researchers include Tsyr-Huei Chiou, Department of Biological Sciences, University of Maryland Baltimore County, Baltimore, MD; Sonja Kleinlogel, Sensory Neurobiology Group, Vision Touch and Hearing Research Centre, School of Biomedical Sciences and Queensland Brain Institute, University of Queensland, Brisbane, Queensland, Australia; Tom Cronin, Department of Biological Sciences, University of Maryland Baltimore County, Baltimore, MD; Roy Calwell, Department of Integrative Biology, University of California, Berkeley, Berkeley, CA; Birte Loeffler, Sensory Neurobiology Group, Vision Touch and Hearing Research Centre, School of Biomedical Sciences and Queensland Brain Institute, University of Queensland, Brisbane, Queensland, Australia; Afsheen Siddiqi, Department of Biological Sciences, University of Maryland Baltimore County, Baltimore, MD;

Alan Goldizen, Sensory Neurobiology Group, Vision Touch and Hearing Research Centre, School of Biomedical Sciences and Queensland Brain Institute, University of Queensland, Brisbane, Queensland, Australia; and Justin Marshall, Sensory Neurobiology Group, Vision Touch and Hearing Research Centre, School of Biomedical Sciences and Queensland Brain Institute, University of Queensland, Brisbane, Queensland, Australia.

This work was supported by grants from the Asian Office of Aerospace Research and Development, the Air Force Office of Scientific Research, the Australian Research Council, the National Science Foundation, and the Swiss National Foundation.

Journal reference: Chiou et al.: “Circular Polarization Vision in a Stomatopod Crustacean.” Publishing in Current Biology 18, 1–6, March 25, 2008. DOI 10.1016/j.cub.2008.02.066.

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Researchers at The University of Manchester and Chernogolovka, Russia have discovered the world’s first single-atom-thick fabric, which reveals the existence of a new class of materials and may lead to computers made from a single molecule. The research is to be published in Science on 22 October.

The team led by Professor Andre Geim at The University of Manchester, has succeeded in extracting individual planes of carbon atoms from graphite crystals, which has resulted in the production of the thinnest possible fabric - graphene. The resulting atomic sheet is stable, highly flexible and strong and remarkably conductive. The nanofabric belongs to the family of fullerene molecules, which were discovered during the last two decades, but is the first two-dimensional fullerene.

The researchers concentrate on the electronic properties of carbon nanofabric. By employing the standard microfabrication techniques used, for instance, in manufacturing of computer chips, the team has demonstrated an ambipolar field-effect transistor, which works under ambient conditions. They found that the nanofabric exhibits a remarkable quality such that electrons can travel without any scattering over submicron distances, which is important for making very-fast-switching transistors.

In the quest to make the computer chip more powerful and fast, engineers strive to produce smaller transistors, shortening the paths electrons have to travel to switch the devices on and off. Ultimately, scientists envisage transistors made from a single molecule, and this work brings that vision ever nearer.

In terms of applications, the sort of quality demonstrated by graphene can only be compared with that demonstrated by some nanotubes. Professor Geim commented: “As carbon nanotubes are basically made from rolled-up narrow stripes of graphene, any of the thousands of applications currently considered for nanotubes renowned for their unique properties can also apply to graphene itself.”

Although the researchers are currently dealing with patches of graphene that are about ten microns across Professor Geim commented: “Computer engineers will need graphene wafers a few inches in size, before considering graphene as |”the next big thing”. However, all the omens are good, as there are no fundamental limitations on the lateral size of carbon nanofabric.” Dr Novoselov added: “Only ten years ago carbon nanotubes were less than a micron long. Now, scientists can make nanotubes several centimetres long, and similar progress can reasonably be expected for carbon nanofabric too”.

David Glover from University of Manchester Intellectual Property Ltd commented: “This is clearly an exciting breakthrough with huge potential, and with development graphene could soon compete in many niche markets where low energy consumption and high electron mobility are paramount requirements”.

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