An index assessing the health of the oceans gives a global score of 60 out of 100. But the idea that a single number can encompass both environmental status and the benefits that the oceans provide for humans may prove controversial. See Article p.615

doi: 10.1038/488594a

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ImmunologyLicensed in the lungs p.595

In multiple sclerosis, the body's own immune cells attack the brain and spinal cord. But how they get there from peripheral tissues has been a mystery. Surprisingly, the lungs might be a key transit point. See Letter p.675

doi: 10.1038/488595a

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Ocean scienceAncient burial at sea p.596

A study reveals cyclic changes in the rate of burial of biogenic calcium carbonate at the Pacific ocean floor 43 million to 33 million years ago, as Earth exited a warm 'greenhouse' state to become an ice-capped planet. See Article p.609

doi: 10.1038/488596a

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OpticsMixing waves in a diamond p.598

Use of an ultra-high-intensity X-ray laser has allowed X-ray and optical waves to be mixed in a diamond sample. The effect paves the way to studying the microscopic optical response of materials on an atomic scale. See Article p.603

doi: 10.1038/488598a

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EpigeneticsActors in the cell reprogramming drama p.599

The transformation of skin cells into stem cells is a fascinating but poorly understood process. At last, the molecular characters underlying the initial steps have been revealed. See Letter p.652

doi: 10.1038/488599a

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AstronomyCollision course p.600

Four billion years from now, the Andromeda galaxy will have a close encounter with the Milky Way. The two galaxies will commence a dance of disruption that will, over the course of another two billion years, lead to their complete union.

doi: 10.1038/nature11482

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MicrobiologyAntibiotics and adiposity p.601

Mice receiving low doses of certain antibiotics gain weight and accumulate fat. This could be because some gut bacteria survive the treatment better than others, shifting digestion towards greater energy provision. See Article p.621

doi: 10.1038/488601a

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Articles

X-ray and optical wave mixing p.603

Light–matter interactions are ubiquitous, and underpin a wide range of basic research fields and applied technologies. Although optical interactions have been intensively studied, their microscopic details are often poorly understood and have so far not been directly measurable. X-ray and optical wave mixing was proposed nearly half a century ago as an atomic-scale probe of optical interactions but has not yet been observed owing to a lack of sufficiently intense X-ray sources. Here we use an X-ray laser to demonstrate X-ray and optical sum-frequency generation. The underlying nonlinearity is a reciprocal-space probe of the optically induced charges and associated microscopic fields that arise in an illuminated material. To within the experimental errors, the measured efficiency is consistent with first-principles calculations of microscopic optical polarization in diamond. The ability to probe optical interactions on the atomic scale offers new opportunities in both basic and applied areas of science.

doi: 10.1038/nature11340

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A Cenozoic record of the equatorial Pacific carbonate compensation depth p.609

Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0–3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

doi: 10.1038/nature11360

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An index to assess the health and benefits of the global ocean p.615

The ocean plays a critical role in supporting human well-being, from providing food, livelihoods and recreational opportunities to regulating the global climate. Sustainable management aimed at maintaining the flow of a broad range of benefits from the ocean requires a comprehensive and quantitative method to measure and monitor the health of coupled human–ocean systems. We created an index comprising ten diverse public goals for a healthy coupled human–ocean system and calculated the index for every coastal country. Globally, the overall index score was 60 out of 100 (range 36–86), with developed countries generally performing better than developing countries, but with notable exceptions. Only 5% of countries scored higher than 70, whereas 32% scored lower than 50. The index provides a powerful tool to raise public awareness, direct resource management, improve policy and prioritize scientific research.

doi: 10.1038/nature11397

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Antibiotics in early life alter the murine colonic microbiome and adiposity p.621

Antibiotics administered in low doses have been widely used as growth promoters in the agricultural industry since the 1950s, yet the mechanisms for this effect are unclear. Because antimicrobial agents of different classes and varying activity are effective across several vertebrate species, we proposed that such subtherapeutic administration alters the population structure of the gut microbiome as well as its metabolic capabilities. We generated a model of adiposity by giving subtherapeutic antibiotic therapy to young mice and evaluated changes in the composition and capabilities of the gut microbiome. Administration of subtherapeutic antibiotic therapy increased adiposity in young mice and increased hormone levels related to metabolism. We observed substantial taxonomic changes in the microbiome, changes in copies of key genes involved in the metabolism of carbohydrates to short-chain fatty acids, increases in colonic short-chain fatty acid levels, and alterations in the regulation of hepatic metabolism of lipids and cholesterol. In this model, we demonstrate the alteration of early-life murine metabolic homeostasis through antibiotic manipulation.

doi: 10.1038/nature11400

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