In This Issue

APPLIED PHYSICAL SCIENCES

Weak ties bind human communications networks

Whether coworker, friend, relative, or casual acquaintance, everyone is part of someone's “network.” Social networks undoubtedly affect the flow of information, but the laws that govern their structure and function remain unclear. The abundant electronic databases that track mobile phone calls and e-mail transmissions now make it possible to recreate such networks and determine the factors that influence their behavior. J.-P. Onnela et al. report the construction of a weighted communication network from the call records of millions of mobile phone users. The reconstruction showed an unexpected link between the stability of a network and the strength of the ties between its members. To their surprise, the authors found that removing strong ties (e.g., contacts one might have within one's group of close friends) had little effect on network stability, but severing the weak ties, such as those between different communities, caused a rapid collapse of the network. The structure of this one-to-one communication network can “trap” information and hinder its efficient global spread. — M.M.

“Structure and tie strengths in mobile communication networks” by J.-P. Onnela, J. Saramäki, J. Hyvönen, G. Szabó, D. Lazer, K. Kaski, J. Kertész, and A.-L. Barabási (see pages 7332–7336)

ECOLOGY

Fast growth reduces mercury concentration

Methylmercury is a potent neurotoxin that becomes more concentrated as it rises through animals in the food chain. Water fleas and other zooplankton are a major source of methylmercury for lake fish. When fed nutritious, high-quality food, zooplankton end up with low concentrations of methylmercury. The result suggests ways of slowing the passage of the toxin in the food chain. Roxanne Karimi et al. found that water fleas accumulate much less methylmercury when feeding on nutritious algae with a low carbon-to-phosphorus ratio. Juvenile water fleas were fed either phosphorus-rich or phosphorus-poor algae for 5 days. During this period, the water fleas that were fed the more nutritious, phosphorus-rich algae grew 3.5 times faster and had one-third the concentration of methylmercury. The fleas accumulated less methylmercury because they grew to adulthood faster and ingested slightly less food. The rapid growth diluted the concentration of methylmercury in the fleas' bodies. When zooplankton grow rapidly by feeding on high-quality food, the amount of methylmercury they accumulate and transfer through the food chain will decrease, according to the authors. — P.D.

“Stoichiometric controls of mercury dilution by growth” by Roxanne Karimi, Celia Y. Chen, Paul C. Pickhardt, Nicholas S. Fisher, and Carol L. Folt (see pages 7477–7482)

MEDICAL SCIENCES

Public health interventions reduced flu deaths in 1918

The onset and intensity of the 1918 influenza pandemic varied in different U.S. cities. Independent studies by Richard Hatchett et al. and Martin Bootsma and Neil Ferguson gathered historical data on the timing and nature of nonpharmacological interventions (NPIs), such as school closures and quarantine of patients, used by U.S. cities during the 1918 influenza pandemic and tested whether NPIs correlated with fewer influenza deaths. Both studies found that early implementation of NPIs correlated with reduced weekly peak death rates and, to a lesser extent, with fewer total deaths. Hatchett et al. further report that the number of deaths during second waves of infections, which many cities experienced after NPIs were relaxed, was inversely correlated with the number of deaths during the first wave. Bootsma and Ferguson developed an epidemic model that incorporates the observed timing of NPIs and reproduces the patterns of deaths in different cities. The model suggests that second waves of infections occurred if early interventions were, paradoxically, too effective. By preventing infections, NPIs kept herd immunity low and, in the absence of a vaccine, the population remained vulnerable to renewed spread of the virus after NPIs were relaxed. The studies conclude that early implementation of NPIs, before 1% of the population has been infected, can prevent infection of more people than necessary to establish herd immunity and thereby reduce deaths by 30–40%. More influenza deaths could be avoided by using NPIs aggressively until a vaccine becomes available to protect the population. — F.H.

“Public health interventions and epidemic intensity during the 1918 influenza pandemic” by Richard J. Hatchett, Carter E. Mecher, and Marc Lipsitch (see pages 7582–7587)

and

“The effect of public health measures on the 1918 influenza pandemic in U.S. cities” by Martin C. J. Bootsma and Neil M. Ferguson (see pages 7588–7593)

MICROBIOLOGY

Syntrophs live an unconventional life

In anaerobic environments, the breakdown of some organic compounds is thermodynamically unfavorable, but necessary to complete the carbon cycle. A group of microbes, called syntrophs, has evolved to perform these difficult steps. These syntrophs work cooperatively with other microbes to recycle organic matter into methane and carbon dioxide. Michael McInerney et al. report the genome sequence of Syntrophus aciditrophicus and describe how elements of its genetic program enable it to live an unconventional life. One of the thermodynamically unfavorable reactions that syntrophs like S. aciditrophicus must carry out is reverse electron transport. McInerney et al. found several genes that appear to participate in this process. The bacterium has limited means for traditional metabolism and instead metabolizes fatty acids and aromatic acids. This reliance on nontraditional energy sources sets S. aciditrophicus apart from most other organisms. — T.H.D.

“The genome of Syntrophus aciditrophicus: Life at the thermodynamic limit of microbial growth” by Michael J. McInerney, Lars Rohlin, Housna Mouttaki, UnMi Kim, Rebecca S. Krupp, Luis Rios-Hernandez, Jessica Sieber, Christopher G. Struchtemeyer, Anamitra Bhattacharyya, John W. Campbell, and Robert P. Gunsalus (see pages 7600–7605)

PSYCHOLOGY

Making generalizations takes time and sleep

The ability to make a generalization based on existing information is a key property of human cognition and is critical for enabling humans to make judgments in novel situations. Jeffrey Ellenbogen et al. show that this so-called relational memory works best after an “offline” period. By using a group of 56 subjects, the authors randomly showed the participants a series of relationships, i.e., A>B, D>E, C>D, E>F, and B>C. This sequence contained the following hidden metastructure: A>B>C>D>E>F. The learning session was followed by a rest period of 20 min, 12 h, or 24 h. The authors subsequently tested each group's relational memory by showing them the following inference pairs: B>D, C>E, and B>E. Only the groups with the 12- or 24-h offline period developed associative links, enabling the ability for such inferential judgments. If the 12- or 24-h period included sleep, relational memory was boosted even further. — B.T.

“Human relational memory requires time and sleep” by Jeffrey M. Ellenbogen, Peter T. Hu, Jessica D. Payne, Debra Titone, and Matthew P. Walker (see pages 7723–7728)