EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES
Water flowing into a moulin and down toward the bed of Store Glacier, Greenland.
Basal melting of the Greenland Ice Sheet
Subglacial water drainage controls ice sheet flow and ice discharge into the ocean, but these drainage systems are poorly characterized due to a lack of observations. Tun Jan Young et al. calculated a time series of basal melt rates for the Store Glacier in western Greenland by using measurements from an autonomous phase-sensitive radio-echo sounding instrument and temperature measurements from nearby boreholes. The radar-based instrument recorded vertical displacement of internal layers and the ice-bed interface at 4-hour intervals over 4 months. The results suggest that basal melt rates averaged 14 mm per day and peaked at 57 mm per day in August, when basal water temperatures reached 0.88 °C. The measured basal melt rate is two orders of magnitude higher than previous estimates for an ice sheet and is comparable to solar-driven melting at a glacier’s surface. The authors modeled heat flows in the subglacial hydrological system and estimated that the unexpectedly warm basal conditions were generated by the conversion of gravitational potential energy from surface meltwater into heat at the glacier bed. According to the authors, surface meltwater is a significant yet overlooked contributor to basal melting in glaciers, particularly in Greenland, where large quantities of surface meltwater descend rapidly from considerable heights. — M.H.
ENGINEERING
Digital synthesis of programmable multimaterial structures.
Digital synthesis of programmable multimaterial structures
Applications such as soft robotics and flexible electronics rely on programmable structures that can achieve defined mechanical responses. Some complex mechanical responses can only be precisely achieved using multiple materials, but building such multimaterial structures is challenging. Weichen Li et al. developed an algorithm-based design process, termed “digital synthesis,” to synthesize composite structures made of multiple hyperelastic materials that can precisely achieve complex prescribed mechanical responses. The synthesized multimaterial structures contained organic geometry and irregular distribution of different nonlinear materials that seamlessly collaborated to deliver a wide variety of responses, including swift stiffening, large-deformation buckling, multistability, and long force plateaus. The structures were fabricated using two different polydimethylsiloxane elastomers and found to physically and accurately exhibit wide-ranging nonlinear responses. Assembly of the synthesized structures resulted in architectures that exhibited highly complex yet navigable responses. According to the authors, the digital synthesis technique can increase the range of programmable metamaterials and structures that can be designed and synthesized, enabling complex mechanical responses suitable for a range of applications. — S.R.
ENVIRONMENTAL SCIENCES
Wildfires and water supply in the western United States
Wildfires can enhance streamflow due to their effects on vegetation and soil, but the persistence of postdisturbance increases in runoff is unclear. To determine whether increased forest fire activity might affect water availability in the western United States, A. Park Williams et al. used stream-gauge records from 179 river basins in this region, with near-continuous coverage between 1975 and 2020, assessing the strength, duration, and seasonality of changes in streamflow after fires, above and beyond the effects expected from climate alone. In basins where more than 20% of forest area burned, postfire streamflow increased by approximately 30% for an average of 6 years. Postfire runoff enhancements in heavily burned basins occurred across seasons but were most significant in spring and fall. To date, the total forest area burned has been too small to substantially affect runoff at regional scales. However, historical fire–climate relationships combined with climate model projections suggest that widespread continued increases in forest fire area are likely over the next 30 years. The results suggest that burned areas will grow enough over the next three decades to enhance streamflow at regional scales, potentially enhancing water supply but posing runoff-related hazards, according to the authors. — J.W.
ECOLOGY
Factors influencing species richness in grasslands
Plant communities facing an increased availability of resources experience a decline in species richness, but the mechanisms underlying this seemingly paradoxical shift are unclear. Nir Band et al. conducted a global meta-analysis to test three hypotheses: the “biomass-driven competition hypothesis,” according to which an increase in biomass leads to increased interspecific competition; the “niche dimension hypothesis,” according to which increased soil resources reduce resource-based niche partitioning; and the “nitrogen detriment hypothesis,” according to which high levels of nitrogen reduce plant performance through several mechanisms. The authors integrated data from 630 resource enrichment experiments in 99 different sites worldwide and found strong support for the nitrogen detriment hypothesis, with intermediate support for the biomass-driven competition hypothesis, and negligible support for the niche dimension hypothesis. Nitrogen had the largest and most significant effect on species richness of all the tested resources, which included phosphorus, potassium, and water. In addition, nitrogen’s direct effects were much larger than its indirect effects through biomass. The main driver of species loss following resource addition is related to nitrogen, suggesting that minimizing nitrogen inputs may help conserve the diversity of grassland ecosystems, according to the authors. — S.R.
ENVIRONMENTAL SCIENCES
Quantifying the CO2 fertilization effect
Increased atmospheric CO2 concentrations can increase photosynthesis globally. However, the magnitude of the increase in plant photosynthesis is uncertain because CO2 fertilization is not directly observed and is subject to confounding effects. Chi Chen et al. quantified the CO2 fertilization effect by combining observations between 2001 and 2014 from a global network of 68 eddy covariance sites, which measure gas exchange, and a framework for analyzing the observations based on three theories of photosynthetic optimization. The authors found evidence of a strong enhancement of photosynthesis, with a positive annual trend of 9.1 grams of carbon per unit area since 2001. The CO2 fertilization effect accounted for 44% of the plant production enhancement and warming accounted for 28%. Applying the same framework to satellite-derived and meteorological reanalysis data, the authors report a global average photosynthesis enhancement trend of 4.4 grams of carbon per unit area for each year—a measure at least one-third stronger than previous estimates based on global vegetation models and satellite-derived plant production data. According to the authors, the results suggest that the CO2 fertilization effect has likely played a critical role in the global carbon cycle in recent decades. — P.G.
IMMUNOLOGY AND INFLAMMATION
Rickett’s big-footed bat.
Bat salivary toxin and virus transmission
Bats are increasingly recognized as reservoirs of viruses that cause disease outbreaks in humans. However, the biological factors that enable bats to contribute to virus spillover are unclear. Mingqian Fang, Xiaopeng Tang, Juan Zhang, Zhiyi Liao, Gan Wang , et al. identified an immunosuppressant toxin produced in the submandibular salivary glands of Rickett’s big-footed bats (Myotis pilosus) and analyzed the toxin’s effects on host immunity and virus transmission. The authors detected high concentrations of the toxin in the bats’ submandibular salivary glands, representing 1% of all proteins produced. Experiments on human cell cultures and mouse models revealed that the toxin inhibited the activity of several pro-inflammatory enzymes, including leukotriene-A4 hydrolase, which generates leukotriene-B4, a potent chemoattractant involved in antiviral response. In mice infected with the influenza A virus H1N1, the toxin exacerbated viral infectivity and pathology by interfering with the immune response. The results suggest that the toxin facilitates cross-species transmission of pathogens by inhibiting host immunity. However, the toxin did not impede the anti- inflammatory function of leukotriene-A4 hydrolase, suggesting that the toxin is a potential candidate for the development of anti-inflammatory drugs, according to the authors. — M.H.
Journal Club Blog
Ancient herbivorous mammals as well as modern ones, including zebras, display individual dietary specialization. Image credit: Larisa DeSantis.
Dietary specialization is surprisingly common among plant-eating mammals, even for generalist species
Posted on February 18, 2022
Amy McDermott
Humans eat so many different foods that we’re often considered dietary generalists. But at the individual level, diets are fairly consistent—they just vary widely from person to person and region to region.