Day-level associations of physical activity and sedentary time in mother–child dyads across three years: a multi-wave longitudinal study using accelerometers

Journal of Behavioral Medicine - Understanding associations between mothers’ and children’s physical activity and sedentary behavior on more fine-grained timescales can provide insights...     

Routine Laboratory Testing Every 4 Versus Every 6 Weeks for Patients on Maintenance Hemodialysis: A Quality Improvement Project

1. Download high-res image (408KB)
2. Download full-size image

     

An aid to patient positioning after vitrectomy surgery

Graefe's Archive for Clinical and Experimental Ophthalmology -     

Global silicate weathering flux overestimated because of sediment–water cation exchange

Rivers carry the dissolved and solid products of silicate mineral weathering, a process that removes CO2 from the atmosphere and provides a key negative climate feedback over geological timescales. Here we show that, in some river systems, a reactive exchange pool on river suspended particulate matter, bonded weakly to mineral surfaces, increases the mobile cation flux by 50%. The chemistry of both river waters and the exchange pool demonstrates exchange equilibrium, confirmed by Sr isotopes. Global silicate weathering fluxes are calculated based on riverine dissolved sodium (Na⁺) from silicate minerals. The large exchange pool supplies Na⁺ of nonsilicate origin to the dissolved load, especially in catchments with widespread marine sediments, or where rocks have equilibrated with saline basement fluids. We quantify this by comparing the riverine sediment exchange pool and river water chemistry. In some basins, cation exchange could account for the majority of sodium in the river water, significantly reducing estimates of silicate weathering. At a global scale, we demonstrate that silicate weathering fluxes are overestimated by 12 to 28%. This overestimation is greatest in regions of high erosion and high sediment loads where the negative climate feedback has a maximum sensitivity to chemical weathering reactions. In the context of other recent findings that reduce the net CO2 consumption through chemical weathering, the magnitude of the continental silicate weathering fluxes and its implications for solid Earth CO2 degassing fluxes need to be further investigated.

For decades, silicate weathering has been postulated to provide the negative climate feedback on Earth that prevents a runaway greenhouse climate like on Venus (1). Silicate mineral dissolution with carbonic acid converts atmospheric CO2 into carbonate, and releases essential nutrients to the terrestrial and marine biosphere (2). There have been many attempts to quantify the silicate weathering flux (3), mostly assuming that riverine dissolved sodium (Na+) is derived only from silicate minerals and rock salt. Here we show that there is a major addition of nonsilicate Na+ to the critical zone from ancient seawater, weakly bonded to sedimentary rocks and supplied to waters via the cation exchange process. The implication is not only that the silicate weathering flux is overestimated at a global scale, but that this nonsilicate Na+ is most important in regions previously thought to have the highest silicate weathering fluxes (so called weathering-limited regions) and greatest climate sensitivity.Cation exchange is a rapid chemical reaction between cations in the dissolved phase and mineral surfaces, particularly clays (4). Major and trace cations such as calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), and strontium (Sr2+) form the cation exchange pool, which balances negative charges on river-borne clay particle surfaces. This exchange takes place on interlayer sites, between the tetrahedral and octahedral layers, or on exposed surfaces (4). The importance of the cation exchange pool is well recognized in soils and aquifers (4, 5), has significant implications for enhanced weathering (6), and has been proposed as an important mechanism for buffering the composition of river waters (7–9). However, data on the riverine exchange pool are only available for two large river systems [Amazon and Ganges-Brahmaputra (10, 11)], despite its significance in providing a source of elements that are immediately bioavailable (12), and their potential for biasing the quantification of silicate weathering (9).It is increasingly recognized that rapidly reactive phases have a strong influence on the chemistry of river waters (13, 14). Cation exchange is a rapid reaction occurring continuously in soils, as riverine freshwaters evolve downstream interacting with particulate matter, and when they mix with seawater (15, 16). Important examples of cation exchange are the “swapping” of divalent cations Ca2+ and Mg2+ with Na+, in particular when there is a major change in water composition such as when fluvial clays reach the ocean,Caclay2++2Nawater+⇋2Naclay++Cawater2+.[1]As a result, marine sediments have an exchange pool that is dominated by Na+ (17). Subsequently, these marine sediments are uplifted and emplaced on the continents where Na+ in the exchange pool is released by cation exchange with Ca-rich fresh waters (9). This has major implications for estimates of silicate weathering fluxes and associated CO2 consumption, because they are calculated using the Na+ content of rivers (3). Cerling et al. (9) proposed that the Na+-rich exchange pool exerts an important control on natural waters, based on charge balance arguments from river water chemistry, but this hypothesis has never been rigorously tested (18) by determining the flux and composition of the exchange pool of rivers around the world.In this contribution, we present a large dataset of fluvial sediment cation exchange capacity (CEC) and composition in several of the world’s largest river basins. By comparing with the concomitant dissolved load chemistry, we demonstrate that 1) the exchange pool in river sediments is in equilibrium with the river water; 2) the fraction of mobile elements in the exchange pool relative to the dissolved pool can be significant, particularly in rapidly eroding, weathering-limited catchments; and 3) given reasonable inferences on the composition of old marine sedimentary rocks, modern-day silicate weathering has been overestimated and carbonate weathering has been underestimated. The results reduce the estimated magnitude of the silicate weathering flux, but increase the supply of base cations (e.g., Ca2+, which can be a limiting nutrient) to the biosphere, suggesting a greater role of organic carbon burial compared with silicate weathering for the long-term atmospheric CO2 sink.     

Interspecies comparison of subchondral bone properties important for cartilage repair

Microfracture repair tissue in young adult humans and in rabbit trochlea is frequently of higher quality than in corresponding ovine or horse models or in the rabbit medial femoral condyle (MFC). This may be related to differences in subchondral properties since repair is initiated from the bone. We tested the hypothesis that subchondral bone from rabbit trochlea and the human MFC are structurally similar. Trochlea and MFC samples from rabbit, sheep, and horse were micro‐CT scanned and histoprocessed. Samples were also collected from normal and lesional areas of human MFC. The subchondral bone of the rabbit trochlea was the most similar to human MFC, where both had a relatively thin bone plate and a more porous and less dense character of subchondral bone. MFC from animals all displayed thicker bone plates, denser and less porous bone and thicker trabeculae, which may be more representative of older or osteoarthritic patients, while both sheep trochlear ridges and the horse lateral trochlea shared some structural features with human MFC. Since several cartilage repair procedures rely on subchondral bone for repair, subchondral properties should be accounted for when choosing animal models to study and test procedures that are intended for human cartilage repair. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 33:63–70, 2015.

     

From the Cover: PNAS Plus: The essential gene set of a photosynthetic organism

Synechococcus elongatus PCC 7942 is a model organism used for studying photosynthesis and the circadian clock, and it is being developed for the production of fuel, industrial chemicals, and pharmaceuticals. To identify a comprehensive set of genes and intergenic regions that impacts fitness in S. elongatus, we created a pooled library of ∼250,000 transposon mutants and used sequencing to identify the insertion locations. By analyzing the distribution and survival of these mutants, we identified 718 of the organism’s 2,723 genes as essential for survival under laboratory conditions. The validity of the essential gene set is supported by its tight overlap with well-conserved genes and its enrichment for core biological processes. The differences noted between our dataset and these predictors of essentiality, however, have led to surprising biological insights. One such finding is that genes in a large portion of the TCA cycle are dispensable, suggesting that S. elongatus does not require a cyclic TCA process. Furthermore, the density of the transposon mutant library enabled individual and global statements about the essentiality of noncoding RNAs, regulatory elements, and other intergenic regions. In this way, a group I intron located in tRNA^Leu, which has been used extensively for phylogenetic studies, was shown here to be essential for the survival of S. elongatus. Our survey of essentiality for every locus in the S. elongatus genome serves as a powerful resource for understanding the organism’s physiology and defines the essential gene set required for the growth of a photosynthetic organism.Determining the sets of genes necessary for survival of diverse organisms has helped to identify the fundamental processes that sustain life across an array of environments (1). This research has also served as the starting point for efforts by synthetic biologists to design organisms from scratch (2, 3). Despite the importance of essential gene sets, they have traditionally been challenging to gather because of the difficulty of observing mutations that result in lethal phenotypes. More recently, the pairing of transposon mutagenesis with next generation sequencing, referred to collectively as transposon sequencing (Tn-seq), has resulted in a dramatic advance in the identification of essential gene sets (4–7). The key characteristic of Tn-seq is the use of high-throughput sequencing to screen for the fitness of every transposon mutant in a pooled population to measure each mutation’s impact on survival. These data can be used to quantitatively ascertain the effect of loss-of-function mutations at any given locus, intragenic or intergenic, in the conditions under which the library is grown (8). Essential gene sets for 42 diverse organisms distributed across all three domains have now been defined, largely through the use of Tn-seq (9). A recently developed variation on Tn-seq, random barcode transposon site sequencing (RB-TnSeq) (10), further minimizes the library preparation and sequencing costs of whole-genome mutant screens.Despite the proliferation of genome-wide essentiality screens, a complete essential gene set has yet to be defined for a photosynthetic organism. A collection of phenotyped Arabidopsis thaliana mutants has been created but extends to only one-tenth of Arabidopsis genes (11). In algae, efforts are underway to produce a Tn-seq–like system in Chlamydomonas reinhardtii; however, the mutant library currently lacks sufficient saturation to determine gene essentiality (12). To date, the essential genes for photoautotrophs have only been estimated by indirect means, such as by comparative genomics (13). The absence of experimentally determined essential gene sets in photosynthetic organisms, despite their importance to the environment and industrial production, is largely because of the difficulty and time required for genetic modification of these organisms.Cyanobacteria comprise an extensively studied and ecologically important photosynthetic phylum. They are responsible for a large portion of marine primary production and have played a foundational role in research to decipher the molecular components of photosynthesis (14, 15). Synechococcus elongatus PCC 7942 is a particularly well-studied member of this phylum because of its genetic tractability and streamlined genome (16). As a result, it has been developed as a model photosynthetic organism and a production platform for a number of fuel products and high-value chemicals (17). Despite the importance of S. elongatus for understanding photosynthesis and industrial production, 40% of its genes have no functional annotation, and only a small portion of those that do have been studied experimentally.Here, we use RB-TnSeq, a method that pairs high-density transposon mutagenesis and pooled mutant screens, to probe the S. elongatus genome for essential genes and noncoding regions. We categorized 96% of 2,723 genes in S. elongatus as either essential (lethal when mutated), beneficial (growth defect when mutated), or nonessential (no phenotype when mutated) under standard laboratory conditions. Furthermore, we determined the genome-wide essentiality of noncoding RNAs (ncRNAs), regulatory regions, and intergenic regions. Our investigation has produced an extensive analysis of the loci essential for the growth of a photosynthetic organism and developed a powerful genomic tool that can be used for additional screens under a wide array of ecologically and industrially relevant growth conditions.