On the combined effects of normobaric hypoxia and bed rest upon bone and mineral metabolism: Results from the PlanHab study

AbstractBone losses are common as a consequence of unloading and also in patients with chronic obstructive pulmonary disease (COPD). Although hypoxia has been implicated as an important factor to drive bone loss, its interaction with unloading remains unresolved. The objective therefore was to assess whether human bone loss caused by unloading could be aggravated by chronic hypoxia.In a cross-over designed study, 14 healthy young men underwent 21-day interventions of bed rest in normoxia (NBR), bed rest in hypoxia (HBR), and hypoxic ambulatory confinement (HAmb). Hypoxic conditions were equivalent to 4000m altitude. Bone metabolism (NTX, P1NP, sclerostin, DKK1) and phospho-calcic homeostasis (calcium and phosphate serum levels and urinary excretion, PTH) were assessed from regular blood samples and 24-hour urine collections, and tibia and femur bone mineral content was assessed by peripheral quantitative computed tomography (pQCT).Urinary NTX excretion increased (P<0.001) to a similar extent in NBR and HBR (P=0.69) and P1NP serum levels decreased (P=0.0035) with likewise no difference between NBR and HBR (P=0.88). Serum total calcium was increased during bed rest by 0.059 (day D05, SE 0.05mM) to 0.091mM (day D21, P<0.001), with no additional effect by hypoxia during bed rest (P=0.199). HAmb led, at least temporally, to increased total serum calcium, to reduced serum phosphate, and to reduced phosphate and calcium excretion.In conclusion, hypoxia did not aggravate bed rest-induced bone resorption, but led to changes in phospho-calcic homeostasis likely caused by hyperventilation. Whether hyperventilation could have mitigated the effects of hypoxia in this study remains to be established

A review of the stable isotope bio-geochemistry of the global silicon cycle and its associated trace elements

Silicon (Si) is the second most abundant element in the Earth's crust and is an important nutrient in the ocean. The global Si cycle plays a critical role in regulating primary productivity and carbon cycling on the continents and in the oceans. Development of the analytical tools used to study the sources, sinks, and fluxes of the global Si cycle (e.g., elemental and stable isotope ratio data for Ge, Si, Zn, etc.) have recently led to major advances in our understanding of the mechanisms and processes that constrain the cycling of Si in the modern environment and in the past. Here, we provide background on the geochemical tools that are available for studying the Si cycle and highlight our current understanding of the marine, freshwater and terrestrial systems. We place emphasis on the geochemistry (e.g., Al/Si, Ge/Si, Zn/Si, δ13C, δ15N, δ18O, δ30Si) of dissolved and biogenic Si, present case studies, such as the Silicic Acid Leakage Hypothesis, and discuss challenges associated with the development of these environmental proxies for the global Si cycle. We also discuss how each system within the global Si cycle might change over time (i.e., sources, sinks, and processes) and the potential technical and conceptual limitations that need to be considered for future studies.The work by JS was supported by the “Laboratoire d’Excellence” LabexMER (ANR-10-LABX-19) and co-funded by a grant from the French government under the program “Investissements d’Avenir,” and by a grant from the Regional Council of Brittany (SAD programme). DJC was partially supported by the Knut and Alice Wallenberg Foundation (KAW Wallenberg Scholar) and the Swedish Research Council. This review article has benefited from funding by the European Union Seventh Framework Programme under grant agreement n◦294146 (project MuSiCC, Marie Curie CIG to DC). GdS is supported by a Marie Skłodowska-Curie Research Fellowship under EU Horizon2020 (GA #708407). JuD was supported by the American Chemical Society Petroleum Research Fund (Grant # 53798-DNI2). CE acknowledges financial support by the Institute for Chemistry and Biology of the Marine Environment (Oldenburg, Germany) and the Max Planck Institute for Marine Microbiology (Bremen, Germany). KH is funded by The Royal Society (UF120084) and the European Research Council (ERC-2015-StG - 678371_ICY-LAB). PG acknowledges funding by the Collaborative Research Centre 754 “ClimateBiogeochemistry interactions in the Tropical Ocean” (www. sfb754.de), supported by the Deutsche Forschungsgemeinschaft (DFG)

The challenges and opportunities of addressing particle size effects in sediment source fingerprinting: A review

publisher: Elsevier articletitle: The challenges and opportunities of addressing particle size effects in sediment source fingerprinting: A review journaltitle: Earth-Science Reviews articlelink: http://dx.doi.org/10.1016/j.earscirev.2017.04.009 content_type: article copyright: © 2017 Elsevier B.V. All rights reserved

Amorphous Silica Transport in the Ganges Basin : Implications for Si Delivery to the Oceans

Rivers transport ∽6 x1012 mol yr-1 of dissolved Si (DSi) from the continents to the oceans. They also carry amorphous silica (ASi), solid phases likely to dissolve in seawater. Unfortunately, the magnitude of this flux is poorly constrained at a global scale. We present 92 new ASi values from suspended particulate matter (SPM) from the Ganges basin. Bulk SPM is ∽1.2% ASi, and mean ASi concentrations are ∽65 μM, of comparable magnitude to DSi concentrations. Our results also indicate a) ASi is not evenly distributed in the water column of large rivers, b) the ASi is not a wholly biogenic Si endmember and c) the ASi flux is, to a first order, a function of the SPM load. Our results suggest that the ASi particulate load is much greater than previously believed, rivaling that of the DSi load with important implications for the global Si cycle and oceanic Si isotopic budget

Linking silicon isotopic signatures with diatom communities

The use of silicon isotope ratios (expressed as δ30Si) as a paleolimnological proxy in lacustrine systems requires a better understanding of the role of lake processes in setting the δ30Si values of dissolved Si (dSi) in water and in diatom biogenic silica (bSi). We determined the δ30Si of modern dSi (δ30SidSi) and bSi (δ30SibSi) in three lakes in Lassen Volcanic National Park (LAVO), California (USA), and produced diatom assemblage compositional data from the modern system and from sediment core samples. In the modern systems, we observe the largest magnitude diatom Si isotope fractionations yet reported, at −3.4 and −3.9‰ for Fragilaria dominated samples. Using statistical approaches designed to condense multivariate ecological data, we can deconvolve assemblage-specific Si isotope fractionations from the combined diatom assemblage-δ30Si data. For example, samples dominated by generally deeper water euplanktic species have low δ30SibSi values (−0.14‰). These data suggest that δ30Si records from LAVO lakes reflect species specific Si isotope fractionations and thus act as paleolimnological proxy for the aquatic-habitat of bSi production. Silicon isotope analysis should be coupled with diatom community composition data and other geochemical proxies for the most robust paleolimnological interpretations. We also construct a Si mass-balance for Manzanita Lake based on elemental fluxes. Despite a short residence time of ∼4 months, it is an efficient Si sink: about 30% of inflowing Si is retained in the lake sediments. An entirely independent Si isotope-based estimate agrees remarkably well. Burial fluxes of bSi derived from radiometrically dated sediment cores yield retention rates of about a factor of three higher, which might suggest groundwater is an important term in the lake Si budget

Linking silicon isotopic signatures with diatom communities

The use of silicon isotope ratios (expressed as δ30Si) as a paleolimnological proxy in lacustrine systems requires a better understanding of the role of lake processes in setting the δ30Si values of dissolved Si (dSi) in water and in diatom biogenic silica (bSi). We determined the δ30Si of modern dSi (δ30SidSi) and bSi (δ30SibSi) in three lakes in Lassen Volcanic National Park (LAVO), California (USA), and produced diatom assemblage compositional data from the modern system and from sediment core samples. In the modern systems, we observe the largest magnitude diatom Si isotope fractionations yet reported, at -3.4 and -3.9‰ for Fragilaria dominated samples. Using statistical approaches designed to condense multivariate ecological data, we can deconvolve assemblage-specific Si isotope fractionations from the combined diatom assemblage-δ30Si data. For example, samples dominated by generally deeper water euplanktic species have low δ30SibSi values ( -0.14‰). These data suggest that δ30Si records from LAVO lakes reflect species specific Si isotope fractionations and thus act as paleolimnological proxy for the aquatic-habitat of bSi production. Silicon isotope analysis should be coupled with diatom community composition data and other geochemical proxies for the most robust paleolimnological interpretations. We also construct a Si mass-balance for Manzanita Lake based on elemental fluxes. Despite a short residence time of ∼4 months, it is an efficient Si sink: about 30% of inflowing Si is retained in the lake sediments. An entirely independent Si isotope-based estimate agrees remarkably well. Burial fluxes of bSi derived from radiometrically dated sediment cores yield retention rates of about a factor of three higher, which might suggest groundwater is an important term in the lake Si budget

Biosilicification Drives a Decline of Dissolved Si in the Oceans through Geologic Time

Biosilicification has driven variation in the global Si cycle over geologic time. The evolution of different eukaryotic lineages that convert dissolved Si (DSi) into mineralized structures (higher plants, siliceous sponges, radiolarians, and diatoms) has driven a secular decrease in DSi in the global ocean leading to the low DSi concentrations seen today. Recent studies, however, have questioned the timing previously proposed for the DSi decreases and the concentration changes through deep time, which would have major implications for the cycling of carbon and other key nutrients in the ocean. Here, we combine relevant genomic data with geological data and present new hypotheses regarding the impact of the evolution of biosilicifying organisms on the DSi inventory of the oceans throughout deep time. Although there is no fossil evidence for true silica biomineralization until the late Precambrian, the timing of the evolution of silica transporter genes suggests that bacterial silicon-related metabolism has been present in the oceans since the Archean with eukaryotic silicon metabolism already occurring in the Neoproterozoic. We hypothesize that biological processes have influenced oceanic DSi concentrations since the beginning of oxygenic photosynthesis

Terrestrial ecosystems buffer inputs through storage and recycling of elements

This study presents a conceptual framework of buffering through storage and recycling of elements in terrestrial ecosystems and reviews the current knowledge about storage and recycling of elements in plants and ecosystems. Terrestrial ecosystems, defined here as plant-soil systems, buffer inputs from the atmosphere and bedrock through storage and recycling of elements, i.e., they dampen and delay their responses to inputs. Our framework challenges conventional paradigms of ecosystem resistance derived from plant community dynamics, and instead shows that element pools and fluxes have an overriding effect on the sensitivity of ecosystems to environmental change. While storage pools allow ecosystems to buffer variability in inputs over short to intermediate periods, recycling of elements enables ecosystems to buffer inputs over longer periods. The conceptual framework presented here improves our ability to predict the responses of ecosystems to environmental change. This is urgently needed to define thresholds which must not be exceeded to guarantee ecosystem functioning. This study provides a framework for future research to explore the extent to which ecosystems buffer variability in inputs

Alkaline-extractable silicon from land to ocean: A challenge for biogenic silicon determination

The biogeochemical cycling of silicon (Si) along the land-to-ocean continuum is studied by a variety of research fields and for a variety of scientific reasons. However, there is an increasing need to refine the methodology and the underlying assumptions used to determine biogenic silica (BSi) concentrations. Recent evidence suggests that contributions of nonbiogenic sources of Si dissolving during alkaline extractions, not corrected by standard silicate mineral dissolution correction protocols, can be substantial. The ratio between dissolved Si and aluminum (Al) monitored continuously during the alkaline extraction can be used to infer the origin of the Si fractions present. In this study, we applied both a continuous analysis method (0.5 M NaOH) and a traditional 0.1 M Na2CO3 extraction to a wide array of samples: (1) terrestrial vegetation, (2) soils from forest, cropland and pasture, (3) lake sediments, (4) suspended particulate matter and sediments from rivers, (5) sediments from estuaries and salt marshes and (6) ocean sediments. Our results indicate that the 0.1 M Na2CO3 extraction protocol can overestimate the BSi content, by simultaneously dissolving Si fractions of nonbiogenic origin that may represent up to 100% of the Si traditionally considered as biogenic, hampering interpretation especially in some deeper soil horizons, rivers and coastal oceanic sediments. Moreover, although the term amorphous Si was coined to reflect a growing awareness of nonbiogenic phases we show it is actually inappropriate in samples where silicate minerals may account for a large part of the extracted Si even after linear mineral correction

Si cycling in transition zones: a study of Si isotopes and biogenic silica accumulation in the Chesapeake Bay through the Holocene

Si fluxes from the continents to the ocean are a key element of the global Si cycle. Due to the ability of coastal ecosystems to process and retain Si, the ‘coastal filter’ has the potential to alter Si fluxes at a global scale. Coastal zones are diverse systems, sensitive to local environmental changes, where Si cycling is currently poorly understood. Here, we present the first palaeoenvironmental study of estuarine biogenic silica (BSi) fluxes and silicon isotope ratios in diatoms (δ30Sidiatom) using hand-picked diatom frustules in two sediment cores (CBdist and CBprox) from the Chesapeake Bay covering the last 12000 and 8000 years, respectively. Constrained by the well-understood Holocene evolution of the Chesapeake Bay, we interpret variations in Si cycling in the context of local climate, vegetation and land use changes. δ30Sidiatom varies between + 0.8 and + 1.7‰ in both sediment cores. A Si mass balance for the Chesapeake Bay suggests much higher rates of Si retention (~ 90%) within the system than seen in other coastal systems. BSi fluxes for both sediment cores co-vary with periods of sea level rise (between 9500 and 7500 a BP) and enhanced erosion due to deforestation (between 250 and 50 a BP). However, differences in δ30Sidiatom and BSi flux between the sites emphasize the importance of the seawater/freshwater mixing ratios and locally variable Si inputs from the catchment. Further, we interpret variations in δ30Sidiatom and the increase in BSi fluxes observed since European settlement (~ 250 a BP) to reflect a growing human influence on the Si cycle in the Chesapeake Bay. Thereby, land use change, especially deforestation, in the catchment is likely the major mechanism