CO2 Sensitivity Research Articles

Soluble adenylyl cyclase in the locus coeruleus

Although it has been demonstrated that the CO2-sensitivity in the locus coeruleus (LC) is mediated by changes in pH, the involvement of HCO3− in the CO2-detection mechanism in these neurons cannot be excluded. In the present work, we characterized sAC for the first time in the LC and we asked whether this enzyme is important in the detection of changes in HCO3−/CO2 levels in these neurons, using an approach that allowed us to isolate CO2 from pH stimulus. sAC mRNA expression and activity were upregulated from 0mM HCO3−/0% CO2 to 24mM HCO3−/5% CO2 in the LC but not in the cortex of the brain. Comparing the effects of sAC and tmAC inhibitors in the LC, we observed that both tmAC and sAC contribute to the generation of cAMP during normocapnic conditions but only sAC contributed to the generation of cAMP during isohydric hypercapnia. Furthermore, activation of tmAC induced an increase in sAC expression in LC, but not cortex.sAC may be involved in CO2 sensitivity in the LC, up to its threshold of saturation, with a particular contribution of this enzyme in situations when low HCO3− concentrations occur. Its role should be further explored in pathological states to determine whether sAC activation with HCO3− alters ventilation.

Effects of prolonged lung inflation or deflation on pulmonary stretch receptor discharge in the alligator (Alligator mississippiensis)

The American alligator (Alligator mississippiensis) is a semi-aquatic diving reptile that has a periodic breathing pattern. Previous work identified pulmonary stretch receptors, that are rapidly and slowly adapting, as well as intrapulmonary chemoreceptors (IPC), sensitive to CO2, that modulate breathing patterns in alligators. The purpose of the present study was to quantify the effects of prolonged lung inflation and deflation (simulated dives) on pulmonary stretch receptors (PSR) and/or IPC discharge characteristics. The effects of airway pressure (0–20cm H2O), hypercapnia (7% CO2), and hypoxia (5% O2) on dynamic and static responses of PSR were studied in juvenile alligators (mean mass=246g) at 24°C. Alligators were initially anesthetized with isoflurane, cranially pithed, tracheotomized and artificially ventilated. Vagal afferent tonic and phasic activity was recorded with platinum hook electrodes. Receptor activity was a mixture of slowly adapting PSR (SAR) and rapidly adapting PSR (RAR) with varying thresholds and degrees of adaptation, without CO2 sensitivity. Receptor activity before, during and after 1min periods of lung inflation and deflation was quantified to examine the effect of simulated breath-hold dives. Some PSR showed a change in dynamic response, exhibiting inhibition for several breaths after prolonged lung inflation. Following 1min deflation, RAR, but not SAR, exhibited a significant potentiation of burst frequency relative to control. For SAR, the post-inflation receptor inhibition was blocked by CO2 and hypoxia; for RAR, the post-inflation inhibition was potentiated by CO2 and blocked by hypoxia. These results suggest that changes in PSR firing following prolonged inflation and deflation may promote post-dive ventilation in alligators. We hypothesize that PSR in alligators may be involved in recovery of breathing patterns and lung volume during pre- and post-diving behavior and apneic periods in diving reptiles.

Diversity of ocean acidification effects on marine N2 fixers

Considering the important role of N2 fixation for primary productivity and CO2 sequestration, it is crucial to assess the response of diazotrophs to ocean acidification. Previous studies on the genus Trichodesmium suggested a strong sensitivity towards ocean acidification. In view of the large functional diversity in N2 fixers, the objective of this study was to improve our knowledge of the CO2 responses of other diazotrophs. To this end, the single-celled Cyanothece sp. and two heterocystous species, Nodularia spumigena and the symbiotic Calothrix rhizosoleniae, were acclimated to two pCO2 levels (380 vs. 980 μatm). Growth rates, cellular composition (carbon, nitrogen and chlorophyll a) as well as carbon and N2 fixation rates (14C incorporation, acetylene reduction) were measured and compared to literature data on different N2 fixers. The three species investigated in this study responded differently to elevated pCO2, showing enhanced, decreased as well as unaltered growth and production rates. For instance, Cyanothece increased production rates with pCO2, which is in line with the general view that N2 fixers benefit from ocean acidification. Due to lowered growth and production of Nodularia, nitrogen input to the Baltic Sea might decrease in the future. In Calothrix, no significant changes in growth or production could be observed, even though N2 fixation was stimulated under elevated pCO2. Reviewing literature data confirmed a large variability in CO2 sensitivity across diazotrophs. The contrasting response patterns in our and previous studies were discussed with regard to the carbonate chemistry in the respective natural habitats, the mode of N2 fixation as well as differences in cellular energy limitation between the species. The group-specific CO2 sensitivities will impact differently on future biogeochemical cycles of open-ocean environments and systems like the Baltic Sea and should therefore be considered in models estimating climate feedback effects.

Emergent constraints on climate‐carbon cycle feedbacks in the CMIP5 Earth system models

AbstractAn emergent linear relationship between the long‐term sensitivity of tropical land carbon storage to climate warming (γLT) and the short‐term sensitivity of atmospheric carbon dioxide (CO2) to interannual temperature variability (γIAV) has previously been identified by Cox et al. (2013) across an ensemble of Earth system models (ESMs) participating in the Coupled Climate‐Carbon Cycle Model Intercomparison Project (C4MIP). Here we examine whether such a constraint also holds for a new set of eight ESMs participating in Phase 5 of the Coupled Model Intercomparison Project. A wide spread in tropical land carbon storage is found for the quadrupling of atmospheric CO2, which is of the order of 252 ± 112 GtC when carbon‐climate feedbacks are enabled. Correspondingly, the spread in γLT is wide (−49 ± 40 GtC/K) and thus remains one of the key uncertainties in climate projections. A tight correlation is found between the long‐term sensitivity of tropical land carbon and the short‐term sensitivity of atmospheric CO2 (γLT versus γIAV), which enables the projections to be constrained with observations. The observed short‐term sensitivity of CO2 (−4.4 ± 0.9 GtC/yr/K) sharpens the range of γLT to −44 ± 14 GtC/K, which overlaps with the probability density function derived from the C4MIP models (−53 ± 17 GtC/K) by Cox et al. (2013), even though the lines relating γLT and γIAV differ in the two cases. Emergent constraints of this type provide a means to focus ESM evaluation against observations on the metrics most relevant to projections of future climate change.

Potential bullfrog homologue to the chemosensitive mammalian retrotrapezoid nucleus (1092.4)

Vertebrate breathing is controlled by a highly conserved brainstem neural network. Air breathing is generally driven by elevated CO2 in body tissues/fluids and is present in all vertebrate classes. Bullfrogs transition from water to air breathing during metamorphosis, and exhibit central CO2 sensitivity throughout development. In mammals, many sites contribute to this process, and their relative contributions vary according to CO2/pH level, arousal state, and development. Potential central CO2 sensors have been extensively investigated and described in mammals but not amphibians. The retrotrapezoid nucleus (RTN) lies on the medullary surface near the facial nucleus and is crucial for chemosensitivity in developing mammals. The locus coeruleus and classical “rostral” and “caudal” medullary chemosensitive regions have been identified in frogs. Furthermore, the “rostral” and “caudal” sites correspond to the locations of the mammalian RTN and Pre‐Botzinger Complex, respectively, suggesting potential homology. Immunohistochemical markers for the mammalian RTN include presence of Phox2b transcription factor and vesicular glutamate transporter 2 (VGLUT2), and absence of tyrosine hydroxylase (TH), the enzymatic marker of dopamine synthesis. We tested the hypothesis that a chemosensitive region homologous to the mammalian RTN is present in bullfrog. We identified this area by staining for RTN markers: presence of Phox2b and VGLUT2, and absence of TH. We describe this region in different developmental stages and identify metamorphic changes. We also assessed chemosensitivity of this region with cFos, a marker of neural activity. Establishing this as a homologous chemosensory site between amphibians and mammals provides insight into evolutionary origins of chemosensory development and function.Grant Funding Source: Supported by NSF IOS‐1022442

Utilizing transgenic rats with eGFP‐tagged serotonergic (5‐HT) neurons to assess differential gene expression associated with age‐dependent changes in cellular CO2/H+ chemosensitivity (1092.7)

The ventilatory response to hypercapnia in rodents increases throughout postnatal development, which parallels the increases in cellular CO2 sensitivity of serotonergic (5‐HT) neurons in primary cultures. Herein we begin to test the hypothesis that specific changes in gene expression in 5‐HT neurons during development contribute to the changes in cellular and/or organismal CO2/pH sensitivity. To begin to test this, acutely dissociated medullary raphe neurons obtained from adult (P56) transgenic rats (n=2) that express enhanced green fluorescent protein (eGFP) in cells expressing Pet‐1 (ePet‐1:eGFP rats) were sorted into pools (n= 5000 cells) using Fluorescent‐Assisted Cell Sorting (FACS). Total RNA was then extracted, and RNA quality and abundance measured before quantifying transcript abundance (Δ ct) with qRT‐PCR of 5‐HT neuron‐specific (slc6a4 (SERT); slc18a2 ( VMAT2); Tph2, and Trh) or non‐5‐HT specific raphe (Gad2 and Hprt) genes. We found high expression of 5‐HT neuron‐specific genes in the eGFP+ cell pools, including TRH (∆ct 6.94), TPH (∆ct 6.95), SERT (∆ct 3.18), and VMAT2 (∆ct 2.27), with little or no detectable expression of these genes in eGFP‐ cell pools. Expression levels of Gad2 and the housekeeping gene Hprt were comparable in both eGFP+ and eGFP‐ cells. These data indicate effective sorting of 5‐HT and non‐5‐HT raphe neurons in adult transgenic rats, and allow for further studies utilizing RNA sequencing to identify, measure and compare age‐related differences in 5‐HT neuron and non‐5‐HT neuron raphe transcriptomes.Grant Funding Source: NIH HL097033

KCC and NKCC activity affect spike frequency adaptation in CO2‐sensitive avian intrapulmonary chemoreceptors (1092.3)

IPC are respiratory chemoreceptors that exhibit SFA, have phasic and tonic CO2 sensitivity, and help control breathing in birds. In IPC, large changes in PCO2, pHi, and [HCO3]i occur with each breath. Here we test the hypothesis that Cl‐ transmembrane flux may be involved in IPC CO2 transduction to balance intracellular [HCO3‐] changes. Furosemide (FU), an equipotent inhibitor of both K+ ‐Cl‐ cotransport (KCC), which allows for Cl‐ efflux driven by the K+ gradient, and Na+ ‐K+ ‐2Cl‐ cotransport (NKCC), which allows for Cl‐ influx driven by the Na+ gradient, was given to anesthetized Anas platyrhynchos (N=6) while recording single‐unit IPC. FU progressively reduced SFA at 10‐40 mg/kg (p蠄0.0001). In contrast, tonic IPC discharge was decreased at low FU doses (0.5‐5 mg/kg, p蠄0.0001) but increased at higher dosages (10‐40 mg/kg, p蠄0.0001). Results suggest that IPC SFA and tonic CO2 sensitivity are influenced by KCC and/or NKCC, presumably by reducing ability of the cell to produce Cl‐ counter movements required by phasic [HCO3‐] changes.Grant Funding Source: Supported by NIH R15 HL087269‐02

Adaptation of Agricultural and Food Systems to a Changing Climate and Increasing Urbanization

Climate change will pose risks for the world’s food supply in the coming decades; this comes at a time when the global demand for food is expected to soar based on 2050 world population estimates. It is important to recognize that climate change will necessitate temporal and geographical shifts in food production, but will most likely not result in the collapse of our food systems. However, because of differences in the severity of how climate change will affect agriculture, regional and temporal changes in production and harvest-time will challenge the existing and sometimes outdated agricultural infrastructure with respect to collection, storage, transportation, and distribution of food. Increasing regional and global urbanization will further perturb these systems. Adaptation to climate change with respect to crop and food animal production will have to occur at multiple temporal, seasonal, and geospatial levels. Other major adaptation measures will have to occur with respect to crop selection, genetics, CO2 and temperature sensitivity, and resilience of crops and food animals, water resources, and mitigation of invasive species. Technology, including sophisticated Geographic Information Systems (GIS)-based modeling, coupled with publically available soil and weather data that help farmers optimize production and conservation will be essential toward adaptation. Communication of this type of localized technical information to agricultural stakeholders by national, federal, and state entities is beginning to occur in order to help farmers adapt and prepare for extreme events associated with climate change. As the largest agricultural state in the USA, California has developed a robust mitigation and adaptation strategy that may be useful for other nation-states.

Strong shift from HCO3 (-) to CO 2 uptake in Emiliania huxleyi with acidification: new approach unravels acclimation versus short-term pH effects.

Effects of ocean acidification on Emiliania huxleyi strain RCC 1216 (calcifying, diploid life-cycle stage) and RCC 1217 (non-calcifying, haploid life-cycle stage) were investigated by measuring growth, elemental composition, and production rates under different pCO2 levels (380 and 950 μatm). In these differently acclimated cells, the photosynthetic carbon source was assessed by a 14C disequilibrium assay, conducted over a range of ecologically relevant pH values (7.9–8.7). In agreement with previous studies, we observed decreased calcification and stimulated biomass production in diploid cells under high pCO2, but no CO2-dependent changes in biomass production for haploid cells. In both life-cycle stages, the relative contributions of CO2 and HCO3 − uptake depended strongly on the assay pH. At pH values ≤ 8.1, cells preferentially used CO2 (≥ 90 % CO2), whereas at pH values ≥ 8.3, cells progressively increased the fraction of HCO3 − uptake (~45 % CO2 at pH 8.7 in diploid cells; ~55 % CO2 at pH 8.5 in haploid cells). In contrast to the short-term effect of the assay pH, the pCO2 acclimation history had no significant effect on the carbon uptake behavior. A numerical sensitivity study confirmed that the pH-modification in the 14C disequilibrium method yields reliable results, provided that model parameters (e.g., pH, temperature) are kept within typical measurement uncertainties. Our results demonstrate a high plasticity of E. huxleyi to rapidly adjust carbon acquisition to the external carbon supply and/or pH, and provide an explanation for the paradoxical observation of high CO2 sensitivity despite the apparently high HCO3 − usage seen in previous studies.Electronic supplementary materialThe online version of this article (doi:10.1007/s11120-014-9984-9) contains supplementary material, which is available to authorized users.

Divers revisited: The ventilatory response to carbon dioxide in experienced scuba divers

To investigate the ventilatory response to CO2 in hyperoxia, hypoxia, and during exercise amongst experienced scuba divers and matched controls. Two studies were performed. The first investigated the CO2 sensitivity in rest and exercise using CO2 rebreathing in hyperoxia at a workload typical for diving with divers (n=11) and controls (n=11). The second study examined the respiratory drive of divers (n=10) and controls (n=10) whilst breathing four different gas mixtures balanced with N2 (ambient air; 25% O2/6% CO2; 13% O2; 13% O2/6% CO2) to assess the combined response to hypercapnia and moderate hypoxia. Exercise at a load typical for diving was found to have no effect on the ventilatory sensitivity to CO2 in divers (rest: 1.49±0.33; exercise: 1.22±0.55 [l/min×mmHg(-1)]) and controls (rest: 2.08±0.71; exercise: 2.05±0.98 [l/min×mmHg(-1)]) while differences in sensitivity remained between the groups. Inhalation of the four gas mixtures revealed the tested oxygen pressures caused no significant alteration in the ventilatory sensitivity to CO2 in divers and controls. Experienced divers possess a lower ventilatory response to CO2 which was not affected by exercise or the tested oxygen pressures suggesting a dominant adaptation of central CO2 sensitivity.

Evaluation of snow depth and snow cover fraction simulated by two versions of the flexible global ocean-atmosphere-land system model

Based on historical runs, one of the core experiments of the fifth phase of the Coupled Model Intercomparison Project (CMIP5), the snow depth (SD) and snow cover fraction (SCF) simulated by two versions of the Flexible Global Ocean-Atmosphere-Land System (FGOALS) model, Grid-point Version 2 (g2) and Spectral Version 2 (s2), were validated against observational data. The results revealed that the spatial pattern of SD and SCF over the Northern Hemisphere (NH) are simulated well by both models, except over the Tibetan Plateau, with the average spatial correlation coefficient over all months being around 0.7 and 0.8 for SD and SCF, respectively. Although the onset of snow accumulation is captured well by the two models in terms of the annual cycle of SD and SCF, g2 overestimates SD/SCF over most mid- and high-latitude areas of the NH. Analysis showed that g2 produces lower temperatures than s2 because it considers the indirect effects of aerosols in its atmospheric component, which is the primary driver for the SD/SCF difference between the two models. In addition, both models simulate the significant decreasing trend of SCF well over (30°-70°N) in winter during the period 1971–94. However, as g2 has a weak response to an increase in the concentration of CO2 and lower climate sensitivity, it presents weaker interannual variation compared to s2.

Oxygen and substrate availability interactively control the temperature sensitivity of CO2 and N2O emission from soil

We investigated how oxygen availability, substrate amount, and quality affect the temperature dependency of enzymatic processes involved in the production of carbon dioxide (CO2) and nitrous oxide (N2O). Three substrates differing in microbial degradability (glucose with potassium nitrate, glycine, and phenylalanine) were added to a mountain grassland soil at a range of concentrations. Soils were incubated at 21 and 1 % of O2 content and at 10 and 20 °C. Oxygen availability was a main factor controlling the reaction rates and temperature sensitivity of CO2 and N2O production. The temperature sensitivity of CO2 production was higher under aerobic versus oxygen-limited conditions, and the opposite dependency was observed for the N2O production. Substrate availability was a second factor affecting the temperature sensitivity of the processes leading to the production of these gases. The temperature response was reduced under substrate limitation. Apparent activation energy for aerobic CO2 production was similar (E a ~ 30 kJ mol−1) for tested substrates, while E a for anaerobic N2O production increased in the order phenylalanine < glycine < glucose + NO3 − having values 45, 75, and 106 kJ mol−1, respectively. Commonly, the temperature sensitivity of N2O production (2 < Q 10 < 4.5) was much higher than that for CO2 (Q 10 ≤ 1.5).

Response of atmospheric CO2 to the abrupt cooling event 8200 years ago

AbstractAtmospheric CO2 records for the centennial scale cooling event 8200 years ago (8.2 ka event) may help us understand climate‐carbon cycle feedbacks under interglacial conditions, which are important for understanding future climate, but existing records do not provide enough detail. Here we present a new CO2 record from the Siple Dome ice core, Antarctica, that covers 7.4–9.0 ka with 8 to 16 year resolution. We observe a small, about 1–2 ppm, increase of atmospheric CO2 during the 8.2 ka event. The increase is not significant when compared to other centennial variations in the Holocene that are not linked to large temperature changes. Our results do not agree with leaf stomata records that suggest a CO2 decrease of up to ~25 ppm and imply that the sensitivity of atmospheric CO2 to the primarily Northern Hemisphere cooling of the 8.2 ka event was limited.

POSS-based luminescent porous polymers for carbon dioxide sorption and nitroaromatic explosives detection

Luminescent porous polymers based on octavinylsilsequioxane and triphenylamine units show tunable porosity and luminescence, a moderate uptake of CO2 and high sensitivity for TNT.

Chapter 1 - Physiological and pathophysiological interactions between the respiratory central pattern generator and the sympathetic nervous system

Respiratory modulation seen in the sympathetic nerve activity (SNA) implies that the respiratory and sympathetic networks interact. During hypertension elicited by chronic intermittent hypoxia (CIH), the SNA displays an enhanced respiratory modulation reflecting strengthened interactions between the networks. In this chapter, we review a series of experimental and modeling studies that help elucidate possible mechanisms of sympatho-respiratory coupling. We conclude that this coupling significantly contributes to both the sympathetic baroreflex and the augmented sympathetic activity after exposure to CIH. This conclusion is based on the following findings. (1) Baroreceptor activation results in perturbation of the respiratory pattern via transient activation of postinspiratory neurons in the Bötzinger complex (BötC). The same BötC neurons are involved in the respiratory modulation of SNA, and hence provide an additional pathway for the sympathetic baroreflex. (2) Under hypercapnia, phasic activation of abdominal motor nerves (AbN) is accompanied by synchronous discharges in SNA due to the common source of this rhythmic activity in the retrotrapezoid nucleus (RTN). CIH conditioning increases the CO2 sensitivity of central chemoreceptors in the RTN which results in the emergence of AbN and SNA discharges under normocapnic conditions similar to those observed during hypercapnia in naïve animals. Thus, respiratory-sympathetic interactions play an important role in defining sympathetic output and significantly contribute to the sympathetic activity and hypertension under certain physiological or pathophysiological conditions, and the theoretical framework presented may be instrumental in understanding of malfunctioning control of sympathetic activity in a variety of disease states.

Effect of Increased CO2 And Temperature on Growth, Photosynthesis And Lipid Content of Tropical Algae

Increasing temperatures causally linked with anthropogenically elevated atmospheric carbon dioxide (CO2 ) levels characterise current ‘global warming’. Very little is known about how these factors will affect tropical microalgae. The aim of this study was to investigate the CO2 and temperature sensitivity of the photosynthetic process of two tropical Chlorella strains (Chlorella vulgaris UMACC 001 and Chlorella vulgaris UMACC 014). We grew cultures of the two strains under four conditions: (1) 28°C and 375 ppm CO2 (control), (2) 28°C and 750 ppm CO2 (high CO2 ), (3) 32°C and 375 ppm CO2 (high temperature), and (4) 32°C and 750 ppm CO2 (combined factors). Based on specifi c growth rate (μ), both Chlorella strains grew best under the combined factors of high CO2 level (750 ppm) and high temperature (32°C). Elevated CO2 stimulated the production of lipid in both strains. (Keywords: Chlorella, temperature, CO2 , photosynthesis )

Physiological and genetic analysis of CO2-induced breakdown of self-incompatibility in Brassica rapa

Self-incompatibility (SI) of the Brassicaceae family can be overcome by CO2 gas treatment. This method has been used for decades as an effective means to obtain a large amount of inbred seeds which can then be used for F1 hybrid seed production; however, the molecular mechanism by which CO2 alters the SI pathway has not been elucidated. In this study, to obtain new insights into the mechanism of CO2-induced SI breakdown, the focus was on two inbred lines of Brassica rapa (syn. campestris) with different CO2 sensitivity. Physiological examination using X-ray microanalysis suggested that SI breakdown in the CO2-sensitive line was accompanied by a significant accumulation of calcium at the pollen–stigma interface. Pre-treatment of pollen or pistil with CO2 gas before pollination showed no effect on the SI reaction, suggesting that some physiological process after pollination is necessary for SI to be overcome. Genetic analyses using F1 progeny of a CO2-sensitive×CO2-insensitive cross suggested that CO2 sensitivity is a semi-dominant trait in these lines. Analysis of F2 progeny suggested that CO2 sensitivity could be a quantitative trait, which is controlled by more than one gene. Quantitative trait locus (QTL) analyses identified two major loci, BrSIO1 and BrSIO2, which work additively in overcoming SI during CO2 treatment. No QTL was detected at the loci previously shown to affect SI stability, suggesting that CO2 sensitivity is determined by novel genes. The QTL data presented here should be useful for determining the responsible genes, and for the marker-assisted selection of desirable parental lines with stable but CO2-sensitive SI in F1 hybrid breeding.

High-resolution mineral dust and sea ice proxy records from the Talos Dome ice core

Abstract. In this study we report on new non-sea salt calcium (nssCa2+, mineral dust proxy) and sea salt sodium (ssNa+, sea ice proxy) records along the East Antarctic Talos Dome deep ice core in centennial resolution reaching back 150 thousand years (ka) before present. During glacial conditions nssCa2+ fluxes in Talos Dome are strongly related to temperature as has been observed before in other deep Antarctic ice core records, and has been associated with synchronous changes in the main source region (southern South America) during climate variations in the last glacial. However, during warmer climate conditions Talos Dome mineral dust input is clearly elevated compared to other records mainly due to the contribution of additional local dust sources in the Ross Sea area. Based on a simple transport model, we compare nssCa2+ fluxes of different East Antarctic ice cores. From this multi-site comparison we conclude that changes in transport efficiency or atmospheric lifetime of dust particles do have a minor effect compared to source strength changes on the large-scale concentration changes observed in Antarctic ice cores during climate variations of the past 150 ka. Our transport model applied on ice core data is further validated by climate model data. The availability of multiple East Antarctic nssCa2+ records also allows for a revision of a former estimate on the atmospheric CO2 sensitivity to reduced dust induced iron fertilisation in the Southern Ocean during the transition from the Last Glacial Maximum to the Holocene (T1). While a former estimate based on the EPICA Dome C (EDC) record only suggested 20 ppm, we find that reduced dust induced iron fertilisation in the Southern Ocean may be responsible for up to 40 ppm of the total atmospheric CO2 increase during T1. During the last interglacial, ssNa+ levels of EDC and EPICA Dronning Maud Land (EDML) are only half of the Holocene levels, in line with higher temperatures during that period, indicating much reduced sea ice extent in the Atlantic as well as the Indian Ocean sector of the Southern Ocean. In contrast, Holocene ssNa+ flux in Talos Dome is about the same as during the last interglacial, indicating that there was similar ice cover present in the Ross Sea area during MIS 5.5 as during the Holocene.

Climate sensitivities of two versions of FGOALS model to idealized radiative forcing

Projections of future climate change by climate system models depend on the sensitivities of models to specified greenhouse gases. To reveal and understand the different climate sensitivities of two versions of LASG/IAP climate system model FGOALS-g2 and FGOALS-s2, we investigate the global mean surface air temperature responses to idealized CO2 forcing by using the output of abruptly quadrupling CO2 experiments. The Gregory-style regression method is used to estimate the “radiative forcing” of quadrupled CO2 and equilibrium sensitivity. The model response is separated into a fast-response stage associated with the CO2 forcing during the first 20 years, and a slow-response stage post the first 20 years. The results show that the radiative forcing of CO2 is overestimated due to the positive water-vapor feedback and underestimated due to the fast cloud processes. The rapid response of water vapor in FGOALS-s2 is responsible for the stronger radiative forcing of CO2. The climate sensitivity, defined as the equilibrium temperature change under doubled CO2 forcing, is about 3.7 K in FGOALS-g2 and 4.5 K in FGOALS-s2. The larger sensitivity of FGOALS-s2 is due mainly to the weaker negative longwave clear-sky feedback and stronger positive shortwave clear-sky feedback at the fast-response stage, because of the more rapid response of water vapor increase and sea-ice decrease in FGOALS-s2 than in FGOALS-g2. At the slow-response stage, similar to the fast-response stage, net negative clear-sky feedback is weaker in FGOALS-s2. Nevertheless, the total negative feedback is larger in FGOALS-s2 due to a larger negative shortwave cloud feedback that involves a larger response of total cloud fraction and condensed water path increase. The uncertainties of estimated forcing and net feedback mainly come from the shortwave cloud processes.

CO2 directly modulates connexin 26 by formation of carbamate bridges between subunits

Homeostatic regulation of the partial pressure of CO2 (PCO2) is vital for life. Sensing of pH has been proposed as a sufficient proxy for determination of PCO2 and direct CO2-sensing largely discounted. Here we show that connexin 26 (Cx26) hemichannels, causally linked to respiratory chemosensitivity, are directly modulated by CO2. A 'carbamylation motif', present in CO2-sensitive connexins (Cx26, Cx30, Cx32) but absent from a CO2-insensitive connexin (Cx31), comprises Lys125 and four further amino acids that orient Lys125 towards Arg104 of the adjacent subunit of the connexin hexamer. Introducing the carbamylation motif into Cx31 created a mutant hemichannel (mCx31) that was opened by increases in PCO2. Mutation of the carbamylation motif in Cx26 and mCx31 destroyed CO2 sensitivity. Course-grained computational modelling of Cx26 demonstrated that the proposed carbamate bridge between Lys125 and Arg104 biases the hemichannel to the open state. Carbamylation of Cx26 introduces a new transduction principle for physiological sensing of CO2. DOI: http://dx.doi.org/10.7554/eLife.01213.001.