An experiment to assess the effects of diatom dissolution on oxygen isotope ratios

Rationale: Current studies which use the oxygen isotope composition from diatom silica (δ18Odiatom) as a palaeoclimate proxy assume that the δ18Odiatom value reflects the isotopic composition of the water in which the diatom formed. However, diatoms dissolve post mortem, preferentially losing less silicified structures in the water column and during/after burial into sediments. The impact of dissolution on δ18Odiatom values and potential misinterpretation of the palaeoclimate record are evaluated. Methods: Diatom frustules covering a range of ages (6 samples from the Miocene to the Holocene), environments and species were exposed to a weak alkaline solution for 48 days at two temperatures (20 °C and 4 °C), mimicking natural dissolution post mucilage removal. Following treatment, dissolution was assessed using scanning electron microscope images and a qualitative diatom dissolution index. The diatoms were subsequently analysed for their δ18O values using step-wise fluorination and isotope ratio mass spectrometry. Results: Variable levels of diatom dissolution were observed between the six samples; in all cases higher temperatures resulted in more frustule degradation. Dissolution was most evident in younger samples, probably as a result of the more porous nature of the silica. The degree of diatom dissolution does not directly equate to changes in the isotope ratios; the δ18Odiatom value was, however, lower after dissolution, but in only half the samples was this reduction outside the analytical error (2σ analytical error = 0.46‰). Conclusions: We have shown that dissolution can have a small negative impact on δ18Odiatom values, causing reductions of up to 0.59‰ beyond analytical error (0.46‰) at natural environmental temperatures. These findings need to be considered in palaeoenvironmental reconstructions using δ18Odiatom values, especially when interpreting variations in these values of <1‰

The biochemical mechanisms of action of tigliane and phorbol esters

Initial studies adding TPA and Sap A to Human Mononuclear cell (HMNC) cultures found that the mitogenic action of Sap A was abolished if cells were incubated for 3 days prior to phorbol ester addition. Previously the phorbol ester receptor has been found to be a family of protein kinases (PKC's). The relationship between biochemical action and biological effect was investigated using rat brain as the enzyme source. Phorbol ester-stimulated kinase activities in rat brain hydroxylapatite FPLC fractions were investigated using a modified PS/Triton micellar assay. Proteins in the elution profile were separated by SDS-PAGE and immuno-blotted with antisera specific for PKC α, β1, γ, δ and ϵ to identify the PKC isotypes present.These investigations were coordinated with phorbol ester activation studies on PKC Isotypes (α, β1, γ, δ and ϵ) purified from bovine brain. This methodology was then applied to screen eluted proteins from subpopulations of HMNC as well as Daudi cells and mouse macrophages. These approaches revealed several phorbol ester-sensitive protein kinase activities, which could not be identified as PKC α, β1, γ, δ and ϵ Isotypes by elution or immunological definition, in crudely fractionated brain and cell extracts. Some of these activities demonstrated tissue specificity when elution positions characteristic of brain were compared to those seen for cultured cells. Preparations of HMNC's were also found to contain an entirely novel kinase activity which was stimulatable by Resiniferatoxin (Rx, a daphnane ester) only in the absence of added calcium (termed Rx-Kinase). A similar activity was also isolated, in greater quantity, from starch-elicited mouse peritoneal macrophages under identical conditions. Fractions of mouse Rx-Kinase activity were found to potently activate the reconstituted mouse NADPH oxidase system to generate superoxide in vitro In the presence of Rx and the absence of calcium (i.e. conditions corresponding to its histone kinase requirements in vitro).This suggests major differences between this putative phorbol ester receptor and the PKC family of isotypes

Experimental assessment and implications of long‐term within‐trap mineralization of seston in lake trapping studies

Sediment trapping is a widely accepted technique in lake studies for analyzing seasonal limnological events and can provide insight into ecological succession as well as the seasonal dynamics of organic and inorganic fluxes. More recently, organic flux measurement from traps has been especially important in estimating whole‐lake C sequestration as a basis for regional and global upscaling of C budgets across lake types. However, in‐trap mineralization or dissolution of components of collected sediment (seston) has not been systematically examined, and thus a reliable correction factor for in‐trap losses of various sedimentary fluxes (especially those involving organic carbon) is still unknown. This experimental study assesses the loss of algal biomass representative of a 6‐month carousel‐type (closed) sediment trap deployment in a deep, eutrophic lake under cold (~ 5°C) and anoxic ambient conditions typical of the hypolimnion in stratifying, temperate lakes. Results show a loss of organic matter (OM) at a consistent rate over 180 d, reducing the fraction of initial OM content by approximately a third after 180 d of deployment (linear regression of OM fraction loss = −0.001864t ). The significance of these findings is demonstrated by application to published trap data; at Rostherne Mere, UK, which implies that annual OM fluxes are underestimated by 18.2% on average (range 13.7–23.2%). This highlights the far‐reaching implications for lake sediment trap methodology and our understanding of seston taphonomy, suggesting a mineralization correction factor for OM should be applied to traps deployed for longer than 1 week. With loss correction factored in, this study supports the reliability of longer‐term (i.e., ~ 6 months) sediment trap deployment

Diatom-based models for inferring past water chemistry in western Ugandan crater lakes

Diatom surface sediment samples and corresponding water chemistry were collected from 56 lakes across a natural conductivity gradient in western Uganda (reflecting a regional climatic gradient of effective moisture) to explore factors controlling diatom distribution. Here we develop a regional training set from these crater lakes to test the hypothesis that this approach, by providing more appropriate and closer analogues, can improve the accuracy of palaeo-conductivity reconstructions, and so environmental inferences in these lake systems compared to larger training sets. We compare this output to models based on larger, but geographically and limnologically diverse training sets, using the European Diatom Database Initiative (EDDI) database. The relationships between water chemistry and diatom distributions were explored using canonical correspondence analysis (CCA) and partial CCA. Variance partitioning indicated that conductivity accounted for a significant and independent portion of this variation. A transfer function was developed for conductivity (r jack 2 = 0.74). Prediction errors, estimated using jack-knifing, are low for the conductivity model (0.256 log10 units). The resulting model was applied to a sedimentary sequence from Lake Kasenda, western Uganda. Comparison of conductivity reconstructions using the Ugandan crater lake training set and the East Africa training set (EDDI) highlighted a number of differences in the optima of key diatom taxa, which lead to differences in reconstructed values and could lead to misinterpretation of the fossil record. This study highlights issues of how far transfer functions based on continental-scale lake datasets such as the EDDI pan-African models should be used and the benefits that may be obtained from regional training sets

Diatom dissolution in saline lake sediments: an experimental study in the Great Plains of North America

Environmental reconstructions are limited by the quality of the original data from which they are derived. In situations where microfossils are poorly preserved problems may arise, both through taxonomic uncertainty and more subtly from the alteration of the death assemblage as a result of the differential robustness of species. Diatom dissolution tends to be a particular problem in saline lakes. Laboratory-based dissolution experiments on fresh, modern diatoms collected from lakes in North and South Dakota and Saskatchewan were carried out to establish the abundance and composition changes of assemblages as dissolution progressed. Analysis of experimental results demonstrates that species exhibit regular dissolution relationships and can be ranked according to susceptibility to dissolution. Changes in valve morphology for selected key taxa were categorised under scanning electron and light microscopy into 'dissolution stages'. These data provide the basis for developing dissolution indices for individual taxa and assemblages, which can be related to absolute abundance changes of diatom valves. Experimental data were applied to two separate weighted averaging (WA) transfer functions to predict measured dissolution parameters (such as dissolved silica) and to a salinity transfer function developed from the Northern Great Plains. In the former case, models incorporating dissolution stage counting were more accurate and robust (as validated by jackknifing). Species and samples were downweighted according to species robustness (dissolution rank) within the WA transfer function. Downweighting either, or both, species and samples in the transfer function algorithm lead to minor improvements in model performance in terms of both r2 and standard error (as RMSE), despite incomplete coverage of species. A short core from Spiritwood Lake, North Dakota, was used to test the differences variable weighting had on reconstructed salinity. Results suggest Spiritwood Lake is only responsive to more extreme climatic events, and has remained fresh (<0.5g/l TDS) or subsaline (0.5-<3g/l TDS) throughout the last 150 years. The approach of variable sample and species weighting to the rest of the NGP surface sediment assemblage training set may improve the model further, which could be tested at sites with an historical record of salinity

TPA and resiniferatoxin-mediated activation of NADPH-oxidase A possible role for Rx-kinase augmentation of PKC

AbstractThe non-tumour promoting irritant, resiniferatoxin, was capable of activating the NADPH-oxidase respiratory burst of starch-elicited, but not resident mouse peritoneal macrophages in vitro. Unlike TPA, the response was synergised by incubation with zymosan. The Rx-stimulated NADPH-oxidase activity in a cell-free assay was selectively enhanced in the presence of exogenous Rx-kinase rather than PKC and in the absence of Ca2+. Since resiniferatoxin is a poor activator of PKC, it is probable that the Ca2+-independent Rx-kinase plays a role in activation of the macrophage respiratory burst following stimulation by zymosan

Experimental Diatom Dissolution and the Quantification of Microfossil Preservation in Sediments

Four laboratory experiments on fresh, modern diatoms collected from lakes in the Northern Great Plains of North America were carried out to assess the effects of dissolution on diatom abundance and composition. Marked differences in mean dissolution susceptibility exist between species, despite sometimes significant intra- specific variation between heterovalves. Twenty-four taxa were ranked according to susceptibility to dissolution using an exponential decay model of valve abundance. This dissolution ranking was used to derive two weighted indices of sample preservation. A third index (F) was based on a simple binary classification of valve morphology into dissolved and pristine categories, as distinguished by light microscopy (LM). When compared against rank indices and a measure of species diversity, this diatom dissolution index was found to be the best predictor of the progress of dissolution as estimated by total valve abundance or biogenic silica (BiSi) loss. Strong empirical relationships between F index values and diatom abundance (r2 = 0.84, n = 32) and BiSi (r2 = 0.89, n = 32) were developed and applied to a diatom sequence from a short core from Devils Lake, North Dakota, and compared to diatom-inferred and observed salinity at this site. The F index is a simple, effective diagnostic tool to assess important aspects of diatom preservation. The index can provide insight into Si cycling and record changes in conditions pertinent to diatom dissolution, and has a role in validation of transfer functions or other inferences derived from compositional data

Understanding the delayed prescribing of antibiotics for respiratory tract infection in primary care: a qualitative analysis

OBJECTIVE: To identify general practitioner (GP) views and understanding on the use of delayed prescribing in primary care.DESIGN: Qualitative semistructured telephone interview study.SETTING: Primary care general practices in England.PARTICIPANTS: 32 GPs from identified high-prescribing and low-prescribing general practices in England.METHOD: Semistructured telephone interviews were conducted with GPs identified from practices within clinical commissioning groups with the highest and lowest prescribing rates in England. A thematic analysis of the data was conducted to generate themes.RESULTS: All GPs had a good understanding of respiratory tract infection (RTI) management and how the delayed prescribing approach could be used in primary care. However, GPs highlighted factors that were influential as to whether delayed prescribing was successfully carried out during the consultation. These included the increase in evidence of antimicrobial resistance, and GPs' prior experiences of using delayed prescribing during the consultation. The patient-practitioner relationship could also influence treatment outcomes for RTI, and a lack of an agreed prescribing strategy within and between practices was considered to be of significance to GPs. Participants expressed that a lack of feedback on prescribing data at an individual and practice level made it difficult to know if delayed prescribing strategies were successful in reducing unnecessary consumption. GPs agreed that coherent and uniform training and guidelines would be of some benefit to ensure consistent prescribing throughout the UK.CONCLUSIONS: Delayed prescribing is encouraged in primary care, but is not always implemented successfully. Greater uniformity within and between practices in the UK is needed to operationalise delayed prescribing, as well as providing feedback on the uptake of antibiotics. Finally, GPs may need further guidance on how to answer the concerns of patients without interpreting these questions as a demand for antibiotics, as well as educating the patient about antimicrobial resistance and supporting a good patient-practitioner relationship